Published by the Free Software Foundation
Copyright © 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with the Invariant Sections being “GNU General Public License” and “Funding Free Software”, the Front-Cover texts being (a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in the section entitled “GNU Free Documentation License”.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU software. Copies published by the Free Software Foundation raise funds for GNU development.
typeof
void- and Function-Pointers
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enum Types
This manual documents how to use the GNU compilers, as well as their features and incompatibilities, and how to report bugs. It corresponds to GCC version 3.3.5. The internals of the GNU compilers, including how to port them to new targets and some information about how to write front ends for new languages, are documented in a separate manual. See Introduction (GNU Compiler Collection (GCC) Internals).
Several versions of the compiler (C, C++, Objective-C, Ada, Fortran, Java and treelang) are integrated; this is why we use the name “GNU Compiler Collection”. GCC can compile programs written in any of these languages. The Ada, Fortran, Java and treelang compilers are described in separate manuals.
“GCC” is a common shorthand term for the GNU Compiler Collection. This is both the most general name for the compiler, and the name used when the emphasis is on compiling C programs (as the abbreviation formerly stood for “GNU C Compiler”).
When referring to C++ compilation, it is usual to call the compiler “G++”. Since there is only one compiler, it is also accurate to call it “GCC” no matter what the language context; however, the term “G++” is more useful when the emphasis is on compiling C++ programs.
Similarly, when we talk about Ada compilation, we usually call the compiler “GNAT”, for the same reasons.
We use the name “GCC” to refer to the compilation system as a whole, and more specifically to the language-independent part of the compiler. For example, we refer to the optimization options as affecting the behavior of “GCC” or sometimes just “the compiler”.
Front ends for other languages, such as Mercury and Pascal exist but have not yet been integrated into GCC. These front ends, like that for C++, are built in subdirectories of GCC and link to it. The result is an integrated compiler that can compile programs written in C, C++, Objective-C, or any of the languages for which you have installed front ends.
In this manual, we only discuss the options for the C, Objective-C, and C++ compilers and those of the GCC core. Consult the documentation of the other front ends for the options to use when compiling programs written in other languages.
G++ is a compiler, not merely a preprocessor. G++ builds object code directly from your C++ program source. There is no intermediate C version of the program. (By contrast, for example, some other implementations use a program that generates a C program from your C++ source.) Avoiding an intermediate C representation of the program means that you get better object code, and better debugging information. The GNU debugger, GDB, works with this information in the object code to give you comprehensive C++ source-level editing capabilities (see C and C++ (Debugging with GDB)).
For each language compiled by GCC for which there is a standard, GCC attempts to follow one or more versions of that standard, possibly with some exceptions, and possibly with some extensions.
GCC supports three versions of the C standard, although support for the most recent version is not yet complete.
The original ANSI C standard (X3.159-1989) was ratified in 1989 and published in 1990. This standard was ratified as an ISO standard (ISO/IEC 9899:1990) later in 1990. There were no technical differences between these publications, although the sections of the ANSI standard were renumbered and became clauses in the ISO standard. This standard, in both its forms, is commonly known as C89, or occasionally as C90, from the dates of ratification. The ANSI standard, but not the ISO standard, also came with a Rationale document. To select this standard in GCC, use one of the options -ansi, -std=c89 or -std=iso9899:1990; to obtain all the diagnostics required by the standard, you should also specify -pedantic (or -pedantic-errors if you want them to be errors rather than warnings). See Options Controlling C Dialect.
Errors in the 1990 ISO C standard were corrected in two Technical Corrigenda published in 1994 and 1996. GCC does not support the uncorrected version.
An amendment to the 1990 standard was published in 1995. This
amendment added digraphs and __STDC_VERSION__ to the language,
but otherwise concerned the library. This amendment is commonly known
as AMD1; the amended standard is sometimes known as C94 or
C95. To select this standard in GCC, use the option
-std=iso9899:199409 (with, as for other standard versions,
-pedantic to receive all required diagnostics).
A new edition of the ISO C standard was published in 1999 as ISO/IEC 9899:1999, and is commonly known as C99. GCC has incomplete support for this standard version; see http://gcc.gnu.org/gcc-3.3/c99status.html for details. To select this standard, use -std=c99 or -std=iso9899:1999. (While in development, drafts of this standard version were referred to as C9X.)
Errors in the 1999 ISO C standard were corrected in a Technical Corrigendum published in 2001. GCC does not support the uncorrected version.
By default, GCC provides some extensions to the C language that on rare occasions conflict with the C standard. See Extensions to the C Language Family. Use of the -std options listed above will disable these extensions where they conflict with the C standard version selected. You may also select an extended version of the C language explicitly with -std=gnu89 (for C89 with GNU extensions) or -std=gnu99 (for C99 with GNU extensions). The default, if no C language dialect options are given, is -std=gnu89; this will change to -std=gnu99 in some future release when the C99 support is complete. Some features that are part of the C99 standard are accepted as extensions in C89 mode.
The ISO C standard defines (in clause 4) two classes of conforming
implementation. A conforming hosted implementation supports the
whole standard including all the library facilities; a conforming
freestanding implementation is only required to provide certain
library facilities: those in <float.h>, <limits.h>,
<stdarg.h>, and <stddef.h>; since AMD1, also those in
<iso646.h>; and in C99, also those in <stdbool.h> and
<stdint.h>. In addition, complex types, added in C99, are not
required for freestanding implementations. The standard also defines
two environments for programs, a freestanding environment,
required of all implementations and which may not have library
facilities beyond those required of freestanding implementations,
where the handling of program startup and termination are
implementation-defined, and a hosted environment, which is not
required, in which all the library facilities are provided and startup
is through a function int main (void) or int main (int,
char *[]). An OS kernel would be a freestanding environment; a
program using the facilities of an operating system would normally be
in a hosted implementation.
GCC aims towards being usable as a conforming freestanding
implementation, or as the compiler for a conforming hosted
implementation. By default, it will act as the compiler for a hosted
implementation, defining __STDC_HOSTED__ as 1 and
presuming that when the names of ISO C functions are used, they have
the semantics defined in the standard. To make it act as a conforming
freestanding implementation for a freestanding environment, use the
option -ffreestanding; it will then define
__STDC_HOSTED__ to 0 and not make assumptions about the
meanings of function names from the standard library, with exceptions
noted below. To build an OS kernel, you may well still need to make
your own arrangements for linking and startup.
See Options Controlling C Dialect.
GCC does not provide the library facilities required only of hosted implementations, nor yet all the facilities required by C99 of freestanding implementations; to use the facilities of a hosted environment, you will need to find them elsewhere (for example, in the GNU C library). See Standard Libraries.
Most of the compiler support routines used by GCC are present in
libgcc, but there are a few exceptions. GCC requires the
freestanding environment provide memcpy, memmove,
memset and memcmp. Some older ports of GCC are
configured to use the BSD bcopy, bzero and bcmp
functions instead, but this is deprecated for new ports.
Finally, if __builtin_trap is used, and the target does
not implement the trap pattern, then GCC will emit a call
to abort.
For references to Technical Corrigenda, Rationale documents and information concerning the history of C that is available online, see http://gcc.gnu.org/readings.html
There is no formal written standard for Objective-C. The most authoritative manual is “Object-Oriented Programming and the Objective-C Language”, available at a number of web sites
There is no standard for treelang, which is a sample language front end for GCC. Its only purpose is as a sample for people wishing to write a new language for GCC. The language is documented in gcc/treelang/treelang.texi which can be turned into info or HTML format.
See GNAT Reference Manual (GNAT Reference Manual), for information on standard conformance and compatibility of the Ada compiler.
See The GNU Fortran Language (Using and Porting GNU Fortran), for details of the Fortran language supported by GCC.
See Compatibility with the Java Platform (GNU gcj), for details of compatibility between gcj and the Java Platform.
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The “overall options” allow you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists of object files output by the assembler.
Other options are passed on to one stage of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them.
Most of the command line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages.
See Compiling C++ Programs, for a summary of special options for compiling C++ programs.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dr is very different from -d -r.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified.
Many options have long names starting with -f or with -W—for example, -fforce-mem, -fstrength-reduce, -Wformat and so on. Most of these have both positive and negative forms; the negative form of -ffoo would be -fno-foo. This manual documents only one of these two forms, whichever one is not the default.
See Option Index, for an index to GCC's options.
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
-c -S -E -o file -pipe -pass-exit-codes
-x language -v -### --help --target-help --version
-ansi -std=standard -aux-info filename
-fno-asm -fno-builtin -fno-builtin-function
-fhosted -ffreestanding -fms-extensions
-trigraphs -no-integrated-cpp -traditional -traditional-cpp
-fallow-single-precision -fcond-mismatch
-fsigned-bitfields -fsigned-char
-funsigned-bitfields -funsigned-char
-fwritable-strings
-fabi-version=n -fno-access-control -fcheck-new
-fconserve-space -fno-const-strings -fdollars-in-identifiers
-fno-elide-constructors
-fno-enforce-eh-specs -fexternal-templates
-falt-external-templates
-ffor-scope -fno-for-scope -fno-gnu-keywords
-fno-implicit-templates
-fno-implicit-inline-templates
-fno-implement-inlines -fms-extensions
-fno-nonansi-builtins -fno-operator-names
-fno-optional-diags -fpermissive
-frepo -fno-rtti -fstats -ftemplate-depth-n
-fuse-cxa-atexit -fvtable-gc -fno-weak -nostdinc++
-fno-default-inline -Wabi -Wctor-dtor-privacy
-Wnon-virtual-dtor -Wreorder
-Weffc++ -Wno-deprecated
-Wno-non-template-friend -Wold-style-cast
-Woverloaded-virtual -Wno-pmf-conversions
-Wsign-promo -Wsynth
-fconstant-string-class=class-name
-fgnu-runtime -fnext-runtime -gen-decls
-Wno-protocol -Wselector -Wundeclared-selector
-fmessage-length=n
-fdiagnostics-show-location=[once|every-line]
-fsyntax-only -pedantic -pedantic-errors
-w -W -Wall -Waggregate-return
-Wcast-align -Wcast-qual -Wchar-subscripts -Wcomment
-Wconversion -Wno-deprecated-declarations
-Wdisabled-optimization -Wno-div-by-zero -Werror
-Wfloat-equal -Wformat -Wformat=2
-Wformat-nonliteral -Wformat-security
-Wimplicit -Wimplicit-int
-Wimplicit-function-declaration
-Werror-implicit-function-declaration
-Wimport -Winline -Wno-endif-labels
-Wlarger-than-len -Wlong-long
-Wmain -Wmissing-braces
-Wmissing-format-attribute -Wmissing-noreturn
-Wno-multichar -Wno-format-extra-args -Wno-format-y2k
-Wno-import -Wnonnull -Wpacked -Wpadded
-Wparentheses -Wpointer-arith -Wredundant-decls
-Wreturn-type -Wsequence-point -Wshadow
-Wsign-compare -Wstrict-aliasing
-Wswitch -Wswitch-default -Wswitch-enum
-Wsystem-headers -Wtrigraphs -Wundef -Wuninitialized
-Wunknown-pragmas -Wunreachable-code
-Wunused -Wunused-function -Wunused-label -Wunused-parameter
-Wunused-value -Wunused-variable -Wwrite-strings
-Wbad-function-cast -Wmissing-declarations
-Wmissing-prototypes -Wnested-externs
-Wstrict-prototypes -Wtraditional
-dletters -dumpspecs -dumpmachine -dumpversion
-fdump-unnumbered -fdump-translation-unit[-n]
-fdump-class-hierarchy[-n]
-fdump-tree-original[-n]
-fdump-tree-optimized[-n]
-fdump-tree-inlined[-n]
-feliminate-dwarf2-dups -fmem-report
-fprofile-arcs -frandom-seed=n
-fsched-verbose=n -ftest-coverage -ftime-report
-g -glevel -gcoff -gdwarf -gdwarf-1 -gdwarf-1+ -gdwarf-2
-ggdb -gstabs -gstabs+ -gvms -gxcoff -gxcoff+
-p -pg -print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib
-print-prog-name=program -print-search-dirs -Q
-save-temps -time
-falign-functions=n -falign-jumps=n
-falign-labels=n -falign-loops=n
-fbranch-probabilities -fcaller-saves -fcprop-registers
-fcse-follow-jumps -fcse-skip-blocks -fdata-sections
-fdelayed-branch -fdelete-null-pointer-checks
-fexpensive-optimizations -ffast-math -ffloat-store
-fforce-addr -fforce-mem -ffunction-sections
-fgcse -fgcse-lm -fgcse-sm -floop-optimize -fcrossjumping
-fif-conversion -fif-conversion2
-finline-functions -finline-limit=n -fkeep-inline-functions
-fkeep-static-consts -fmerge-constants -fmerge-all-constants
-fmove-all-movables -fnew-ra -fno-branch-count-reg
-fno-default-inline -fno-defer-pop
-fno-function-cse -fno-guess-branch-probability
-fno-inline -fno-math-errno -fno-peephole -fno-peephole2
-funsafe-math-optimizations -ffinite-math-only
-fno-trapping-math -fno-zero-initialized-in-bss
-fomit-frame-pointer -foptimize-register-move
-foptimize-sibling-calls -fprefetch-loop-arrays
-freduce-all-givs -fregmove -frename-registers
-freorder-blocks -freorder-functions
-frerun-cse-after-loop -frerun-loop-opt
-fschedule-insns -fschedule-insns2
-fno-sched-interblock -fno-sched-spec -fsched-spec-load
-fsched-spec-load-dangerous -fsignaling-nans
-fsingle-precision-constant -fssa -fssa-ccp -fssa-dce
-fstrength-reduce -fstrict-aliasing -ftracer -fthread-jumps
-funroll-all-loops -funroll-loops
--param name=value
-O -O0 -O1 -O2 -O3 -Os
-$ -Aquestion=answer
-A-question[=answer]
-C -dD -dI -dM -dN
-Dmacro[=defn] -E -H
-idirafter dir
-include file -imacros file
-iprefix file -iwithprefix dir
-iwithprefixbefore dir -isystem dir
-M -MM -MF -MG -MP -MQ -MT -nostdinc -P -remap
-trigraphs -undef -Umacro -Wp,option
-Wa,option
object-file-name -llibrary
-nostartfiles -nodefaultlibs -nostdlib
-s -static -static-libgcc -shared -shared-libgcc -symbolic
-Wl,option -Xlinker option
-u symbol
-Bprefix -Idir -I- -Ldir -specs=file
-V version -b machine
M680x0 Options
-m68000 -m68020 -m68020-40 -m68020-60 -m68030 -m68040
-m68060 -mcpu32 -m5200 -m68881 -mbitfield -mc68000 -mc68020
-mfpa -mnobitfield -mrtd -mshort -msoft-float -mpcrel
-malign-int -mstrict-align
M68hc1x Options
-m6811 -m6812 -m68hc11 -m68hc12 -m68hcs12
-mauto-incdec -minmax -mlong-calls -mshort
-msoft-reg-count=count
VAX Options
-mg -mgnu -munix
SPARC Options
-mcpu=cpu-type
-mtune=cpu-type
-mcmodel=code-model
-m32 -m64
-mapp-regs -mbroken-saverestore -mcypress
-mfaster-structs -mflat
-mfpu -mhard-float -mhard-quad-float
-mimpure-text -mlittle-endian -mlive-g0 -mno-app-regs
-mno-faster-structs -mno-flat -mno-fpu
-mno-impure-text -mno-stack-bias -mno-unaligned-doubles
-msoft-float -msoft-quad-float -msparclite -mstack-bias
-msupersparc -munaligned-doubles -mv8
-threads -pthreads
ARM Options
-mapcs-frame -mno-apcs-frame
-mapcs-26 -mapcs-32
-mapcs-stack-check -mno-apcs-stack-check
-mapcs-float -mno-apcs-float
-mapcs-reentrant -mno-apcs-reentrant
-msched-prolog -mno-sched-prolog
-mlittle-endian -mbig-endian -mwords-little-endian
-malignment-traps -mno-alignment-traps
-msoft-float -mhard-float -mfpe
-mthumb-interwork -mno-thumb-interwork
-mcpu=name -march=name -mfpe=name
-mstructure-size-boundary=n
-mabort-on-noreturn
-mlong-calls -mno-long-calls
-msingle-pic-base -mno-single-pic-base
-mpic-register=reg
-mnop-fun-dllimport
-mpoke-function-name
-mthumb -marm
-mtpcs-frame -mtpcs-leaf-frame
-mcaller-super-interworking -mcallee-super-interworking
MN10200 Options
-mrelax
MN10300 Options
-mmult-bug -mno-mult-bug
-mam33 -mno-am33
-mno-crt0 -mrelax
M32R/D Options
-m32rx -m32r -mcode-model=model-type
-msdata=sdata-type -G num
M88K Options
-m88000 -m88100 -m88110 -mbig-pic
-mcheck-zero-division -mhandle-large-shift
-midentify-revision -mno-check-zero-division
-mno-ocs-debug-info -mno-ocs-frame-position
-mno-optimize-arg-area -mno-serialize-volatile
-mno-underscores -mocs-debug-info
-mocs-frame-position -moptimize-arg-area
-mserialize-volatile -mshort-data-num -msvr3
-msvr4 -mtrap-large-shift -muse-div-instruction
-mversion-03.00 -mwarn-passed-structs
RS/6000 and PowerPC Options
-mcpu=cpu-type
-mtune=cpu-type
-mpower -mno-power -mpower2 -mno-power2
-mpowerpc -mpowerpc64 -mno-powerpc
-maltivec -mno-altivec
-mpowerpc-gpopt -mno-powerpc-gpopt
-mpowerpc-gfxopt -mno-powerpc-gfxopt
-mnew-mnemonics -mold-mnemonics
-mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc
-m64 -m32 -mxl-call -mno-xl-call -mpe
-msoft-float -mhard-float -mmultiple -mno-multiple
-mstring -mno-string -mupdate -mno-update
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable
-mno-relocatable -mrelocatable-lib -mno-relocatable-lib
-mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian
-mcall-aix -mcall-sysv -mcall-netbsd
-maix-struct-return -msvr4-struct-return
-mabi=altivec -mabi=no-altivec
-mabi=spe -mabi=no-spe
-misel=yes -misel=no
-mprototype -mno-prototype
-msim -mmvme -mads -myellowknife -memb -msdata
-msdata=opt -mvxworks -mwindiss -G num -pthread
Darwin Options
-all_load -allowable_client -arch -arch_errors_fatal
-arch_only -bind_at_load -bundle -bundle_loader
-client_name -compatibility_version -current_version
-dependency-file -dylib_file -dylinker_install_name
-dynamic -dynamiclib -exported_symbols_list
-filelist -flat_namespace -force_cpusubtype_ALL
-force_flat_namespace -headerpad_max_install_names
-image_base -init -install_name -keep_private_externs
-multi_module -multiply_defined -multiply_defined_unused
-noall_load -nomultidefs -noprebind -noseglinkedit
-pagezero_size -prebind -prebind_all_twolevel_modules
-private_bundle -read_only_relocs -sectalign
-sectobjectsymbols -whyload -seg1addr
-sectcreate -sectobjectsymbols -sectorder
-seg_addr_table -seg_addr_table_filename -seglinkedit
-segprot -segs_read_only_addr -segs_read_write_addr
-single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined
-unexported_symbols_list -weak_reference_mismatches -whatsloaded
RT Options
-mcall-lib-mul -mfp-arg-in-fpregs -mfp-arg-in-gregs
-mfull-fp-blocks -mhc-struct-return -min-line-mul
-mminimum-fp-blocks -mnohc-struct-return
MIPS Options
-mabicalls -march=cpu-type -mtune=cpu=type
-mcpu=cpu-type -membedded-data -muninit-const-in-rodata
-membedded-pic -mfp32 -mfp64 -mfused-madd -mno-fused-madd
-mgas -mgp32 -mgp64
-mgpopt -mhalf-pic -mhard-float -mint64 -mips1
-mips2 -mips3 -mips4 -mlong64 -mlong32 -mlong-calls -mmemcpy
-mmips-as -mmips-tfile -mno-abicalls
-mno-embedded-data -mno-uninit-const-in-rodata
-mno-embedded-pic -mno-gpopt -mno-long-calls
-mno-memcpy -mno-mips-tfile -mno-rnames -mno-stats
-mrnames -msoft-float
-m4650 -msingle-float -mmad
-mstats -EL -EB -G num -nocpp
-mabi=32 -mabi=n32 -mabi=64 -mabi=eabi
-mfix7000 -mno-crt0 -mflush-func=func -mno-flush-func
-mbranch-likely -mno-branch-likely
i386 and x86-64 Options
-mcpu=cpu-type -march=cpu-type
-mfpmath=unit -masm=dialect -mno-fancy-math-387
-mno-fp-ret-in-387 -msoft-float -msvr3-shlib
-mno-wide-multiply -mrtd -malign-double
-mpreferred-stack-boundary=num
-mmmx -msse -msse2 -msse3 -m3dnow
-mthreads -mno-align-stringops -minline-all-stringops
-mpush-args -maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mregparm=num -momit-leaf-frame-pointer
-mno-red-zone
-mcmodel=code-model
-m32 -m64
HPPA Options
-march=architecture-type
-mbig-switch -mdisable-fpregs -mdisable-indexing
-mfast-indirect-calls -mgas -mgnu-ld -mhp-ld
-mjump-in-delay -mlinker-opt -mlong-calls
-mlong-load-store -mno-big-switch -mno-disable-fpregs
-mno-disable-indexing -mno-fast-indirect-calls -mno-gas
-mno-jump-in-delay -mno-long-load-store
-mno-portable-runtime -mno-soft-float
-mno-space-regs -msoft-float -mpa-risc-1-0
-mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime
-mschedule=cpu-type -mspace-regs -msio -mwsio
-nolibdld -static -threads
Intel 960 Options
-mcpu-type -masm-compat -mclean-linkage
-mcode-align -mcomplex-addr -mleaf-procedures
-mic-compat -mic2.0-compat -mic3.0-compat
-mintel-asm -mno-clean-linkage -mno-code-align
-mno-complex-addr -mno-leaf-procedures
-mno-old-align -mno-strict-align -mno-tail-call
-mnumerics -mold-align -msoft-float -mstrict-align
-mtail-call
DEC Alpha Options
-mno-fp-regs -msoft-float -malpha-as -mgas
-mieee -mieee-with-inexact -mieee-conformant
-mfp-trap-mode=mode -mfp-rounding-mode=mode
-mtrap-precision=mode -mbuild-constants
-mcpu=cpu-type -mtune=cpu-type
-mbwx -mmax -mfix -mcix
-mfloat-vax -mfloat-ieee
-mexplicit-relocs -msmall-data -mlarge-data
-mmemory-latency=time
DEC Alpha/VMS Options
-mvms-return-codes
H8/300 Options
-mrelax -mh -ms -mn -mint32 -malign-300
SH Options
-m1 -m2 -m3 -m3e
-m4-nofpu -m4-single-only -m4-single -m4
-m5-64media -m5-64media-nofpu
-m5-32media -m5-32media-nofpu
-m5-compact -m5-compact-nofpu
-mb -ml -mdalign -mrelax
-mbigtable -mfmovd -mhitachi -mnomacsave
-mieee -misize -mpadstruct -mspace
-mprefergot -musermode
System V Options
-Qy -Qn -YP,paths -Ym,dir
ARC Options
-EB -EL
-mmangle-cpu -mcpu=cpu -mtext=text-section
-mdata=data-section -mrodata=readonly-data-section
TMS320C3x/C4x Options
-mcpu=cpu -mbig -msmall -mregparm -mmemparm
-mfast-fix -mmpyi -mbk -mti -mdp-isr-reload
-mrpts=count -mrptb -mdb -mloop-unsigned
-mparallel-insns -mparallel-mpy -mpreserve-float
V850 Options
-mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace
-mtda=n -msda=n -mzda=n
-mapp-regs -mno-app-regs
-mdisable-callt -mno-disable-callt
-mv850e
-mv850 -mbig-switch
NS32K Options
-m32032 -m32332 -m32532 -m32081 -m32381
-mmult-add -mnomult-add -msoft-float -mrtd -mnortd
-mregparam -mnoregparam -msb -mnosb
-mbitfield -mnobitfield -mhimem -mnohimem
AVR Options
-mmcu=mcu -msize -minit-stack=n -mno-interrupts
-mcall-prologues -mno-tablejump -mtiny-stack
MCore Options
-mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates
-mno-relax-immediates -mwide-bitfields -mno-wide-bitfields
-m4byte-functions -mno-4byte-functions -mcallgraph-data
-mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim
-mlittle-endian -mbig-endian -m210 -m340 -mstack-increment
MMIX Options
-mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu
-mabi=mmixware -mzero-extend -mknuthdiv -mtoplevel-symbols
-melf -mbranch-predict -mno-branch-predict -mbase-addresses
-mno-base-addresses -msingle-exit -mno-single-exit
IA-64 Options
-mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic
-mvolatile-asm-stop -mb-step -mregister-names -mno-sdata
-mconstant-gp -mauto-pic -minline-float-divide-min-latency
-minline-float-divide-max-throughput
-minline-int-divide-min-latency
-minline-int-divide-max-throughput -mno-dwarf2-asm
-mfixed-range=register-range
D30V Options
-mextmem -mextmemory -monchip -mno-asm-optimize
-masm-optimize -mbranch-cost=n -mcond-exec=n
S/390 and zSeries Options
-mhard-float -msoft-float -mbackchain -mno-backchain
-msmall-exec -mno-small-exec -mmvcle -mno-mvcle
-m64 -m31 -mdebug -mno-debug
CRIS Options
-mcpu=cpu -march=cpu -mtune=cpu
-mmax-stack-frame=n -melinux-stacksize=n
-metrax4 -metrax100 -mpdebug -mcc-init -mno-side-effects
-mstack-align -mdata-align -mconst-align
-m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt
-melf -maout -melinux -mlinux -sim -sim2
-mmul-bug-workaround -mno-mul-bug-workaround
PDP-11 Options
-mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10
-mbcopy -mbcopy-builtin -mint32 -mno-int16
-mint16 -mno-int32 -mfloat32 -mno-float64
-mfloat64 -mno-float32 -mabshi -mno-abshi
-mbranch-expensive -mbranch-cheap
-msplit -mno-split -munix-asm -mdec-asm
Xstormy16 Options
-msim
Xtensa Options
-mbig-endian -mlittle-endian
-mdensity -mno-density
-mmac16 -mno-mac16
-mmul16 -mno-mul16
-mmul32 -mno-mul32
-mnsa -mno-nsa
-mminmax -mno-minmax
-msext -mno-sext
-mbooleans -mno-booleans
-mhard-float -msoft-float
-mfused-madd -mno-fused-madd
-mserialize-volatile -mno-serialize-volatile
-mtext-section-literals -mno-text-section-literals
-mtarget-align -mno-target-align
-mlongcalls -mno-longcalls
FRV Options
-mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64
-mhard-float -msoft-float -malloc-cc -mfixed-cc
-mdword -mno-dword -mdouble -mno-double
-mmedia -mno-media -mmuladd -mno-muladd -mlibrary-pic
-macc-4 -macc-8 -mpack -mno-pack -mno-eflags
-mcond-move -mno-cond-move -mscc -mno-scc
-mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch
-mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec
-mno-nested-cond-exec -mtomcat-stats
-mcpu=cpu
-fcall-saved-reg -fcall-used-reg
-ffixed-reg -fexceptions
-fnon-call-exceptions -funwind-tables
-fasynchronous-unwind-tables
-finhibit-size-directive -finstrument-functions
-fno-common -fno-ident -fno-gnu-linker
-fpcc-struct-return -fpic -fPIC
-freg-struct-return -fshared-data -fshort-enums
-fshort-double -fshort-wchar -fvolatile
-fvolatile-global -fvolatile-static
-fverbose-asm -fpack-struct -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym
-fargument-alias -fargument-noalias
-fargument-noalias-global -fleading-underscore
-ftls-model=model
-ftrapv -fbounds-check
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that order. The first three stages apply to an individual source file, and end by producing an object file; linking combines all the object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
.c.i.ii.m.mi.h.cc.cp.cxx.cpp.c++.C.f.for.FOR.F.fpp.FPP.rSee Options Controlling the Kind of Output (Using and Porting GNU Fortran), for more details of the handling of
Fortran input files.
.ads.adb.s.SYou can specify the input language explicitly with the -x option:
-x language c c-header cpp-output
c++ c++-cpp-output
objective-c objc-cpp-output
assembler assembler-with-cpp
ada
f77 f77-cpp-input ratfor
java
treelang
-x none-pass-exit-codesIf you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all.
-cBy default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are
ignored.
-SBy default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-EInput files which don't require preprocessing are ignored.
-o fileSince only one output file can be specified, it does not make sense to use -o when compiling more than one input file, unless you are producing an executable file as output.
If -o is not specified, the default is to put an executable file
in a.out, the object file for source.suffix in
source.o, its assembler file in source.s, and
all preprocessed C source on standard output.
-v-###-pipe--help--target-help--versionC++ source files conventionally use one of the suffixes .C, .cc, .cpp, .c++, .cp, or .cxx; preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as for compiling C programs (usually with the name gcc).
However, C++ programs often require class libraries as well as a compiler that understands the C++ language—and under some circumstances, you might want to compile programs from standard input, or otherwise without a suffix that flags them as C++ programs. g++ is a program that calls GCC with the default language set to C++, and automatically specifies linking against the C++ library. On many systems, g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. See Options Controlling C Dialect, for explanations of options for languages related to C. See Options Controlling C++ Dialect, for explanations of options that are meaningful only for C++ programs.
The following options control the dialect of C (or languages derived from C, such as C++ and Objective-C) that the compiler accepts:
-ansiThis turns off certain features of GCC that are incompatible with ISO
C90 (when compiling C code), or of standard C++ (when compiling C++ code),
such as the asm and typeof keywords, and
predefined macros such as unix and vax that identify the
type of system you are using. It also enables the undesirable and
rarely used ISO trigraph feature. For the C compiler,
it disables recognition of C++ style // comments as well as
the inline keyword.
The alternate keywords __asm__, __extension__,
__inline__ and __typeof__ continue to work despite
-ansi. You would not want to use them in an ISO C program, of
course, but it is useful to put them in header files that might be included
in compilations done with -ansi. Alternate predefined macros
such as __unix__ and __vax__ are also available, with or
without -ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, -pedantic is required in addition to -ansi. See Warning Options.
The macro __STRICT_ANSI__ is predefined when the -ansi
option is used. Some header files may notice this macro and refrain
from declaring certain functions or defining certain macros that the
ISO standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions which would normally be built in but do not have semantics
defined by ISO C (such as alloca and ffs) are not built-in
functions with -ansi is used. See Other built-in functions provided by GCC, for details of the functions
affected.
-std=Even when this option is not specified, you can still use some of the
features of newer standards in so far as they do not conflict with
previous C standards. For example, you may use __restrict__ even
when -std=c99 is not specified.
The -std options specifying some version of ISO C have the same
effects as -ansi, except that features that were not in ISO C90
but are in the specified version (for example, // comments and
the inline keyword in ISO C99) are not disabled.
See Language Standards Supported by GCC, for details of
these standard versions.
-aux-info filenameBesides declarations, the file indicates, in comments, the origin of
each declaration (source file and line), whether the declaration was
implicit, prototyped or unprototyped (I, N for new or
O for old, respectively, in the first character after the line
number and the colon), and whether it came from a declaration or a
definition (C or F, respectively, in the following
character). In the case of function definitions, a K&R-style list of
arguments followed by their declarations is also provided, inside
comments, after the declaration.
-fno-asmasm, inline or typeof as a
keyword, so that code can use these words as identifiers. You can use
the keywords __asm__, __inline__ and __typeof__
instead. -ansi implies -fno-asm.
In C++, this switch only affects the typeof keyword, since
asm and inline are standard keywords. You may want to
use the -fno-gnu-keywords flag instead, which has the same
effect. In C99 mode (-std=c99 or -std=gnu99), this
switch only affects the asm and typeof keywords, since
inline is a standard keyword in ISO C99.
-fno-builtin-fno-builtin-functionGCC normally generates special code to handle certain built-in functions
more efficiently; for instance, calls to alloca may become single
instructions that adjust the stack directly, and calls to memcpy
may become inline copy loops. The resulting code is often both smaller
and faster, but since the function calls no longer appear as such, you
cannot set a breakpoint on those calls, nor can you change the behavior
of the functions by linking with a different library.
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named this is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fhostedmain has a return
type of int. Examples are nearly everything except a kernel.
This is equivalent to -fno-freestanding.
-ffreestandingmain. The most obvious example is an OS kernel.
This is equivalent to -fno-hosted.
See Language Standards Supported by GCC, for details of
freestanding and hosted environments.
-fms-extensions-trigraphs-no-integrated-cppThe semantics of this option will change if "cc1", "cc1plus", and "cc1obj" are merged.
-traditional-traditional-cpp-fcond-mismatch-funsigned-charchar be unsigned, like unsigned char.
Each kind of machine has a default for what char should
be. It is either like unsigned char by default or like
signed char by default.
Ideally, a portable program should always use signed char or
unsigned char when it depends on the signedness of an object.
But many programs have been written to use plain char and
expect it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let you
make such a program work with the opposite default.
The type char is always a distinct type from each of
signed char or unsigned char, even though its behavior
is always just like one of those two.
-fsigned-charchar be signed, like signed char.
Note that this is equivalent to -fno-unsigned-char, which is
the negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields-funsigned-bitfields-fno-signed-bitfields-fno-unsigned-bitfieldssigned or unsigned. By
default, such a bit-field is signed, because this is consistent: the
basic integer types such as int are signed types.
-fwritable-stringsWriting into string constants is a very bad idea; “constants” should be constant.
This section describes the command-line options that are only meaningful
for C++ programs; but you can also use most of the GNU compiler options
regardless of what language your program is in. For example, you
might compile a file firstClass.C like this:
g++ -g -frepo -O -c firstClass.C
In this example, only -frepo is an option meant only for C++ programs; you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=nThe default is version 1.
-fno-access-control-fcheck-newoperator new is non-null
before attempting to modify the storage allocated. This check is
normally unnecessary because the C++ standard specifies that
operator new will only return 0 if it is declared
throw(), in which case the compiler will always check the
return value even without this option. In all other cases, when
operator new has a non-empty exception specification, memory
exhaustion is signalled by throwing std::bad_alloc. See also
new (nothrow).
-fconserve-spacemain() has
completed, you may have an object that is being destroyed twice because
two definitions were merged.
This option is no longer useful on most targets, now that support has
been added for putting variables into BSS without making them common.
-fno-const-stringschar * instead of type const
char *. By default, G++ uses type const char * as required by
the standard. Even if you use -fno-const-strings, you cannot
actually modify the value of a string constant, unless you also use
-fwritable-strings.
This option might be removed in a future release of G++. For maximum
portability, you should structure your code so that it works with
string constants that have type const char *.
-fdollars-in-identifiers-fno-elide-constructors-fno-enforce-eh-specs-fexternal-templatesThis option is deprecated.
-falt-external-templatesThis option is deprecated.
-ffor-scope-fno-for-scopeThe default if neither flag is given to follow the standard,
but to allow and give a warning for old-style code that would
otherwise be invalid, or have different behavior.
-fno-gnu-keywordstypeof as a keyword, so that code can use this
word as an identifier. You can use the keyword __typeof__ instead.
-ansi implies -fno-gnu-keywords.
-fno-implicit-templates-fno-implicit-inline-templates-fno-implement-inlines-fms-extensions-fno-nonansi-builtinsffs, alloca, _exit,
index, bzero, conjf, and other related functions.
-fno-operator-namesand, bitand,
bitor, compl, not, or and xor as
synonyms as keywords.
-fno-optional-diags-fpermissive-frepo-fno-rtti-fstats-ftemplate-depth-n-fuse-cxa-atexit__cxa_atexit function rather than the atexit function.
This option is required for fully standards-compliant handling of static
destructors, but will only work if your C library supports
__cxa_atexit.
-fvtable-gcThis optimization requires GNU as and GNU ld. Not all systems support
this option. -Wl,--gc-sections is ignored without -static.
-fno-weak-nostdinc++In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-fno-default-inline-Wabi (C++ only)You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.
The known incompatibilities at this point include:
struct A { virtual void f(); int f1 : 1; };
struct B : public A { int f2 : 1; };
In this case, G++ will place B::f2 into the same byte
asA::f1; other compilers will not. You can avoid this problem
by explicitly padding A so that its size is a multiple of the
byte size on your platform; that will cause G++ and other compilers to
layout B identically.
struct A { virtual void f(); char c1; };
struct B { B(); char c2; };
struct C : public A, public virtual B {};
In this case, G++ will not place B into the tail-padding for
A; other compilers will. You can avoid this problem by
explicitly padding A so that its size is a multiple of its
alignment (ignoring virtual base classes); that will cause G++ and other
compilers to layout C identically.
union U { int i : 4096; };
Assuming that an int does not have 4096 bits, G++ will make the
union too small by the number of bits in an int.
struct A {};
struct B {
A a;
virtual void f ();
};
struct C : public B, public A {};
G++ will place the A base class of C at a nonzero offset;
it should be placed at offset zero. G++ mistakenly believes that the
A data member of B is already at offset zero.
typename or
template template parameters can be mangled incorrectly.
template <typename Q>
void f(typename Q::X) {}
template <template <typename> class Q>
void f(typename Q<int>::X) {}
Instantiations of these templates may be mangled incorrectly.
-Wctor-dtor-privacy (C++ only)-Wnon-virtual-dtor (C++ only)-Wreorder (C++ only) struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler will rearrange the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall.
The following -W... options are not affected by -Wall.
-Weffc++ (C++ only)operator= return a reference to *this.
Also warn about violations of the following style guidelines from Scott Meyers' More Effective C++ book:
&&, ||, or ,.
When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use grep -v
to filter out those warnings.
-Wno-deprecated (C++ only)-Wno-non-template-friend (C++ only)-Wold-style-cast (C++ only)-Woverloaded-virtual (C++ only) struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the A class version of f is hidden in B, and code
like:
B* b;
b->f();
will fail to compile.
-Wno-pmf-conversions (C++ only)-Wsign-promo (C++ only)-Wsynth (C++ only) struct A {
operator int ();
A& operator = (int);
};
main ()
{
A a,b;
a = b;
}
In this example, G++ will synthesize a default A& operator = (const A&);, while cfront will use the user-defined operator =.
This section describes the command-line options that are only meaningful
for Objective-C programs, but you can also use most of the GNU compiler
options regardless of what language your program is in. For example,
you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C programs; you can use the other options with any language supported by GCC.
Here is a list of options that are only for compiling Objective-C programs:
-fconstant-string-class=class-name@"...". The default
class name is NXConstantString.
-fgnu-runtime-fnext-runtime__NEXT_RUNTIME__ is predefined if (and only if) this option is
used.
-gen-decls-Wno-protocol-Wno-protocol option, then
methods inherited from the superclass are considered to be implemented,
and no warning is issued for them.
-Wselector@selector(...)
expression, and a corresponding method for that selector has been found
during compilation. Because these checks scan the method table only at
the end of compilation, these warnings are not produced if the final
stage of compilation is not reached, for example because an error is
found during compilation, or because the -fsyntax-only option is
being used.
-Wundeclared-selector@selector(...) expression referring to an
undeclared selector is found. A selector is considered undeclared if no
method with that name has been declared before the
@selector(...) expression, either explicitly in an
@interface or @protocol declaration, or implicitly in
an @implementation section. This option always performs its
checks as soon as a @selector(...) expression is found,
while -Wselector only performs its checks in the final stage of
compilation. This also enforces the coding style convention
that methods and selectors must be declared before being used.
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). The options described below can be used to control the diagnostic messages formatting algorithm, e.g. how many characters per line, how often source location information should be reported. Right now, only the C++ front end can honor these options. However it is expected, in the near future, that the remaining front ends would be able to digest them correctly.
-fmessage-length=n-fdiagnostics-show-location=once-fdiagnostics-show-location=every-lineWarnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there may have been an error.
You can request many specific warnings with options beginning -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example, -Wno-implicit. This manual lists only one of the two forms, whichever is not the default.
The following options control the amount and kinds of warnings produced by GCC; for further, language-specific options also refer to C++ Dialect Options and Objective-C Dialect Options.
-fsyntax-only-pedanticValid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few will require -ansi or a -std option specifying the required version of ISO C). However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are rejected.
-pedantic does not cause warning messages for use of the
alternate keywords whose names begin and end with __. Pedantic
warnings are also disabled in the expression that follows
__extension__. However, only system header files should use
these escape routes; application programs should avoid them.
See Alternate Keywords.
Some users try to use -pedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want: it finds some non-ISO practices, but not all—only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful in some instances, but would require considerable additional work and would be quite different from -pedantic. We don't have plans to support such a feature in the near future.
Where the standard specified with -std represents a GNU
extended dialect of C, such as gnu89 or gnu99, there is a
corresponding base standard, the version of ISO C on which the GNU
extended dialect is based. Warnings from -pedantic are given
where they are required by the base standard. (It would not make sense
for such warnings to be given only for features not in the specified GNU
C dialect, since by definition the GNU dialects of C include all
features the compiler supports with the given option, and there would be
nothing to warn about.)
-pedantic-errors-w-Wno-import-Wchar-subscriptschar. This is a common cause
of error, as programmers often forget that this type is signed on some
machines.
-Wcomment-Wformatprintf and scanf, etc., to make sure that
the arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string make
sense. This includes standard functions, and others specified by format
attributes (see Function Attributes), in the printf,
scanf, strftime and strfmon (an X/Open extension,
not in the C standard) families.
The formats are checked against the format features supported by GNU
libc version 2.2. These include all ISO C90 and C99 features, as well
as features from the Single Unix Specification and some BSD and GNU
extensions. Other library implementations may not support all these
features; GCC does not support warning about features that go beyond a
particular library's limitations. However, if -pedantic is used
with -Wformat, warnings will be given about format features not
in the selected standard version (but not for strfmon formats,
since those are not in any version of the C standard). See Options Controlling C Dialect.
Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull.
-Wformat is included in -Wall. For more control over some
aspects of format checking, the options -Wno-format-y2k,
-Wno-format-extra-args, -Wno-format-zero-length,
-Wformat-nonliteral, -Wformat-security, and
-Wformat=2 are available, but are not included in -Wall.
-Wno-format-y2kstrftime
formats which may yield only a two-digit year.
-Wno-format-extra-argsprintf or scanf format function. The C standard specifies
that such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with $ operand number specifications, normally
warnings are still given, since the implementation could not know what
type to pass to va_arg to skip the unused arguments. However,
in the case of scanf formats, this option will suppress the
warning if the unused arguments are all pointers, since the Single
Unix Specification says that such unused arguments are allowed.
-Wno-format-zero-length-Wformat-nonliteralva_list.
-Wformat-securityprintf and scanf functions where the
format string is not a string literal and there are no format arguments,
as in printf (foo);. This may be a security hole if the format
string came from untrusted input and contains %n. (This is
currently a subset of what -Wformat-nonliteral warns about, but
in future warnings may be added to -Wformat-security that are not
included in -Wformat-nonliteral.)
-Wformat=2-Wnonnullnonnull function attribute.
-Wnonnull is included in -Wall and -Wformat. It
can be disabled with the -Wno-nonnull option.
-Wimplicit-int-Wimplicit-function-declaration-Werror-implicit-function-declaration-Wimplicit-Wmain-Wmissing-braces int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
-WparenthesesAlso warn about constructions where there may be confusion to which
if statement an else branch belongs. Here is an example of
such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C, every else branch belongs to the innermost possible if
statement, which in this example is if (b). This is often not
what the programmer expected, as illustrated in the above example by
indentation the programmer chose. When there is the potential for this
confusion, GCC will issue a warning when this flag is specified.
To eliminate the warning, add explicit braces around the innermost
if statement so there is no way the else could belong to
the enclosing if. The resulting code would look like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
-Wsequence-pointThe C standard defines the order in which expressions in a C program are
evaluated in terms of sequence points, which represent a partial
ordering between the execution of parts of the program: those executed
before the sequence point, and those executed after it. These occur
after the evaluation of a full expression (one which is not part of a
larger expression), after the evaluation of the first operand of a
&&, ||, ? : or , (comma) operator, before a
function is called (but after the evaluation of its arguments and the
expression denoting the called function), and in certain other places.
Other than as expressed by the sequence point rules, the order of
evaluation of subexpressions of an expression is not specified. All
these rules describe only a partial order rather than a total order,
since, for example, if two functions are called within one expression
with no sequence point between them, the order in which the functions
are called is not specified. However, the standards committee have
ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C standard specifies that “Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.”. If a program breaks these rules, the results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are a = a++;, a[n]
= b[n++] and a[i++] = i;. Some more complicated cases are not
diagnosed by this option, and it may give an occasional false positive
result, but in general it has been found fairly effective at detecting
this sort of problem in programs.
The present implementation of this option only works for C programs. A future implementation may also work for C++ programs.
The C standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle cases.
Links to discussions of the problem, including proposed formal
definitions, may be found on our readings page, at
http://gcc.gnu.org/readings.html.
-Wreturn-typeint. Also warn about any return statement with no
return-value in a function whose return-type is not void.
For C++, a function without return type always produces a diagnostic
message, even when -Wno-return-type is specified. The only
exceptions are main and functions defined in system headers.
-Wswitchswitch statement has an index of enumeral type
and lacks a case for one or more of the named codes of that
enumeration. (The presence of a default label prevents this
warning.) case labels outside the enumeration range also
provoke warnings when this option is used.
-Wswitch-defaultswitch statement does not have a default
case.
-Wswitch-enumswitch statement has an index of enumeral type
and lacks a case for one or more of the named codes of that
enumeration. case labels outside the enumeration range also
provoke warnings when this option is used.
-Wtrigraphs-Wunused-function-Wunused-labelTo suppress this warning use the unused attribute
(see Variable Attributes).
-Wunused-parameterTo suppress this warning use the unused attribute
(see Variable Attributes).
-Wunused-variableTo suppress this warning use the unused attribute
(see Variable Attributes).
-Wunused-valueTo suppress this warning cast the expression to void.
-WunusedIn order to get a warning about an unused function parameter, you must
either specify -W -Wunused or separately specify
-Wunused-parameter.
-Wuninitializedsetjmp call.
These warnings are possible only in optimizing compilation, because they require data flow information that is computed only when optimizing. If you don't specify -O, you simply won't get these warnings.
These warnings occur only for variables that are candidates for
register allocation. Therefore, they do not occur for a variable that
is declared volatile, or whose address is taken, or whose size
is other than 1, 2, 4 or 8 bytes. Also, they do not occur for
structures, unions or arrays, even when they are in registers.
Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.
These warnings are made optional because GCC is not smart enough to see all the reasons why the code might be correct despite appearing to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of y is always 1, 2 or 3, then x is
always initialized, but GCC doesn't know this. Here is
another common case:
{
int save_y;
if (change_y) save_y = y, y = new_y;
...
if (change_y) y = save_y;
}
This has no bug because save_y is used only if it is set.
This option also warns when a non-volatile automatic variable might be
changed by a call to longjmp. These warnings as well are possible
only in optimizing compilation.
The compiler sees only the calls to setjmp. It cannot know
where longjmp will be called; in fact, a signal handler could
call it at any point in the code. As a result, you may get a warning
even when there is in fact no problem because longjmp cannot
in fact be called at the place which would cause a problem.
Some spurious warnings can be avoided if you declare all the functions
you use that never return as noreturn. See Function Attributes.
-Wunknown-pragmas-Wstrict-aliasing-WallThe following -W... options are not implied by -Wall. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning.
-W foo (a)
{
if (a > 0)
return a;
}
static are not the first things in
a declaration. According to the C Standard, this usage is obsolescent.
const.
Such a type qualifier has no effect, since the value returned by a
function is not an lvalue. (But don't warn about the GNU extension of
volatile void return types. That extension will be warned about
if -pedantic is specified.)
x.h:
struct s { int f, g; };
struct t { struct s h; int i; };
struct t x = { 1, 2, 3 };
x.h would be implicitly initialized to zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
-Wno-div-by-zero-Wsystem-headers-Wfloat-equalThe idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you need
to compute (by analyzing the code, or in some other way) the maximum or
likely maximum error that the computation introduces, and allow for it
when performing comparisons (and when producing output, but that's a
different problem). In particular, instead of testing for equality, you
would check to see whether the two values have ranges that overlap; and
this is done with the relational operators, so equality comparisons are
probably mistaken.
-Wtraditional (C only)<limits.h>.
Use of these macros in user code might normally lead to spurious
warnings, however gcc's integrated preprocessor has enough context to
avoid warning in these cases.
switch statement has an operand of type long.
static function declaration follows a static one.
This construct is not accepted by some traditional C compilers.
__STDC__ to avoid missing
initializer warnings and relies on default initialization to zero in the
traditional C case.
PARAMS and
VPARAMS. This warning is also bypassed for nested functions
because that feature is already a gcc extension and thus not relevant to
traditional C compatibility.
-Wundef-Wendif-labels-Wshadow-Wlarger-than-len-Wpointer-arithvoid. GNU C assigns these types a size of 1, for
convenience in calculations with void * pointers and pointers
to functions.
-Wbad-function-cast (C only)int malloc() is cast to anything *.
-Wcast-qualconst char * is cast
to an ordinary char *.
-Wcast-alignchar * is cast to
an int * on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wwrite-stringsconst
char[length] so that
copying the address of one into a non-const char *
pointer will get a warning; when compiling C++, warn about the
deprecated conversion from string constants to char *.
These warnings will help you find at
compile time code that can try to write into a string constant, but
only if you have been very careful about using const in
declarations and prototypes. Otherwise, it will just be a nuisance;
this is why we did not make -Wall request these warnings.
-WconversionAlso, warn if a negative integer constant expression is implicitly
converted to an unsigned type. For example, warn about the assignment
x = -1 if x is unsigned. But do not warn about explicit
casts like (unsigned) -1.
-Wsign-compare-Waggregate-return-Wstrict-prototypes (C only)-Wmissing-prototypes (C only)-Wmissing-declarations (C only)-Wmissing-noreturnnoreturn.
Note these are only possible candidates, not absolute ones. Care should
be taken to manually verify functions actually do not ever return before
adding the noreturn attribute, otherwise subtle code generation
bugs could be introduced. You will not get a warning for main in
hosted C environments.
-Wmissing-format-attributeformat attributes. Note these are only possible
candidates, not absolute ones. GCC will guess that format
attributes might be appropriate for any function that calls a function
like vprintf or vscanf, but this might not always be the
case, and some functions for which format attributes are
appropriate may not be detected. This option has no effect unless
-Wformat is enabled (possibly by -Wall).
-Wno-multichar-Wno-deprecated-declarationsdeprecated attribute.
(see Function Attributes, see Variable Attributes,
see Type Attributes.)
-Wpackedf.x in struct bar
will be misaligned even though struct bar does not itself
have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wpadded-Wredundant-decls-Wnested-externs (C only)extern declaration is encountered within a function.
-Wunreachable-codeThis option is intended to warn when the compiler detects that at least a whole line of source code will never be executed, because some condition is never satisfied or because it is after a procedure that never returns.
It is possible for this option to produce a warning even though there are circumstances under which part of the affected line can be executed, so care should be taken when removing apparently-unreachable code.
For instance, when a function is inlined, a warning may mean that the line is unreachable in only one inlined copy of the function.
This option is not made part of -Wall because in a debugging
version of a program there is often substantial code which checks
correct functioning of the program and is, hopefully, unreachable
because the program does work. Another common use of unreachable
code is to provide behavior which is selectable at compile-time.
-WinlineThe compiler uses a variety of heuristics to determine whether or not
to inline a function. For example, the compiler takes into account
the size of the function being inlined and the the amount of inlining
that has already been done in the current function. Therefore,
seemingly insignificant changes in the source program can cause the
warnings produced by -Winline to appear or disappear.
-Wlong-long-Wdisabled-optimization-WerrorGCC has various special options that are used for debugging either your program or GCC:
-gOn most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but will probably make other debuggers crash or refuse to read the program. If you want to control for certain whether to generate the extra information, use -gstabs+, -gstabs, -gxcoff+, -gxcoff, -gdwarf-1+, -gdwarf-1, or -gvms (see below).
Unlike most other C compilers, GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally produce surprising results: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values were already at hand; some statements may execute in different places because they were moved out of loops.
Nevertheless it proves possible to debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.
The following options are useful when GCC is generated with the
capability for more than one debugging format.
-ggdb-gstabs-gstabs+-gcoff-gxcoff-gxcoff+-gdwarfThis option is deprecated.
-gdwarf+This option is deprecated.
-gdwarf-2-gvms-glevel-ggdblevel-gstabslevel-gcofflevel-gxcofflevel-gvmslevelLevel 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, but no information about local variables and no line numbers.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3.
Note that in order to avoid confusion between DWARF1 debug level 2,
and DWARF2, neither -gdwarf nor -gdwarf-2 accept
a concatenated debug level. Instead use an additional -glevel
option to change the debug level for DWARF1 or DWARF2.
-feliminate-dwarf2-dups-p-pg-Q-ftime-report-fmem-report-fprofile-arcsFor profile-directed block ordering, compile the program with -fprofile-arcs plus optimization and code generation options, generate the arc profile information by running the program on a selected workload, and then compile the program again with the same optimization and code generation options plus -fbranch-probabilities (see Options that Control Optimization).
The other use of -fprofile-arcs is for use with gcov, when it is used with the -ftest-coverage option.
With -fprofile-arcs, for each function of your program GCC
creates a program flow graph, then finds a spanning tree for the graph.
Only arcs that are not on the spanning tree have to be instrumented: the
compiler adds code to count the number of times that these arcs are
executed. When an arc is the only exit or only entrance to a block, the
instrumentation code can be added to the block; otherwise, a new basic
block must be created to hold the instrumentation code.
-ftest-coverage.bb.bbgUse -ftest-coverage with -fprofile-arcs; the latter option adds instrumentation to the program, which then writes execution counts to another data file:
.daCoverage data will map better to the source files if
-ftest-coverage is used without optimization.
-dlettersADDRESSOF codes, to file.10.addressof.
-fdump-unnumbered-fdump-translation-unit (C and C++ only)-fdump-translation-unit-options (C and C++ only)-fdump-class-hierarchy (C++ only)-fdump-class-hierarchy-options (C++ only)-fdump-tree-switch (C++ only)-fdump-tree-switch-options (C++ only)The following tree dumps are possible:
-frandom-seed=stringThe string should be different for every file you compile.
-fsched-verbose=nFor n greater than zero, -fsched-verbose outputs the
same information as -dRS. For n greater than one, it
also output basic block probabilities, detailed ready list information
and unit/insn info. For n greater than two, it includes RTL
at abort point, control-flow and regions info. And for n over
four, -fsched-verbose also includes dependence info.
-save-temps-time # cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the “user time,” that is time spent
executing the program itself. The second number is “system time,”
time spent executing operating system routines on behalf of the program.
Both numbers are in seconds.
-print-file-name=library-print-multi-directory-print-multi-lib-print-prog-name=program-print-libgcc-file-nameThis is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do
gcc -nostdlib files... `gcc -print-libgcc-file-name`
-print-search-dirsThis is useful when gcc prints the error message
installation problem, cannot exec cpp0: No such file or directory.
To resolve this you either need to put cpp0 and the other compiler
components where gcc expects to find them, or you can set the environment
variable GCC_EXEC_PREFIX to the directory where you installed them.
Don't forget the trailing '/'.
See Environment Variables.
-dumpmachine-dumpversion-dumpspecsThese options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you would expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed.
-O-O1With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.
-O turns on the following optimization flags:
-fdefer-pop
-fmerge-constants
-fthread-jumps
-floop-optimize
-fcrossjumping
-fif-conversion
-fif-conversion2
-fdelayed-branch
-fguess-branch-probability
-fcprop-registers
-O also turns on -fomit-frame-pointer on machines
where doing so does not interfere with debugging.
-O2-O2 turns on all optimization flags specified by -O. It also turns on the following optimization flags:
-fforce-mem
-foptimize-sibling-calls
-fstrength-reduce
-fcse-follow-jumps -fcse-skip-blocks
-frerun-cse-after-loop -frerun-loop-opt
-fgcse -fgcse-lm -fgcse-sm
-fdelete-null-pointer-checks
-fexpensive-optimizations
-fregmove
-fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec
-fcaller-saves
-fpeephole2
-freorder-blocks -freorder-functions
-fstrict-aliasing
-falign-functions -falign-jumps
-falign-loops -falign-labels
Please note the warning under -fgcse about
invoking -O2 on programs that use computed gotos.
-O3-O0-Os-Os disables the following optimization flags:
-falign-functions -falign-jumps -falign-loops
-falign-labels -freorder-blocks -fprefetch-loop-arrays
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the forms is listed—the one you typically will use. You can figure out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when “fine-tuning” of optimizations to be performed is desired.
-fno-default-inline-fno-defer-popDisabled at levels -O, -O2, -O3, -Os.
-fforce-memEnabled at levels -O2, -O3, -Os.
-fforce-addr-fomit-frame-pointerOn some machines, such as the VAX, this flag has no effect, because
the standard calling sequence automatically handles the frame pointer
and nothing is saved by pretending it doesn't exist. The
machine-description macro FRAME_POINTER_REQUIRED controls
whether a target machine supports this flag. See Register Usage (GNU Compiler Collection (GCC) Internals).
Enabled at levels -O, -O2, -O3, -Os.
-foptimize-sibling-callsEnabled at levels -O2, -O3, -Os.
-fno-inlineinline keyword. Normally this option
is used to keep the compiler from expanding any functions inline.
Note that if you are not optimizing, no functions can be expanded inline.
-finline-functionsIf all calls to a given function are integrated, and the function is
declared static, then the function is normally not output as
assembler code in its own right.
Enabled at level -O3.
-finline-limit=nInlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters as follows:
max-inline-insnsmax-inline-insns-singlemax-inline-insns-automin-inline-insnsmax-inline-insns-rtlUsing -finline-limit=600 thus results in the default settings for these parameters. See below for a documentation of the individual parameters controlling inlining.
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way, it represents a count
of assembly instructions and as such its exact meaning might change from one
release to an another.
-fkeep-inline-functionsstatic, nevertheless output a separate run-time
callable version of the function. This switch does not affect
extern inline functions.
-fkeep-static-constsstatic const when optimization isn't turned
on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to
check if the variable was referenced, regardless of whether or not
optimization is turned on, use the -fno-keep-static-consts option.
-fmerge-constantsThis option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constantsThis option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating point
types. Languages like C or C++ require each non-automatic variable to
have distinct location, so using this option will result in non-conforming
behavior.
-fno-branch-count-regThe default is -fbranch-count-reg, enabled when
-fstrength-reduce is enabled.
-fno-function-cseThis option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bssThis option turns off this behavior because some programs explicitly rely on variables going to the data section. E.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that.
The default is -fzero-initialized-in-bss.
-fstrength-reduceEnabled at levels -O2, -O3, -Os.
-fthread-jumpsEnabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumpsif statement with an
else clause, CSE will follow the jump when the condition
tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocksif statement with no else clause,
-fcse-skip-blocks causes CSE to follow the jump around the
body of the if.
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loopEnabled at levels -O2, -O3, -Os.
-frerun-loop-optEnabled at levels -O2, -O3, -Os.
-fgcseNote: When compiling a program using computed gotos, a GCC extension, you may get better runtime performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lmEnabled by default when gcse is enabled.
-fgcse-smEnabled by default when gcse is enabled.
-floop-optimizeEnabled at levels -O, -O2, -O3, -Os.
-fcrossjumpingEnabled at levels -O, -O2, -O3, -Os.
-fif-conversionif-conversion2.
Enabled at levels -O, -O2, -O3, -Os.
-fif-conversion2Enabled at levels -O, -O2, -O3, -Os.
-fdelete-null-pointer-checksIn some environments, this assumption is not true, and programs can safely dereference null pointers. Use -fno-delete-null-pointer-checks to disable this optimization for programs which depend on that behavior.
Enabled at levels -O2, -O3, -Os.
-fexpensive-optimizationsEnabled at levels -O2, -O3, -Os.
-foptimize-register-move-fregmoveNote -fregmove and -foptimize-register-move are the same optimization.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branchEnabled at levels -O, -O2, -O3, -Os.
-fschedule-insnsEnabled at levels -O2, -O3, -Os.
-fschedule-insns2Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock-fno-sched-spec-fsched-spec-load-fsched-spec-load-dangerous-fcaller-savesThis option is always enabled by default on certain machines, usually those which have no call-preserved registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fmove-all-movables-freduce-all-givsNote: When compiling programs written in Fortran, -fmove-all-movables and -freduce-all-givs are enabled by default when you use the optimizer.
These options may generate better or worse code; results are highly dependent on the structure of loops within the source code.
These two options are intended to be removed someday, once they have helped determine the efficacy of various approaches to improving loop optimizations.
Please let us (gcc@gcc.gnu.org and fortran@gnu.org)
know how use of these options affects
the performance of your production code.
We're very interested in code that runs slower
when these options are enabled.
-fno-peephole-fno-peephole2-fpeephole is enabled by default.
-fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probabilitySometimes gcc will opt to use a randomized model to guess branch probabilities, when none are available from either profiling feedback (-fprofile-arcs) or __builtin_expect. This means that different runs of the compiler on the same program may produce different object code.
In a hard real-time system, people don't want different runs of the compiler to produce code that has different behavior; minimizing non-determinism is of paramount import. This switch allows users to reduce non-determinism, possibly at the expense of inferior optimization.
The default is -fguess-branch-probability at levels
-O, -O2, -O3, -Os.
-freorder-blocksEnabled at levels -O2, -O3.
-freorder-functionstext.hot for most frequently executed functions and
text.unlikely for unlikely executed functions. Reordering is done by
the linker so object file format must support named sections and linker must
place them in a reasonable way.
Also profile feedback must be available in to make this option effective. See -fprofile-arcs for details.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasingunsigned int can alias an int, but not a
void* or a double. A character type may alias any other
type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most recently written to (called “type-punning”) is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So, the code above will work as expected. However, this code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Every language that wishes to perform language-specific alias analysis
should define a function that computes, given an tree
node, an alias set for the node. Nodes in different alias sets are not
allowed to alias. For an example, see the C front-end function
c_get_alias_set.
Enabled at levels -O2, -O3, -Os.
-falign-functions-falign-functions=n-fno-align-functions and -falign-functions=1 are equivalent and mean that functions will not be aligned.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-labels-falign-labels=n-fno-align-labels and -falign-labels=1 are equivalent and mean that labels will not be aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment.
Enabled at levels -O2, -O3.
-falign-loops-falign-loops=n-fno-align-loops and -falign-loops=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps-falign-jumps=n-fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops will not be aligned.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-frename-registersEnabled at levels -O3.
-fno-cprop-registersDisabled at levels -O, -O2, -O3, -Os.
The following options control compiler behavior regarding floating point arithmetic. These options trade off between speed and correctness. All must be specifically enabled.
-ffloat-storeThis option prevents undesirable excess precision on machines such as
the 68000 where the floating registers (of the 68881) keep more
precision than a double is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does only
good, but a few programs rely on the precise definition of IEEE floating
point. Use -ffloat-store for such programs, after modifying
them to store all pertinent intermediate computations into variables.
-ffast-mathThis option causes the preprocessor macro __FAST_MATH__ to be defined.
This option should never be turned on by any -O option since
it can result in incorrect output for programs which depend on
an exact implementation of IEEE or ISO rules/specifications for
math functions.
-fno-math-errnoThis option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -fmath-errno.
-funsafe-math-optimizationsThis option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -fno-unsafe-math-optimizations.
-ffinite-math-onlyThis option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications.
The default is -fno-finite-math-only.
-fno-trapping-mathThis option should never be turned on by any -O option since it can result in incorrect output for programs which depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
-fsignaling-nansThis option causes the preprocessor macro __SUPPORT_SNAN__ to
be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that affect signaling NaN behavior.
-fsingle-precision-constantThe following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code.
-fbranch-probabilitiesWith -fbranch-probabilities, GCC puts a
REG_BR_PROB note on each JUMP_INSN and CALL_INSN.
These can be used to improve optimization. Currently, they are only
used in one place: in reorg.c, instead of guessing which path a
branch is mostly to take, the REG_BR_PROB values are used to
exactly determine which path is taken more often.
-fnew-ra-ftracer-funroll-loops-funroll-all-loops-fprefetch-loop-arraysDisabled at level -Os.
-ffunction-sections-fdata-sectionsUse these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format and SPARC processors running Solaris 2 have linkers with such optimizations. AIX may have these optimizations in the future.
Only use these options when there are significant benefits from doing
so. When you specify these options, the assembler and linker will
create larger object and executable files and will also be slower.
You will not be able to use gprof on all systems if you
specify this option and you may have problems with debugging if
you specify both this option and -g.
-fssa-fssa-ccp-fssa-dce--param name=valueThe names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases.
In each case, the value is an integer. The allowable choices for name are given in the following table:
max-crossjump-edgesmax-delay-slot-insn-searchmax-delay-slot-live-searchmax-gcse-memorymax-gcse-passesmax-pending-list-lengthmax-inline-insns-singlemax-inline-insns-automax-inline-insnsmax-inline-slopemin-inline-insnsmax-inline-insns-rtlmax-unrolled-insnshot-bb-count-fractionhot-bb-frequency-fractiontracer-dynamic-coveragetracer-dynamic-coverage-feedbackThe tracer-dynamic-coverage-feedback is used only when profile
feedback is available. The real profiles (as opposed to statically estimated
ones) are much less balanced allowing the threshold to be larger value.
tracer-max-code-growthtracer-min-branch-ratiotracer-min-branch-ratiotracer-min-branch-ratio-feedbackSimilarly to tracer-dynamic-coverage two values are present, one for
compilation for profile feedback and one for compilation without. The value
for compilation with profile feedback needs to be more conservative (higher) in
order to make tracer effective.
ggc-min-expandThe default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when
RAM >= 1GB. If getrlimit is available, the notion of "RAM" is
the smallest of actual RAM, RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If
GCC is not able to calculate RAM on a particular platform, the lower
bound of 30% is used. Setting this parameter and
ggc-min-heapsize to zero causes a full collection to occur at
every opportunity. This is extremely slow, but can be useful for
debugging.
ggc-min-heapsizeThe default is RAM/8, with a lower bound of 4096 (four megabytes) and an
upper bound of 131072 (128 megabytes). If getrlimit is
available, the notion of "RAM" is the smallest of actual RAM,
RLIMIT_RSS, RLIMIT_DATA and RLIMIT_AS. If GCC is not able to calculate
RAM on a particular platform, the lower bound is used. Setting this
parameter very large effectively disables garbage collection. Setting
this parameter and ggc-min-expand to zero causes a full
collection to occur at every opportunity.
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation.
You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and let the driver handle the options instead.
-D name1.
-D name=definitionIf you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you will need to quote the option. With sh and csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they
are given on the command line. All -imacros file and
-include file options are processed after all
-D and -U options.
-U name-undef-I dir-o file-Wall-Wcomment-Wcomments-Wtrigraphs-Wtraditional-Wimport-Wundef-Wunused-macrosBuilt-in macros, macros defined on the command line, and macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then CPP will report it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif
-Wendif-labels #if FOO
...
#else FOO
...
#endif FOO
The second and third FOO should be in comments, but often are not
in older programs. This warning is on by default.
-Werror-Wsystem-headers-w-pedantic-pedantic-errors-MUnless specified explicitly (with -MT or -MQ), the object file name consists of the basename of the source file with any suffix replaced with object file suffix. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT (see Environment Variables). Debug output will still be sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses
warnings with an implicit -w.
-MMThis implies that the choice of angle brackets or double quotes in an
#include directive does not in itself determine whether that
header will appear in -MM dependency output. This is a
slight change in semantics from GCC versions 3.0 and earlier.
-MF fileWhen used with the driver options -MD or -MMD,
-MF overrides the default dependency output file.
-MG#include directive without prepending any path. -MG
also suppresses preprocessed output, as a missing header file renders
this useless.
This feature is used in automatic updating of makefiles.
-MPThis is typical output:
test.o: test.c test.h
test.h:
-MT targetAn -MT option will set the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target $$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with
-MQ.
-MDIf -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file (but see -MF), but if used without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate
a dependency output file as a side-effect of the compilation process.
-MMD-x c-x c++-x objective-c-x assembler-with-cppNote: Previous versions of cpp accepted a -lang option
which selected both the language and the standards conformance level.
This option has been removed, because it conflicts with the -l
option.
-std=standard-ansistandard may be one of:
iso9899:1990c89The -ansi option is equivalent to -std=c89.
iso9899:199409iso9899:1999c99iso9899:199xc9xgnu89gnu99gnu9xc++98gnu++98-I-#include "file"; they are not searched for
#include <file>. If additional directories are
specified with -I options after the -I-, those
directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current
file directory as the first search directory for #include "file".
-nostdinc-nostdinc++-include file#include "file" appeared as the first
line of the primary source file. However, the first directory searched
for file is the preprocessor's working directory instead of
the directory containing the main source file. If not found there, it
is searched for in the remainder of the #include "..." search
chain as normal.
If multiple -include options are given, the files are included
in the order they appear on the command line.
-imacros fileAll files specified by -imacros are processed before all files
specified by -include.
-idirafter dir-iprefix prefix-iwithprefix dir-iwithprefixbefore dirUse of these options is discouraged.
-isystem dir-fpreprocessed-fpreprocessed is implicit if the input file has one of the
extensions .i, .ii or .mi. These are the
extensions that GCC uses for preprocessed files created by
-save-temps.
-ftabstop=width-fno-show-column-A predicate=answer-A -predicate=answer-dCHARS touch foo.h; cpp -dM foo.h
will show all the predefined macros.
-P-CYou should be prepared for side effects when using -C; it
causes the preprocessor to treat comments as tokens in their own right.
For example, comments appearing at the start of what would be a
directive line have the effect of turning that line into an ordinary
source line, since the first token on the line is no longer a #.
-CCIn addition to the side-effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
-traditional-cpp-trigraphsThe nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
-remap--help--target-help-v-H-version--versionYou can pass options to the assembler.
-Wa,optionThese options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step.
-c-S-E-llibrary-l libraryIt makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded.
The linker searches a standard list of directories for the library, which is actually a file named liblibrary.a. The linker then uses this file as if it had been specified precisely by name.
The directories searched include several standard system directories plus any that you specify with -L.
Normally the files found this way are library files—archive files
whose members are object files. The linker handles an archive file by
scanning through it for members which define symbols that have so far
been referenced but not defined. But if the file that is found is an
ordinary object file, it is linked in the usual fashion. The only
difference between using an -l option and specifying a file name
is that -l surrounds library with lib and .a
and searches several directories.
-lobjc-nostartfiles-nodefaultlibs-nostdlibOne of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal subroutines
that GCC uses to overcome shortcomings of particular machines, or special
needs for some languages.
(See Interfacing to GCC Output (GNU Compiler Collection (GCC) Internals),
for more discussion of libgcc.a.)
In most cases, you need libgcc.a even when you want to avoid
other standard libraries. In other words, when you specify -nostdlib
or -nodefaultlibs you should usually specify -lgcc as well.
This ensures that you have no unresolved references to internal GCC
library subroutines. (For example, __main, used to ensure C++
constructors will be called; see collect2 (GNU Compiler Collection (GCC) Internals).)
-s-static-shared-shared-libgcc-static-libgccThere are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc.
Therefore, the G++ and GCJ drivers automatically add -shared-libgcc whenever you build a shared library or a main executable, because C++ and Java programs typically use exceptions, so this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find that they will not always be linked with the shared libgcc. If GCC finds, at its configuration time, that you have a GNU linker that does not support option --eh-frame-hdr, it will link the shared version of libgcc into shared libraries by default. Otherwise, it will take advantage of the linker and optimize away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch
exceptions, you must link it using the G++ or GCJ driver, as appropriate
for the languages used in the program, or using the option
-shared-libgcc, such that it is linked with the shared
libgcc.
-symbolic-Xlinker optionIf you want to pass an option that takes an argument, you must use
-Xlinker twice, once for the option and once for the argument.
For example, to pass -assert definitions, you must write
-Xlinker -assert -Xlinker definitions. It does not work to write
-Xlinker "-assert definitions", because this passes the entire
string as a single argument, which is not what the linker expects.
-Wl,option-u symbolThese options specify directories to search for header files, for libraries and for parts of the compiler:
-IdirIf a standard system include directory, or a directory specified with
-isystem, is also specified with -I, the -I
option will be ignored. The directory will still be searched but as a
system directory at its normal position in the system include chain.
This is to ensure that GCC's procedure to fix buggy system headers and
the ordering for the include_next directive are not inadvertently changed.
If you really need to change the search order for system directories,
use the -nostdinc and/or -isystem options.
-I-If additional directories are specified with -I options after the -I-, these directories are searched for all #include directives. (Ordinarily all -I directories are used this way.)
In addition, the -I- option inhibits the use of the current directory (where the current input file came from) as the first search directory for #include "file". There is no way to override this effect of -I-. With -I. you can specify searching the directory which was current when the compiler was invoked. That is not exactly the same as what the preprocessor does by default, but it is often satisfactory.
-I- does not inhibit the use of the standard system directories
for header files. Thus, -I- and -nostdinc are
independent.
-Ldir-BprefixThe compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ (see Target Options).
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B was not specified, the driver tries two standard prefixes, which are /usr/lib/gcc/ and /usr/local/lib/gcc-lib/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.
The compiler will check to see if the path provided by the -B refers to a directory, and if necessary it will add a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to includes files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.
The run-time support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX. See Environment Variables.
As a special kludge, if the path provided by -B is
[dir/]stageN/, where N is a number in the range 0 to
9, then it will be replaced by [dir/]include. This is to help
with boot-strapping the compiler.
-specs=filegcc is a driver program. It performs its job by invoking a sequence of other programs to do the work of compiling, assembling and linking. GCC interprets its command-line parameters and uses these to deduce which programs it should invoke, and which command-line options it ought to place on their command lines. This behavior is controlled by spec strings. In most cases there is one spec string for each program that GCC can invoke, but a few programs have multiple spec strings to control their behavior. The spec strings built into GCC can be overridden by using the -specs= command-line switch to specify a spec file.
Spec files are plaintext files that are used to construct spec strings. They consist of a sequence of directives separated by blank lines. The type of directive is determined by the first non-whitespace character on the line and it can be one of the following:
%command%include <file>%include_noerr <file>%rename old_name new_name*[spec_name]:[suffix]: .ZZ:
z-compile -input %i
This says that any input file whose name ends in .ZZ should be passed to the program z-compile, which should be invoked with the command-line switch -input and with the result of performing the %i substitution. (See below.)
As an alternative to providing a spec string, the text that follows a suffix directive can be one of the following:
@language .ZZ:
@c++
Says that .ZZ files are, in fact, C++ source files.
#name name compiler not installed on this system.
GCC already has an extensive list of suffixes built into it. This directive will add an entry to the end of the list of suffixes, but since the list is searched from the end backwards, it is effectively possible to override earlier entries using this technique.
GCC has the following spec strings built into it. Spec files can override these strings or create their own. Note that individual targets can also add their own spec strings to this list.
asm Options to pass to the assembler
asm_final Options to pass to the assembler post-processor
cpp Options to pass to the C preprocessor
cc1 Options to pass to the C compiler
cc1plus Options to pass to the C++ compiler
endfile Object files to include at the end of the link
link Options to pass to the linker
lib Libraries to include on the command line to the linker
libgcc Decides which GCC support library to pass to the linker
linker Sets the name of the linker
predefines Defines to be passed to the C preprocessor
signed_char Defines to pass to CPP to say whether char is signed
by default
startfile Object files to include at the start of the link
Here is a small example of a spec file:
%rename lib old_lib
*lib:
--start-group -lgcc -lc -leval1 --end-group %(old_lib)
This example renames the spec called lib to old_lib and then overrides the previous definition of lib with a new one. The new definition adds in some extra command-line options before including the text of the old definition.
Spec strings are a list of command-line options to be passed to their corresponding program. In addition, the spec strings can contain %-prefixed sequences to substitute variable text or to conditionally insert text into the command line. Using these constructs it is possible to generate quite complex command lines.
Here is a table of all defined %-sequences for spec strings. Note that spaces are not generated automatically around the results of expanding these sequences. Therefore you can concatenate them together or combine them with constant text in a single argument.
%%%i%b%B%d%gsuffix%usuffix%Usuffix%jsuffixHOST_BIT_BUCKET, if any, and if it is
writable, and if save-temps is off; otherwise, substitute the name
of a temporary file, just like %u. This temporary file is not
meant for communication between processes, but rather as a junk
disposal mechanism.
%.SUFFIX%w%o%O%pcpp.
%P%I%s%estr%|%(name)%[name]%x{option}%X%Y%Z%v1%v2%v3%aasm spec. This is used to compute the
switches to be passed to the assembler.
%Aasm_final spec. This is a spec string for
passing switches to an assembler post-processor, if such a program is
needed.
%llink spec. This is the spec for computing the
command line passed to the linker. Typically it will make use of the
%L %G %S %D and %E sequences.
%D%M%Llib spec. This is a spec string for deciding which
libraries should be included on the command line to the linker.
%Glibgcc spec. This is a spec string for deciding
which GCC support library should be included on the command line to the linker.
%Sstartfile spec. This is a spec for deciding which
object files should be the first ones passed to the linker. Typically
this might be a file named crt0.o.
%Eendfile spec. This is a spec string that specifies
the last object files that will be passed to the linker.
%Ccpp spec. This is used to construct the arguments
to be passed to the C preprocessor.
%csigned_char spec. This is intended to be used
to tell cpp whether a char is signed. It typically has the definition:
%{funsigned-char:-D__CHAR_UNSIGNED__}
%1cc1 spec. This is used to construct the options to be
passed to the actual C compiler (cc1).
%2cc1plus spec. This is used to construct the options to be
passed to the actual C++ compiler (cc1plus).
%*%:function(args)The following built-in spec functions are provided:
if-existsif-exists spec function takes one argument, an absolute
pathname to a file. If the file exists, if-exists returns the
pathname. Here is a small example of its usage:
*startfile:
crt0%O%s %:if-exists(crti%O%s) crtbegin%O%s
if-exists-elseif-exists-else spec function is similar to the if-exists
spec function, except that it takes two arguments. The first argument is
an absolute pathname to a file. If the file exists, if-exists-else
returns the pathname. If it does not exist, it returns the second argument.
This way, if-exists-else can be used to select one file or another,
based on the existence of the first. Here is a small example of its usage:
*startfile:
crt0%O%s %:if-exists(crti%O%s) \
%:if-exists-else(crtbeginT%O%s crtbegin%O%s)
%{S}-S switch, if that switch was given to GCC.
If that switch was not specified, this substitutes nothing. Note that
the leading dash is omitted when specifying this option, and it is
automatically inserted if the substitution is performed. Thus the spec
string %{foo} would match the command-line option -foo
and would output the command line option -foo.
%W{S}S} but mark last argument supplied within as a file to be
deleted on failure.
%{S*}-S, but which also take an argument. This is used for
switches like -o, -D, -I, etc.
GCC considers -o foo as being
one switch whose names starts with o. %{o*} would substitute this
text, including the space. Thus two arguments would be generated.
%{^S*}S*}, but don't put a blank between a switch and its
argument. Thus %{^o*} would only generate one argument, not two.
%{S*&T*}S*}, but preserve order of S and T options
(the order of S and T in the spec is not significant).
There can be any number of ampersand-separated variables; for each the
wild card is optional. Useful for CPP as %{D*&U*&A*}.
%{<S}-S from the command line. Note—this
command is position dependent. % commands in the spec string
before this option will see -S, % commands in the spec
string after this option will not.
%{S*:X}X if one or more switches whose names start with
-S are specified to GCC. Note that the tail part of the
-S option (i.e. the part matched by the *) will be substituted
for each occurrence of %* within X.
%{S:X}X, but only if the -S switch was given to GCC.
%{!S:X}X, but only if the -S switch was not given to GCC.
%{|S:X}S:X}, but if no S switch, substitute -.
%{|!S:X}S:X}, but if there is an S switch, substitute -.
%{.S:X}X, but only if processing a file with suffix S.
%{!.S:X}X, but only if not processing a file with suffix S.
%{S|P:X}X if either -S or -P was given to GCC. This may be
combined with ! and . sequences as well, although they
have a stronger binding than the |. For example a spec string
like this:
%{.c:-foo} %{!.c:-bar} %{.c|d:-baz} %{!.c|d:-boggle}
will output the following command-line options from the following input command-line options:
fred.c -foo -baz
jim.d -bar -boggle
-d fred.c -foo -baz -boggle
-d jim.d -bar -baz -boggle
The conditional text X in a %{S:X} or
%{!S:X} construct may contain other nested % constructs
or spaces, or even newlines. They are processed as usual, as described
above.
The -O, -f, -m, and -W
switches are handled specifically in these
constructs. If another value of -O or the negated form of a -f, -m, or
-W switch is found later in the command line, the earlier switch
value is ignored, except with {S*} where S is just one
letter, which passes all matching options.
The character | at the beginning of the predicate text is used to indicate that a command should be piped to the following command, but only if -pipe is specified.
It is built into GCC which switches take arguments and which do not. (You might think it would be useful to generalize this to allow each compiler's spec to say which switches take arguments. But this cannot be done in a consistent fashion. GCC cannot even decide which input files have been specified without knowing which switches take arguments, and it must know which input files to compile in order to tell which compilers to run).
GCC also knows implicitly that arguments starting in -l are to be treated as compiler output files, and passed to the linker in their proper position among the other output files.
The usual way to run GCC is to run the executable called gcc, or <machine>-gcc when cross-compiling, or <machine>-gcc-<version> to run a version other than the one that was installed last. Sometimes this is inconvenient, so GCC provides options that will switch to another cross-compiler or version.
-b machineThe value to use for machine is the same as was specified as the
machine type when configuring GCC as a cross-compiler. For
example, if a cross-compiler was configured with configure
i386v, meaning to compile for an 80386 running System V, then you
would specify -b i386v to run that cross compiler.
-V versionThe -V and -b options work by running the <machine>-gcc-<version> executable, so there's no real reason to use them if you can just run that directly.
Earlier we discussed the standard option -b which chooses among different installed compilers for completely different target machines, such as VAX vs. 68000 vs. 80386.
In addition, each of these target machine types can have its own special options, starting with -m, to choose among various hardware models or configurations—for example, 68010 vs 68020, floating coprocessor or none. A single installed version of the compiler can compile for any model or configuration, according to the options specified.
Some configurations of the compiler also support additional special options, usually for compatibility with other compilers on the same platform.
These options are defined by the macro TARGET_SWITCHES in the
machine description. The default for the options is also defined by
that macro, which enables you to change the defaults.
These are the -m options defined for the 68000 series. The default values for these options depends on which style of 68000 was selected when the compiler was configured; the defaults for the most common choices are given below.
-m68000-mc68000Use this option for microcontrollers with a 68000 or EC000 core,
including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356.
-m68020-mc68020-m68881-m68030-m68040This option inhibits the use of 68881/68882 instructions that have to be
emulated by software on the 68040. Use this option if your 68040 does not
have code to emulate those instructions.
-m68060This option inhibits the use of 68020 and 68881/68882 instructions that
have to be emulated by software on the 68060. Use this option if your 68060
does not have code to emulate those instructions.
-mcpu32Use this option for microcontrollers with a
CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334,
68336, 68340, 68341, 68349 and 68360.
-m5200Use this option for microcontroller with a 5200 core, including
the MCF5202, MCF5203, MCF5204 and MCF5202.
-m68020-40-m68020-60-mfpa-msoft-float-mshortint to be 16 bits wide, like short int.
-mnobitfield-mbitfield-mrtdrtd
instruction, which pops their arguments while returning. This
saves one instruction in the caller since there is no need to pop
the arguments there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including printf);
otherwise incorrect code will be generated for calls to those
functions.
In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)
The rtd instruction is supported by the 68010, 68020, 68030,
68040, 68060 and CPU32 processors, but not by the 68000 or 5200.
-malign-int-mno-align-intint, long, long long,
float, double, and long double variables on a 32-bit
boundary (-malign-int) or a 16-bit boundary (-mno-align-int).
Aligning variables on 32-bit boundaries produces code that runs somewhat
faster on processors with 32-bit busses at the expense of more memory.
Warning: if you use the -malign-int switch, GCC will
align structures containing the above types differently than
most published application binary interface specifications for the m68k.
-mpcrel-mno-strict-align-mstrict-alignThese are the -m options defined for the 68hc11 and 68hc12 microcontrollers. The default values for these options depends on which style of microcontroller was selected when the compiler was configured; the defaults for the most common choices are given below.
-m6811-m68hc11-m6812-m68hc12-m68S12-m68hcs12-mauto-incdec-minmax-nominmax-mlong-calls-mno-long-callscall instruction to
call a function and the rtc instruction for returning.
-mshortint to be 16 bits wide, like short int.
-msoft-reg-count=countThese -m options are defined for the VAX:
-munixaobleq and so on)
that the Unix assembler for the VAX cannot handle across long
ranges.
-mgnu-mgThese -m switches are supported on the SPARC:
-mno-app-regs-mapp-regsTo be fully SVR4 ABI compliant at the cost of some performance loss,
specify -mno-app-regs. You should compile libraries and system
software with this option.
-mfpu-mhard-float-mno-fpu-msoft-float-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for
this to work.
-mhard-quad-float-msoft-quad-floatAs of this writing, there are no sparc implementations that have hardware
support for the quad-word floating point instructions. They all invoke
a trap handler for one of these instructions, and then the trap handler
emulates the effect of the instruction. Because of the trap handler overhead,
this is much slower than calling the ABI library routines. Thus the
-msoft-quad-float option is the default.
-mno-flat-mflatWith -mno-flat (the default), the compiler emits save/restore
instructions (except for leaf functions) and is the normal mode of operation.
-mno-unaligned-doubles-munaligned-doublesWith -munaligned-doubles, GCC assumes that doubles have 8 byte
alignment only if they are contained in another type, or if they have an
absolute address. Otherwise, it assumes they have 4 byte alignment.
Specifying this option avoids some rare compatibility problems with code
generated by other compilers. It is not the default because it results
in a performance loss, especially for floating point code.
-mno-faster-structs-mfaster-structsldd and std instructions for copies in structure
assignment, in place of twice as many ld and st pairs.
However, the use of this changed alignment directly violates the SPARC
ABI. Thus, it's intended only for use on targets where the developer
acknowledges that their resulting code will not be directly in line with
the rules of the ABI.
-mimpure-text-mimpure-text suppresses the “relocations remain against allocatable but non-writable sections” linker error message. However, the necessary relocations will trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.
This option is only available on SunOS and Solaris.
-mv8-msparcliteBy default (unless specifically configured for the Fujitsu SPARClite), GCC generates code for the v7 variant of the SPARC architecture.
-mv8 will give you SPARC v8 code. The only difference from v7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC v8 but not in SPARC v7.
-msparclite will give you SPARClite code. This adds the integer
multiply, integer divide step and scan (ffs) instructions which
exist in SPARClite but not in SPARC v7.
These options are deprecated and will be deleted in a future GCC release.
They have been replaced with -mcpu=xxx.
-mcypress-msupersparcWith -mcypress (the default), the compiler optimizes code for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.
With -msupersparc the compiler optimizes code for the SuperSPARC cpu, as used in the SPARCStation 10, 1000 and 2000 series. This flag also enables use of the full SPARC v8 instruction set.
These options are deprecated and will be deleted in a future GCC release.
They have been replaced with -mcpu=xxx.
-mcpu=cpu_typeDefault instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported implementations.
v7: cypress
v8: supersparc, hypersparc
sparclite: f930, f934, sparclite86x
sparclet: tsc701
v9: ultrasparc, ultrasparc3
-mtune=cpu_typeThe same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular cpu implementation. Those are cypress, supersparc, hypersparc, f930, f934, sparclite86x, tsc701, ultrasparc, and ultrasparc3.
These -m switches are supported in addition to the above on the SPARCLET processor.
-mlittle-endian-mlive-g0%g0 as a normal register.
GCC will continue to clobber it as necessary but will not assume
it always reads as 0.
-mbroken-saverestoresave and
restore instructions. Early versions of the SPARCLET processor do
not correctly handle save and restore instructions used with
arguments. They correctly handle them used without arguments. A save
instruction used without arguments increments the current window pointer
but does not allocate a new stack frame. It is assumed that the window
overflow trap handler will properly handle this case as will interrupt
handlers.
These -m switches are supported in addition to the above on SPARC V9 processors in 64-bit environments.
-mlittle-endian-m32-m64-mcmodel=medlow-mcmodel=medmid-mcmodel=medany-mcmodel=embmedany-mstack-bias-mno-stack-biasThese switches are supported in addition to the above on Solaris:
-threads-pthreadsThese -m options are defined for Advanced RISC Machines (ARM) architectures:
-mapcs-frame-mapcs-mapcs-26-mapcs-32-mthumb-interwork-mno-sched-prolog-mhard-float-msoft-float-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for
this to work.
-mlittle-endian-mbig-endian-mwords-little-endian-malignment-trapsThis option is ignored when compiling for ARM architecture 4 or later,
since these processors have instructions to directly access half-word
objects in memory.
-mno-alignment-trapsNote that you cannot use this option to access unaligned word objects, since the processor will only fetch one 32-bit aligned object from memory.
The default setting for most targets is -mno-alignment-traps, since
this produces better code when there are no half-word memory
instructions available.
-mshort-load-bytes-mno-short-load-words-mno-short-load-bytes-mshort-load-words-mcpu=name-mtune=name-march=name-mfpe=number-mfp=number-mstructure-size-boundary=n-mabort-on-noreturnabort at the end of a
noreturn function. It will be executed if the function tries to
return.
-mlong-calls-mno-long-callsEven if this switch is enabled, not all function calls will be turned into long calls. The heuristic is that static functions, functions which have the short-call attribute, functions that are inside the scope of a #pragma no_long_calls directive and functions whose definitions have already been compiled within the current compilation unit, will not be turned into long calls. The exception to this rule is that weak function definitions, functions with the long-call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive, will always be turned into long calls.
This feature is not enabled by default. Specifying
-mno-long-calls will restore the default behavior, as will
placing the function calls within the scope of a #pragma
long_calls_off directive. Note these switches have no effect on how
the compiler generates code to handle function calls via function
pointers.
-mnop-fun-dllimportdllimport attribute.
-msingle-pic-base-mpic-register=reg-mpoke-function-name t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of
pc stored at fp + 0. If the trace function then looks at
location pc - 12 and the top 8 bits are set, then we know that
there is a function name embedded immediately preceding this location
and has length ((pc[-3]) & 0xff000000).
-mthumb-mtpcs-frame-mtpcs-leaf-frame-mcallee-super-interworking-mcaller-super-interworkingThese -m options are defined for Matsushita MN10200 architectures:
-mrelaxThis option makes symbolic debugging impossible.
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug-mno-mult-bug-mam33-mno-am33-mno-crt0-mrelaxThis option makes symbolic debugging impossible.
These -m options are defined for Mitsubishi M32R/D architectures:
-m32rx-m32r-mcode-model=smallld24 instruction), and assume all subroutines
are reachable with the bl instruction.
This is the default.
The addressability of a particular object can be set with the
model attribute.
-mcode-model=mediumseth/add3 instructions to load their addresses), and
assume all subroutines are reachable with the bl instruction.
-mcode-model=largeseth/add3 instructions to load their addresses), and
assume subroutines may not be reachable with the bl instruction
(the compiler will generate the much slower seth/add3/jl
instruction sequence).
-msdata=nonesection attribute has been specified).
This is the default.
The small data area consists of sections .sdata and .sbss.
Objects may be explicitly put in the small data area with the
section attribute using one of these sections.
-msdata=sdata-msdata=use-G numAll modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't the linker will give an error message—incorrect code will not be generated.
These -m options are defined for Motorola 88k architectures:
-m88000-m88100-m88110-mbig-pic-midentify-revisionident directive in the assembler output recording the
source file name, compiler name and version, timestamp, and compilation
flags used.
-mno-underscores-mocs-debug-info-mno-ocs-debug-info-mocs-frame-position-mno-ocs-frame-position-moptimize-arg-area-mno-optimize-arg-area-mshort-data-numr0,
which allows loading a value using a single instruction (rather than the
usual two). You control which data references are affected by
specifying num with this option. For example, if you specify
-mshort-data-512, then the data references affected are those
involving displacements of less than 512 bytes.
-mshort-data-num is not effective for num greater
than 64k.
-mserialize-volatile-mno-serialize-volatileThe order of memory references made by the MC88110 processor does not always match the order of the instructions requesting those references. In particular, a load instruction may execute before a preceding store instruction. Such reordering violates sequential consistency of volatile memory references, when there are multiple processors. When consistency must be guaranteed, GCC generates special instructions, as needed, to force execution in the proper order.
The MC88100 processor does not reorder memory references and so always provides sequential consistency. However, by default, GCC generates the special instructions to guarantee consistency even when you use -m88100, so that the code may be run on an MC88110 processor. If you intend to run your code only on the MC88100 processor, you may use -mno-serialize-volatile.
The extra code generated to guarantee consistency may affect the
performance of your application. If you know that you can safely
forgo this guarantee, you may use -mno-serialize-volatile.
-msvr4-msvr3-msvr4 is the default for the m88k-motorola-sysv4 configuration.
-msvr3 is the default for all other m88k configurations.
-mversion-03.00-mno-check-zero-division-mcheck-zero-divisionSome models of the MC88100 processor fail to trap upon integer division by zero under certain conditions. By default, when compiling code that might be run on such a processor, GCC generates code that explicitly checks for zero-valued divisors and traps with exception number 503 when one is detected. Use of -mno-check-zero-division suppresses such checking for code generated to run on an MC88100 processor.
GCC assumes that the MC88110 processor correctly detects all instances
of integer division by zero. When -m88110 is specified, no
explicit checks for zero-valued divisors are generated, and both
-mcheck-zero-division and -mno-check-zero-division are
ignored.
-muse-div-instructionOn the MC88100 processor the signed integer division instruction div) traps to the operating system on a negative operand. The operating system transparently completes the operation, but at a large cost in execution time. By default, when compiling code that might be run on an MC88100 processor, GCC emulates signed integer division using the unsigned integer division instruction divu), thereby avoiding the large penalty of a trap to the operating system. Such emulation has its own, smaller, execution cost in both time and space. To the extent that your code's important signed integer division operations are performed on two nonnegative operands, it may be desirable to use the div instruction directly.
On the MC88110 processor the div instruction (also known as the divs instruction) processes negative operands without trapping to the operating system. When -m88110 is specified, -muse-div-instruction is ignored, and the div instruction is used for signed integer division.
Note that the result of dividing INT_MIN by −1 is undefined. In
particular, the behavior of such a division with and without
-muse-div-instruction may differ.
-mtrap-large-shift-mhandle-large-shift-mwarn-passed-structsThese -m options are defined for the IBM RS/6000 and PowerPC:
-mpower-mno-power-mpower2-mno-power2-mpowerpc-mno-powerpc-mpowerpc-gpopt-mno-powerpc-gpopt-mpowerpc-gfxopt-mno-powerpc-gfxopt-mpowerpc64-mno-powerpc64Neither architecture is a subset of the other. However there is a large common subset of instructions supported by both. An MQ register is included in processors supporting the POWER architecture.
You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed above.
The -mpower option allows GCC to generate instructions that are found only in the POWER architecture and to use the MQ register. Specifying -mpower2 implies -power and also allows GCC to generate instructions that are present in the POWER2 architecture but not the original POWER architecture.
The -mpowerpc option allows GCC to generate instructions that are found only in the 32-bit subset of the PowerPC architecture. Specifying -mpowerpc-gpopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt implies -mpowerpc and also allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.
If you specify both -mno-power and -mno-powerpc, GCC
will use only the instructions in the common subset of both
architectures plus some special AIX common-mode calls, and will not use
the MQ register. Specifying both -mpower and -mpowerpc
permits GCC to use any instruction from either architecture and to
allow use of the MQ register; specify this for the Motorola MPC601.
-mnew-mnemonics-mold-mnemonicsGCC defaults to the mnemonics appropriate for the architecture in
use. Specifying -mcpu=cpu_type sometimes overrides the
value of these option. Unless you are building a cross-compiler, you
should normally not specify either -mnew-mnemonics or
-mold-mnemonics, but should instead accept the default.
-mcpu=cpu_type-mcpu=common selects a completely generic processor. Code generated under this option will run on any POWER or PowerPC processor. GCC will use only the instructions in the common subset of both architectures, and will not use the MQ register. GCC assumes a generic processor model for scheduling purposes.
-mcpu=power, -mcpu=power2, -mcpu=powerpc, and -mcpu=powerpc64 specify generic POWER, POWER2, pure 32-bit PowerPC (i.e., not MPC601), and 64-bit PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.
The other options specify a specific processor. Code generated under those options will run best on that processor, and may not run at all on others.
The -mcpu options automatically enable or disable other -m options as follows:
-mtune=cpu_type-maltivec-mno-altivec-mabi=spe-mabi=no-spe-misel=yes/no-misel-mfull-toc-mno-fp-in-toc-mno-sum-in-toc-mminimal-tocIf you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run-time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space.
If you still run out of space in the TOC even when you specify both of
these options, specify -mminimal-toc instead. This option causes
GCC to make only one TOC entry for every file. When you specify this
option, GCC will produce code that is slower and larger but which
uses extremely little TOC space. You may wish to use this option
only on files that contain less frequently executed code.
-maix64-maix32long type, and the infrastructure needed to support them.
Specifying -maix64 implies -mpowerpc64 and
-mpowerpc, while -maix32 disables the 64-bit ABI and
implies -mno-powerpc64. GCC defaults to -maix32.
-mxl-call-mno-xl-call-mpe-msoft-float-mhard-float-mmultiple-mno-multiple-mstring-mno-string-mupdate-mno-update-mfused-madd-mno-fused-madd-mno-bit-align-mbit-alignFor example, by default a structure containing nothing but 8
unsigned bit-fields of length 1 would be aligned to a 4 byte
boundary and have a size of 4 bytes. By using -mno-bit-align,
the structure would be aligned to a 1 byte boundary and be one byte in
size.
-mno-strict-align-mstrict-align-mrelocatable-mno-relocatable-mrelocatable-lib-mno-relocatable-lib-mno-toc-mtoc-mlittle-mlittle-endian-mbig-mbig-endian-mcall-sysv-mcall-sysv-eabi-mcall-sysv-noeabi-mcall-aix-mcall-solaris-mcall-linux-mcall-gnu-mcall-netbsd-maix-struct-return-msvr4-struct-return-mabi=altivec-mabi=no-altivec-mprototype-mno-prototype-msim-mmvme-mads-myellowknife-mvxworks-mwindiss-memb-meabi-mno-eabi__eabi is called to from main to set up the eabi
environment, and the -msdata option can use both r2 and
r13 to point to two separate small data areas. Selecting
-mno-eabi means that the stack is aligned to a 16 byte boundary,
do not call an initialization function from main, and the
-msdata option will only use r13 to point to a single
small data area. The -meabi option is on by default if you
configured GCC using one of the powerpc*-*-eabi* options.
-msdata=eabiconst global and static data in the .sdata2 section, which
is pointed to by register r2. Put small initialized
non-const global and static data in the .sdata section,
which is pointed to by register r13. Put small uninitialized
global and static data in the .sbss section, which is adjacent to
the .sdata section. The -msdata=eabi option is
incompatible with the -mrelocatable option. The
-msdata=eabi option also sets the -memb option.
-msdata=sysvr13. Put small uninitialized global and static data in the
.sbss section, which is adjacent to the .sdata section.
The -msdata=sysv option is incompatible with the
-mrelocatable option.
-msdata=default-msdata-msdata-datar13
to address small data however. This is the default behavior unless
other -msdata options are used.
-msdata=none-mno-sdata-G num-mregnames-mno-regnames-mlongcall-mno-longcallshortcall function attribute, or by #pragma longcall(0).
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On these systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, as can the GNU linker for PowerPC/64. It is planned to add this feature to the GNU linker for 32-bit PowerPC systems as well.
In the future, we may cause GCC to ignore all longcall specifications
when the linker is known to generate glue.
-pthreadThese options are defined for all architectures running the Darwin operating system. They are useful for compatibility with other Mac OS compilers.
-all_load-arch_errors_fatal-bind_at_load-bundle-bundle_loader executable-allowable_client client_name-arch_only
-client_name-compatibility_version-current_version-dependency-file-dylib_file-dylinker_install_name-dynamic-dynamiclib-exported_symbols_list-filelist-flat_namespace-force_cpusubtype_ALL-force_flat_namespace-headerpad_max_install_names-image_base-init-install_name-keep_private_externs-multi_module-multiply_defined-multiply_defined_unused-noall_load-nomultidefs-noprebind-noseglinkedit-pagezero_size-prebind-prebind_all_twolevel_modules-private_bundle-read_only_relocs-sectalign-sectobjectsymbols-whyload-seg1addr-sectcreate-sectobjectsymbols-sectorder-seg_addr_table-seg_addr_table_filename-seglinkedit-segprot-segs_read_only_addr-segs_read_write_addr-single_module-static-sub_library-sub_umbrella-twolevel_namespace-umbrella-undefined-unexported_symbols_list-weak_reference_mismatches-whatsloadedThese -m options are defined for the IBM RT PC:
-min-line-mul-mcall-lib-mullmul$$ for integer multiples.
-mfull-fp-blocks-mminimum-fp-blocks-mfp-arg-in-fpregsstdarg.h will not work with floating point operands
if this option is specified.
-mfp-arg-in-gregs-mhc-struct-return-mnohc-struct-returnThese -m options are defined for the MIPS family of computers:
-march=archIn processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r.
GCC defines two macros based on the value of this option. The first is _MIPS_ARCH, which gives the name of target architecture, as a string. The second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. For example, -march=r2000 will set _MIPS_ARCH to "r2000" and define the macro _MIPS_ARCH_R2000.
Note that the _MIPS_ARCH macro uses the processor names given
above. In other words, it will have the full prefix and will not
abbreviate 000 as k. In the case of from-abi,
the macro names the resolved architecture (either "mips1" or
"mips3"). It names the default architecture when no
-march option is given.
-mtune=archWhen this option is not used, GCC will optimize for the processor specified by -march. By using -march and -mtune together, it is possible to generate code that will run on a family of processors, but optimize the code for one particular member of that family.
-mtune defines the macros _MIPS_TUNE and
_MIPS_TUNE_foo, which work in the same way as the
-march ones described above.
-mips1-mips2-mips3-mips4-mips32-mips64-mfused-madd-mno-fused-madd-mfp32-mfp64-mgp32-mgp64-mint64-mlong64-mlong32The default size of ints, longs and pointers depends on the ABI. All
the supported ABIs use 32-bit ints. The n64 ABI uses 64-bit longs, as
does the 64-bit Cygnus EABI; the others use 32-bit longs. Pointers
are the same size as longs, or the same size as integer registers,
whichever is smaller.
-mabi=32-mabi=o64-mabi=n32-mabi=64-mabi=eabi-mabi=meabiNote that there are two embedded ABIs: -mabi=eabi
selects the one defined by Cygnus while -meabi=meabi
selects the one defined by MIPS. Both these ABIs have
32-bit and 64-bit variants. Normally, GCC will generate
64-bit code when you select a 64-bit architecture, but you
can use -mgp32 to get 32-bit code instead.
-mmips-as-mgas-msplit-addresses-mno-split-addresses-mrnames-mno-rnames-mgpopt-mno-gpopt-mstats-mno-stats-mmemcpy-mno-memcpy-mmips-tfile-mno-mips-tfile-msoft-float-mhard-float-mabicalls-mno-abicalls-mlong-calls-mno-long-calls-mhalf-pic-mno-half-pic-membedded-pic-mno-embedded-pic-membedded-data-mno-embedded-data-muninit-const-in-rodata-mno-uninit-const-in-rodata-msingle-float-mdouble-float-mmad-mno-mad-m4650-mips16-mno-mips16-mentry-EL-EB-G num-nocpp-mfix7000-no-crt0-mflush-func=func-mno-flush-func_flush_func(), that is, the address of the
memory range for which the cache is being flushed, the size of the
memory range, and the number 3 (to flush both caches). The default
depends on the target gcc was configured for, but commonly is either
_flush_func or __cpu_flush.
-mbranch-likely-mno-branch-likelyThese -m options are defined for the i386 and x86-64 family of computers:
-mcpu=cpu-typeWhile picking a specific cpu-type will schedule things appropriately
for that particular chip, the compiler will not generate any code that
does not run on the i386 without the -march=cpu-type option
being used. i586 is equivalent to pentium and i686
is equivalent to pentiumpro. k6 and athlon are the
AMD chips as opposed to the Intel ones.
-march=cpu-type-m386-m486-mpentium-mpentiumpro-mfpmath=unitThis is the default choice for i386 compiler.
For i387 you need to use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For x86-64 compiler, these extensions are enabled by default.
The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80bit.
This is the default choice for the x86-64 compiler.
-masm=dialect-mieee-fp-mno-ieee-fp-msoft-floatOn machines where a function returns floating point results in the 80387
register stack, some floating point opcodes may be emitted even if
-msoft-float is used.
-mno-fp-ret-in-387The usual calling convention has functions return values of types
float and double in an FPU register, even if there
is no FPU. The idea is that the operating system should emulate
an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned
in ordinary CPU registers instead.
-mno-fancy-math-387sin, cos and
sqrt instructions for the 387. Specify this option to avoid
generating those instructions. This option is the default on FreeBSD,
OpenBSD and NetBSD. This option is overridden when -march
indicates that the target cpu will always have an FPU and so the
instruction will not need emulation. As of revision 2.6.1, these
instructions are not generated unless you also use the
-funsafe-math-optimizations switch.
-malign-double-mno-align-doubledouble, long double, and
long long variables on a two word boundary or a one word
boundary. Aligning double variables on a two word boundary will
produce code that runs somewhat faster on a Pentium at the
expense of more memory.
Warning: if you use the -malign-double switch,
structures containing the above types will be aligned differently than
the published application binary interface specifications for the 386
and will not be binary compatible with structures in code compiled
without that switch.
-m96bit-long-double-m128bit-long-doublelong double type. The i386
application binary interface specifies the size to be 96 bits,
so -m96bit-long-double is the default in 32 bit mode.
Modern architectures (Pentium and newer) would prefer long double
to be aligned to an 8 or 16 byte boundary. In arrays or structures
conforming to the ABI, this would not be possible. So specifying a
-m128bit-long-double will align long double
to a 16 byte boundary by padding the long double with an additional
32 bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as
its ABI specifies that long double is to be aligned on 16 byte boundary.
Notice that neither of these options enable any extra precision over the x87
standard of 80 bits for a long double.
Warning: if you override the default value for your target ABI, the
structures and arrays containing long double variables will change their size as
well as function calling convention for function taking long double
will be modified. Hence they will not be binary compatible with arrays or
structures in code compiled without that switch.
-msvr3-shlib-mno-svr3-shlibbss or data segments. -msvr3-shlib places them
into bss. These options are meaningful only on System V Release 3.
-mrtdret num
instruction, which pops their arguments while returning. This saves one
instruction in the caller since there is no need to pop the arguments
there.
You can specify that an individual function is called with this calling sequence with the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl. See Function Attributes.
Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including printf);
otherwise incorrect code will be generated for calls to those
functions.
In addition, seriously incorrect code will result if you call a
function with too many arguments. (Normally, extra arguments are
harmlessly ignored.)
-mregparm=numWarning: if you use this switch, and
num is nonzero, then you must build all modules with the same
value, including any libraries. This includes the system libraries and
startup modules.
-mpreferred-stack-boundary=numOn Pentium and PentiumPro, double and long double values
should be aligned to an 8 byte boundary (see -malign-double) or
suffer significant run time performance penalties. On Pentium III, the
Streaming SIMD Extension (SSE) data type __m128 suffers similar
penalties if it is not 16 byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary will most likely misalign the stack. It is recommended that libraries that use callbacks always use the default setting.
This extra alignment does consume extra stack space, and generally
increases code size. Code that is sensitive to stack space usage, such
as embedded systems and operating system kernels, may want to reduce the
preferred alignment to -mpreferred-stack-boundary=2.
-mmmx-mno-mmx-msse-mno-sse-msse2-mno-sse2-msse3-mno-sse3-m3dnow-mno-3dnowSee X86 Built-in Functions, for details of the functions enabled and disabled by these switches.
To have SSE/SSE2 instructions generated automatically from floating-point
code, see -mfpmath=sse.
-mpush-args-mno-push-args-maccumulate-outgoing-args-mthreads-mno-align-stringops-minline-all-stringops-momit-leaf-frame-pointerThese -m switches are supported in addition to the above on AMD x86-64 processors in 64-bit environments.
-m32-m64-mno-red-zone-mcmodel=small-mcmodel=kernel-mcmodel=medium-mcmodel=largeThese -m options are defined for the HPPA family of computers:
-march=architecture-typePA 2.0 support currently requires gas snapshot 19990413 or later. The
next release of binutils (current is 2.9.1) will probably contain PA 2.0
support.
-mpa-risc-1-0-mpa-risc-1-1-mpa-risc-2-0-mbig-switch-mjump-in-delay-mdisable-fpregs-mdisable-indexing-mno-space-regsSuch code is suitable for level 0 PA systems and kernels.
-mfast-indirect-callsThis option will not work in the presence of shared libraries or nested
functions.
-mlong-load-store-mportable-runtime-mgas-mschedule=cpu-type-mlinker-opt-msoft-float-msoft-float changes the calling convention in the output file;
therefore, it is only useful if you compile all of a program with
this option. In particular, you need to compile libgcc.a, the
library that comes with GCC, with -msoft-float in order for
this to work.
-msio_SIO, for server IO. The default is
-mwsio. This generates the predefines, __hp9000s700,
__hp9000s700__ and _WSIO, for workstation IO. These
options are available under HP-UX and HI-UX.
-mgnu-ld-mhp-ld-mlong-callsDistances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it will degrade performance. However, it may be useful in large applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the
assembler and linker, and the type of code being generated. The
impact on systems that support long absolute calls, and long pic
symbol-difference or pc-relative calls should be relatively small.
However, an indirect call is used on 32-bit ELF systems in pic code
and it is quite long.
-nolibdld-staticOn HP-UX 10 and later, the GCC driver adds the necessary options to
link with libdld.sl when the -static option is specified.
This causes the resulting binary to be dynamic. On the 64-bit port,
the linkers generate dynamic binaries by default in any case. The
-nolibdld option can be used to prevent the GCC driver from
adding these link options.
-threadsThese -m options are defined for the Intel 960 implementations:
-mcpu-type-mnumerics-msoft-float-mleaf-procedures-mno-leaf-proceduresbal instruction as well as call. This will result in more
efficient code for explicit calls when the bal instruction can be
substituted by the assembler or linker, but less efficient code in other
cases, such as calls via function pointers, or using a linker that doesn't
support this optimization.
-mtail-call-mno-tail-call-mcomplex-addr-mno-complex-addr-mcode-align-mno-code-align-mic-compat-mic2.0-compat-mic3.0-compat-masm-compat-mintel-asm-mstrict-align-mno-strict-align-mold-align-mlong-double-64These -m options are defined for the DEC Alpha implementations:
-mno-soft-float-msoft-floatNote that Alpha implementations without floating-point operations are
required to have floating-point registers.
-mfp-reg-mno-fp-regs$0 instead of $f0. This is a non-standard calling sequence,
so any function with a floating-point argument or return value called by code
compiled with -mno-fp-regs must also be compiled with that
option.
A typical use of this option is building a kernel that does not use,
and hence need not save and restore, any floating-point registers.
-mieee_IEEE_FP is
defined during compilation. The resulting code is less efficient but is
able to correctly support denormalized numbers and exceptional IEEE
values such as not-a-number and plus/minus infinity. Other Alpha
compilers call this option -ieee_with_no_inexact.
-mieee-with-inexact_IEEE_FP, _IEEE_FP_EXACT is defined as a preprocessor
macro. On some Alpha implementations the resulting code may execute
significantly slower than the code generated by default. Since there is
very little code that depends on the inexact-flag, you should
normally not specify this option. Other Alpha compilers call this
option -ieee_with_inexact.
-mfp-trap-mode=trap-mode-mfp-rounding-mode=rounding-mode-mtrap-precision=trap-precisionOther Alpha compilers provide the equivalent options called
-scope_safe and -resumption_safe.
-mieee-conformant-mbuild-constantsUse this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six).
You would typically use this option to build a shared library dynamic
loader. Itself a shared library, it must relocate itself in memory
before it can find the variables and constants in its own data segment.
-malpha-as-mgas-mbwx-mno-bwx-mcix-mno-cix-mfix-mno-fix-mmax-mno-max-mfloat-vax-mfloat-ieee-mexplicit-relocs-mno-explicit-relocs-msmall-data-mlarge-data.sdata and .sbss sections) and are accessed via
16-bit relocations off of the $gp register. This limits the
size of the small data area to 64KB, but allows the variables to be
directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area
is limited to just below 2GB. Programs that require more than 2GB of
data must use malloc or mmap to allocate the data in the
heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies
-msmall-data and -fPIC implies -mlarge-data.
-mcpu=cpu_typeSupported values for cpu_type are
-mtune=cpu_type-mmemory-latency=timeValid options for time are
These -m options are defined for the DEC Alpha/VMS implementations:
-mvms-return-codesThese -m options are defined for the H8/300 implementations:
-mrelaxld and the H8/300 (Using ld), for a fuller description.
-mh-ms-mn-ms2600-mint32int data 32 bits by default.
-malign-300These -m options are defined for the SH implementations:
-m1-m2-m3-m3e-m4-nofpu-m4-single-only-m4-single-m4-mb-ml-mdalign-mrelax-mbigtableswitch tables. The default is to use
16-bit offsets.
-mfmovdfmovd.
-mhitachi-mnomacsaveMAC register as call-clobbered, even if
-mhitachi is given.
-mieee-misize-mpadstruct-mspace-mprefergot-musermodesh-*-linux*.
These additional options are available on System V Release 4 for compatibility with other compilers on those systems:
-G-Qy.ident assembler directive in the output.
-Qn.ident directives to the output file (this is
the default).
-YP,dirs-Ym,dirThese -m options are defined for TMS320C3x/C4x implementations:
-mcpu=cpu_type-mbig-memory-mbig-msmall-memory-msmall-mbk-mno-bk-mdb-mno-db-mdp-isr-reload-mparanoid-mmpyi-mno-mpyi-mfast-fix-mno-fast-fix-mrptb-mno-rptb-mrpts=count-mno-rpts-mloop-unsigned-mno-loop-unsigned-mti-mregparm-mmemparm-mparallel-insns-mno-parallel-insns-mparallel-mpy-mno-parallel-mpyThese -m options are defined for V850 implementations:
-mlong-calls-mno-long-calls-mno-ep-mepep register, and
use the shorter sld and sst instructions. The -mep
option is on by default if you optimize.
-mno-prolog-function-mprolog-function-mspace-mtda=nep points to. The tiny data
area can hold up to 256 bytes in total (128 bytes for byte references).
-msda=ngp points to. The small data
area can hold up to 64 kilobytes.
-mzda=n-mv850-mbig-switch-mapp-regs-mno-app-regs-mv850eIf neither -mv850 nor -mv850e are defined then a default target processor will be chosen and the relevant __v850*__ preprocessor constant will be defined.
The preprocessor constants __v850 and __v851__ are always
defined, regardless of which processor variant is the target.
-mdisable-calltThese options are defined for ARC implementations:
-EL-EB-mmangle-cpu-mcpu=cpu-mtext=text-section-mdata=data-section-mrodata=readonly-data-sectionsection attribute.
See Variable Attributes.
These are the -m options defined for the 32000 series. The default values for these options depends on which style of 32000 was selected when the compiler was configured; the defaults for the most common choices are given below.
-m32032-m32032-m32332-m32332-m32532-m32532-m32081-m32381-mmulti-addpolyF
and dotF. This option is only available if the -m32381
option is in effect. Using these instructions requires changes to
register allocation which generally has a negative impact on
performance. This option should only be enabled when compiling code
particularly likely to make heavy use of multiply-add instructions.
-mnomulti-addpolyF and dotF. This is the default on all platforms.
-msoft-float-mieee-compare-mno-ieee-compare-mnobitfield-mbitfield-mrtdret instruction.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that
take variable numbers of arguments (including printf);
otherwise incorrect code will be generated for calls to those
functions.
In addition, seriously incorrect code will result if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)
This option takes its name from the 680x0 rtd instruction.
-mregparamThis calling convention is incompatible with the one normally
used on Unix, so you cannot use it if you need to call libraries
compiled with the Unix compiler.
-mnoregparam-msb-mnosb-mhimem-mnohimemThese options are defined for AVR implementations:
-mmcu=mcuInstruction set avr1 is for the minimal AVR core, not supported by the C compiler, only for assembler programs (MCU types: at90s1200, attiny10, attiny11, attiny12, attiny15, attiny28).
Instruction set avr2 (default) is for the classic AVR core with up to 8K program memory space (MCU types: at90s2313, at90s2323, attiny22, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90s8515, at90c8534, at90s8535).
Instruction set avr3 is for the classic AVR core with up to 128K program memory space (MCU types: atmega103, atmega603, at43usb320, at76c711).
Instruction set avr4 is for the enhanced AVR core with up to 8K program memory space (MCU types: atmega8, atmega83, atmega85).
Instruction set avr5 is for the enhanced AVR core with up to 128K program
memory space (MCU types: atmega16, atmega161, atmega163, atmega32, atmega323,
atmega64, atmega128, at43usb355, at94k).
-msize-minit-stack=N-mno-interrupts-mcall-prologues-mno-tablejump-mtiny-stackThese are the -m options defined for the Motorola M*Core processors.
-mhardlit-mno-hardlit-mdiv-mno-div-mrelax-immediate-mno-relax-immediate-mwide-bitfields-mno-wide-bitfields-m4byte-functions-mno-4byte-functions-mcallgraph-data-mno-callgraph-data-mslow-bytes-mno-slow-bytes-mlittle-endian-mbig-endian-m210-m340These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian-mlittle-endian-mgnu-as-mno-gnu-as-mgnu-ld-mno-gnu-ld-mno-pic-mvolatile-asm-stop-mno-volatile-asm-stop-mb-step-mregister-names-mno-register-names-mno-sdata-msdata-mconstant-gp-mauto-pic-minline-float-divide-min-latency-minline-float-divide-max-throughput-minline-int-divide-min-latency-minline-int-divide-max-throughput-mno-dwarf2-asm-mdwarf2-asm-mfixed-range=register-rangeThese -m options are defined for D30V implementations:
-mextmem0x80000000.
-mextmemory-monchip0x0. Also link .data, .bss,
.strings, .rodata, .rodata1, .data1 sections
into onchip data memory, which starts at location 0x20000000.
-mno-asm-optimize-masm-optimize-mbranch-cost=n-mcond-exec=nThese are the -m options defined for the S/390 and zSeries architecture.
-mhard-float-msoft-float-mbackchain-mno-backchain-msmall-exec-mno-small-execbras instruction
to do subroutine calls.
This only works reliably if the total executable size does not
exceed 64k. The default is to use the basr instruction instead,
which does not have this limitation.
-m64-m31-mmvcle-mno-mvclemvcle instruction
to perform block moves. When -mno-mvcle is specified,
use a mvc loop instead. This is the default.
-mdebug-mno-debugThese options are defined specifically for the CRIS ports.
-march=architecture-type-mcpu=architecture-type-mtune=architecture-type-mmax-stack-frame=n-melinux-stacksize=n-metrax4-metrax100-mmul-bug-workaround-mno-mul-bug-workaroundmuls and mulu instructions for CPU
models where it applies. This option is active by default.
-mpdebug-mcc-init-mno-side-effects-mstack-align-mno-stack-align-mdata-align-mno-data-align-mconst-align-mno-const-align-m32-bit-m16-bit-m8-bit-mno-prologue-epilogue-mprologue-epilogue-mno-gotplt-mgotplt-maout-melf-melinux-mlinux-sim-sim2These options are defined for the MMIX:
-mlibfuncs-mno-libfuncs-mepsilon-mno-epsilonrE epsilon register.
-mabi=mmixware-mabi=gnu$0 and up, as opposed to
the GNU ABI which uses global registers $231 and up.
-mzero-extend-mno-zero-extend-mknuthdiv-mno-knuthdiv-mtoplevel-symbols-mno-toplevel-symbolsPREFIX assembly directive.
-melf-mbranch-predict-mno-branch-predict-mbase-addresses-mno-base-addresses-msingle-exit-mno-single-exitThese options are defined for the PDP-11:
-mfpu-msoft-float-mac0-mno-ac0-m40-m45-m10-mbcopy-builtinmovstrhi patterns for copying memory. This is the
default.
-mbcopymovstrhi patterns for copying memory.
-mint16-mno-int32int. This is the default.
-mint32-mno-int16int.
-mfloat64-mno-float32float. This is the default.
-mfloat32-mno-float64float.
-mabshiabshi2 pattern. This is the default.
-mno-abshiabshi2 pattern.
-mbranch-expensive-mbranch-cheap-msplit-mno-split-munix-asm-mdec-asmThese options are defined for Xstormy16:
-msim-mgpr-32-mgpr-64-mfpr-32-mfpr-64-mhard-float-msoft-float-malloc-cc-mfixed-ccicc0 and fcc0.
-mdword-mno-dword-mdouble-mno-double-mmedia-mno-media-mmuladd-mno-muladd-mlibrary-pic-macc-4-macc-8-mpack-mno-pack-mno-eflags-mcond-moveThis switch is mainly for debugging the compiler and will likely be removed
in a future version.
-mno-cond-moveThis switch is mainly for debugging the compiler and will likely be removed
in a future version.
-msccThis switch is mainly for debugging the compiler and will likely be removed
in a future version.
-mno-sccThis switch is mainly for debugging the compiler and will likely be removed
in a future version.
-mcond-execThis switch is mainly for debugging the compiler and will likely be removed
in a future version.
-mno-cond-execThis switch is mainly for debugging the compiler and will likely be removed
in a future version.
-mvliw-branchThis switch is mainly for debugging the compiler and will likely be removed
in a future version.
-mno-vliw-branchThis switch is mainly for debugging the compiler and will likely be removed
in a future version.
-mmulti-cond-exec&& and || in conditional execution
(default).
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
-mno-multi-cond-exec&& and || in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed
in a future version.
-mnested-cond-execThis switch is mainly for debugging the compiler and will likely be removed
in a future version.
-mno-nested-cond-execThis switch is mainly for debugging the compiler and will likely be removed
in a future version.
-mtomcat-stats-mcpu=cpuThe Xtensa architecture is designed to support many different configurations. The compiler's default options can be set to match a particular Xtensa configuration by copying a configuration file into the GCC sources when building GCC. The options below may be used to override the default options.
-mbig-endian-mlittle-endian-mdensity-mno-density-mmac16-mno-mac16-mmul16-mno-mul16-mmul32-mno-mul32-mnsa-mno-nsaNSA) instructions to implement the built-in ffs function.
-mminmax-mno-minmax-msext-mno-sextSEXT)
instruction.
-mbooleans-mno-booleans-mhard-float-msoft-floatfloat
operations. When this option is disabled, GCC generates library calls
to emulate 32-bit floating-point operations using integer instructions.
Regardless of this option, 64-bit double operations are always
emulated with calls to library functions.
-mfused-madd-mno-fused-madd-mserialize-volatile-mno-serialize-volatileMEMW instructions before
volatile memory references to guarantee sequential consistency.
The default is -mserialize-volatile. Use
-mno-serialize-volatile to omit the MEMW instructions.
-mtext-section-literals-mno-text-section-literals-mtarget-align-mno-target-alignLOOP, which the
assembler will always align, either by widening density instructions or
by inserting no-op instructions.
-mlongcalls-mno-longcallsCALL
instruction into an L32R followed by a CALLX instruction.
The default is -mno-longcalls. This option should be used in
programs where the call target can potentially be out of range. This
option is implemented in the assembler, not the compiler, so the
assembly code generated by GCC will still show direct call
instructions—look at the disassembled object code to see the actual
instructions. Note that the assembler will use an indirect call for
every cross-file call, not just those that really will be out of range.
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo would be -fno-foo. In the table below, only one of the forms is listed—the one which is not the default. You can figure out the other form by either removing no- or adding it.
-fbounds-check-ftrapv-fexceptions-fnon-call-exceptionsSIGALRM.
-funwind-tables-fasynchronous-unwind-tables-fpcc-struct-returnstruct and union values in memory like
longer ones, rather than in registers. This convention is less
efficient, but it has the advantage of allowing intercallability between
GCC-compiled files and files compiled with other compilers, particularly
the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return
switch is not binary compatible with code compiled with the
-freg-struct-return switch.
Use it to conform to a non-default application binary interface.
-freg-struct-returnstruct and union values in registers when possible.
This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return
switch is not binary compatible with code compiled with the
-fpcc-struct-return switch.
Use it to conform to a non-default application binary interface.
-fshort-enumsenum type only as many bytes as it needs for the
declared range of possible values. Specifically, the enum type
will be equivalent to the smallest integer type which has enough room.
Warning: the -fshort-enums switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshort-doubledouble as for float.
Warning: the -fshort-double switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshort-wcharWarning: the -fshort-wchar switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshared-dataconst variables of this
compilation be shared data rather than private data. The distinction
makes sense only on certain operating systems, where shared data is
shared between processes running the same program, while private data
exists in one copy per process.
-fno-commonextern) in
two different compilations, you will get an error when you link them.
The only reason this might be useful is if you wish to verify that the
program will work on other systems which always work this way.
-fno-ident-fno-gnu-linker-finhibit-size-directive.size assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This option is
used when compiling crtstuff.c; you should not need to use it
for anything else.
-fverbose-asm-fno-verbose-asm, the default, causes the
extra information to be omitted and is useful when comparing two assembler
files.
-fvolatile-fvolatile-global-fvolatile-static-fpicPosition-independent code requires special support, and therefore works
only on certain machines. For the 386, GCC supports PIC for System V
but not for the Sun 386i. Code generated for the IBM RS/6000 is always
position-independent.
-fPICPosition-independent code requires special support, and therefore works
only on certain machines.
-ffixed-regreg must be the name of a register. The register names accepted
are machine-specific and are defined in the REGISTER_NAMES
macro in the machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-regIt is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-saved-regIt is an error to used this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model will produce disastrous results.
A different sort of disaster will result from the use of this flag for a register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
-fpack-structWarning: the -fpack-struct switch causes GCC to generate
code that is not binary compatible with code generated without that switch.
Additionally, it makes the code suboptimal.
Use it to conform to a non-default application binary interface.
-finstrument-functions__builtin_return_address does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in other functions. The profiling calls will indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use extern inline in your C code, an addressable version of such functions must be provided. (This is normally the case anyways, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)
A function may be given the attribute no_instrument_function, in
which case this instrumentation will not be done. This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling functions
cannot safely be called (perhaps signal handlers, if the profiling
routines generate output or allocate memory).
-fstack-checkNote that this switch does not actually cause checking to be done; the
operating system must do that. The switch causes generation of code
to ensure that the operating system sees the stack being extended.
-fstack-limit-register=reg-fstack-limit-symbol=sym-fno-stack-limitFor instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.
-fargument-alias-fargument-noalias-fargument-noalias-global-fargument-alias specifies that arguments (parameters) may
alias each other and may alias global storage.
-fargument-noalias specifies that arguments do not alias
each other, but may alias global storage.
-fargument-noalias-global specifies that arguments do not
alias each other and do not alias global storage.
Each language will automatically use whatever option is required by
the language standard. You should not need to use these options yourself.
-fleading-underscoreWarning: the -fleading-underscore switch causes GCC to
generate code that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary interface.
Not all targets provide complete support for this switch.
-ftls-model=modelglobal-dynamic,
local-dynamic, initial-exec or local-exec.
The default without -fpic is initial-exec; with
-fpic the default is global-dynamic.
This section describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment.
Note that you can also specify places to search using options such as -B, -I and -L (see Directory Options). These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. See Controlling the Compilation Driver gcc (GNU Compiler Collection (GCC) Internals).
The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that would otherwise be interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value
of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE
and LC_MESSAGES default to the value of the LANG
environment variable. If none of these variables are set, GCC
defaults to traditional C English behavior.
If GCC_EXEC_PREFIX is not set, GCC will attempt to figure out an appropriate prefix to use based on the pathname it was invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.
The default value of GCC_EXEC_PREFIX is
prefix/lib/gcc-lib/ where prefix is the value
of prefix when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are used for linking.
In addition, the prefix is used in an unusual way in finding the
directories to search for header files. For each of the standard
directories whose name normally begins with /usr/local/lib/gcc-lib
(more precisely, with the value of GCC_INCLUDE_DIR), GCC tries
replacing that beginning with the specified prefix to produce an
alternate directory name. Thus, with -Bfoo/, GCC will search
foo/bar where it would normally search /usr/local/lib/bar.
These alternate directories are searched first; the standard directories
come next.
If LANG is not defined, or if it has some other value, then the compiler will use mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters.
Some additional environments variables affect the behavior of the preprocessor.
PATH_SEPARATOR, is target-dependent and
determined at GCC build time. For Windows-based targets it is a
semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. This environment variable is used regardless of which language is being preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line.
In all these variables, an empty element instructs the compiler to
search its current working directory. Empty elements can appear at the
beginning or end of a path. For instance, if the value of
CPATH is :/special/include, that has the same
effect as -I. -I/special/include.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. Or the value can have the form file target, in which case the rules are written to file file using target as the target name.
In other words, this environment variable is equivalent to combining
the options -MM and -MF
(see Preprocessor Options),
with an optional -MT switch too.
The program protoize is an optional part of GCC. You can use
it to add prototypes to a program, thus converting the program to ISO
C in one respect. The companion program unprotoize does the
reverse: it removes argument types from any prototypes that are found.
When you run these programs, you must specify a set of source files as command line arguments. The conversion programs start out by compiling these files to see what functions they define. The information gathered about a file foo is saved in a file named foo.X.
After scanning comes actual conversion. The specified files are all eligible to be converted; any files they include (whether sources or just headers) are eligible as well.
But not all the eligible files are converted. By default,
protoize and unprotoize convert only source and header
files in the current directory. You can specify additional directories
whose files should be converted with the -d directory
option. You can also specify particular files to exclude with the
-x file option. A file is converted if it is eligible, its
directory name matches one of the specified directory names, and its
name within the directory has not been excluded.
Basic conversion with protoize consists of rewriting most
function definitions and function declarations to specify the types of
the arguments. The only ones not rewritten are those for varargs
functions.
protoize optionally inserts prototype declarations at the
beginning of the source file, to make them available for any calls that
precede the function's definition. Or it can insert prototype
declarations with block scope in the blocks where undeclared functions
are called.
Basic conversion with unprotoize consists of rewriting most
function declarations to remove any argument types, and rewriting
function definitions to the old-style pre-ISO form.
Both conversion programs print a warning for any function declaration or definition that they can't convert. You can suppress these warnings with -q.
The output from protoize or unprotoize replaces the
original source file. The original file is renamed to a name ending
with .save (for DOS, the saved filename ends in .sav
without the original .c suffix). If the .save (.sav
for DOS) file already exists, then the source file is simply discarded.
protoize and unprotoize both depend on GCC itself to
scan the program and collect information about the functions it uses.
So neither of these programs will work until GCC is installed.
Here is a table of the options you can use with protoize and
unprotoize. Each option works with both programs unless
otherwise stated.
-B directoryprotoize.
-c compilation-optionsNote that the compilation options must be given as a single argument to
protoize or unprotoize. If you want to specify several
gcc options, you must quote the entire set of compilation options
to make them a single word in the shell.
There are certain gcc arguments that you cannot use, because they
would produce the wrong kind of output. These include -g,
-O, -c, -S, and -o If you include these in
the compilation-options, they are ignored.
-Cprotoize.
-gprotoize.
-i stringprotoize.
unprotoize converts prototyped function definitions to old-style
function definitions, where the arguments are declared between the
argument list and the initial {. By default, unprotoize
uses five spaces as the indentation. If you want to indent with just
one space instead, use -i " ".
-k-lprotoize with -l inserts
a prototype declaration for each function in each block which calls the
function without any declaration. This option applies only to
protoize.
-n-N-p program-q-vIf you need special compiler options to compile one of your program's
source files, then you should generate that file's .X file
specially, by running gcc on that source file with the
appropriate options and the option -aux-info. Then run
protoize on the entire set of files. protoize will use
the existing .X file because it is newer than the source file.
For example:
gcc -Dfoo=bar file1.c -aux-info file1.X
protoize *.c
You need to include the special files along with the rest in the
protoize command, even though their .X files already
exist, because otherwise they won't get converted.
See Protoize Caveats, for more information on how to use
protoize successfully.
A conforming implementation of ISO C is required to document its choice of behavior in each of the areas that are designated “implementation defined.” The following lists all such areas, along with the section number from the ISO/IEC 9899:1999 standard.
Diagnostics consist of all the output sent to stderr by GCC.
The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.
For internal names, all characters are significant. For external names, the number of significant characters are defined by the linker; for almost all targets, all characters are significant.
char object into which has been stored any
character other than a member of the basic execution character set (6.2.5).
signed char or unsigned char has the same range,
representation, and behavior as “plain” char (6.2.5, 6.3.1.1).
GCC supports only two's complement integer types, and all bit patterns are ordinary values.
<math.h> and <complex.h> that return floating-point
results (5.2.4.2.2).
FLT_ROUNDS
(5.2.4.2.2).
FLT_EVAL_METHOD (5.2.4.2.2).
FP_CONTRACT pragma (6.5).
FENV_ACCESS pragma (7.6.1).
FP_CONTRACT pragma (7.12.2).
A cast from pointer to integer discards most-significant bits if the pointer representation is larger than the integer type, sign-extends2 if the pointer representation is smaller than the integer type, otherwise the bits are unchanged.
A cast from integer to pointer discards most-significant bits if the pointer representation is smaller than the integer type, extends according to the signedness of the integer type if the pointer representation is larger than the integer type, otherwise the bits are unchanged.
When casting from pointer to integer and back again, the resulting pointer must reference the same object as the original pointer, otherwise the behavior is undefined. That is, one may not use integer arithmetic to avoid the undefined behavior of pointer arithmetic as proscribed in 6.5.6/8.
register
storage-class specifier are effective (6.7.1).
The register specifier affects code generation only in these ways:
register
storage-class specifier; if register is specified, the variable
may have a shorter lifespan than the code would indicate and may never
be placed in memory.
setjmp doesn't save the registers in
all circumstances. In those cases, GCC doesn't allocate any variables
in registers unless they are marked register.
GCC will not inline any functions if the -fno-inline option is used or if -O0 is used. Otherwise, GCC may still be unable to inline a function for many reasons; the -Winline option may be used to determine if a function has not been inlined and why not.
signed int
bit-field or as an unsigned int bit-field (6.7.2, 6.7.2.1).
_Bool, signed int,
and unsigned int (6.7.2.1).
#include directive are combined into a header
name (6.10.2).
#include processing (6.10.2).
GCC imposes a limit of 200 nested #includes.
STDC #pragma
directive (6.10.6).
__DATE__ and __TIME__ when
respectively, the date and time of translation are not available (6.10.8).
If the date and time are not available, __DATE__ expands to
"??? ?? ????" and __TIME__ expands to
"??:??:??".
The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.
<float.h>, <limits.h>, and <stdint.h>
(5.2.4.2, 7.18.2, 7.18.3).
The behavior of these points are dependent on the implementation of the C library, and are not defined by GCC itself.
GNU C provides several language features not found in ISO standard C.
(The -pedantic option directs GCC to print a warning message if
any of these features is used.) To test for the availability of these
features in conditional compilation, check for a predefined macro
__GNUC__, which is always defined under GCC.
These extensions are available in C and Objective-C. Most of them are also available in C++. See Extensions to the C++ Language, for extensions that apply only to C++.
Some features that are in ISO C99 but not C89 or C++ are also, as extensions, accepted by GCC in C89 mode and in C++.
A compound statement enclosed in parentheses may appear as an expression in GNU C. This allows you to use loops, switches, and local variables within an expression.
Recall that a compound statement is a sequence of statements surrounded by braces; in this construct, parentheses go around the braces. For example:
({ int y = foo (); int z;
if (y > 0) z = y;
else z = - y;
z; })
is a valid (though slightly more complex than necessary) expression
for the absolute value of foo ().
The last thing in the compound statement should be an expression
followed by a semicolon; the value of this subexpression serves as the
value of the entire construct. (If you use some other kind of statement
last within the braces, the construct has type void, and thus
effectively no value.)
This feature is especially useful in making macro definitions “safe” (so that they evaluate each operand exactly once). For example, the “maximum” function is commonly defined as a macro in standard C as follows:
#define max(a,b) ((a) > (b) ? (a) : (b))
But this definition computes either a or b twice, with bad
results if the operand has side effects. In GNU C, if you know the
type of the operands (here let's assume int), you can define
the macro safely as follows:
#define maxint(a,b) \
({int _a = (a), _b = (b); _a > _b ? _a : _b; })
Embedded statements are not allowed in constant expressions, such as the value of an enumeration constant, the width of a bit-field, or the initial value of a static variable.
If you don't know the type of the operand, you can still do this, but you
must use typeof (see Typeof).
Statement expressions are not supported fully in G++, and their fate there is unclear. (It is possible that they will become fully supported at some point, or that they will be deprecated, or that the bugs that are present will continue to exist indefinitely.) Presently, statement expressions do not work well as default arguments.
In addition, there are semantic issues with statement-expressions in C++. If you try to use statement-expressions instead of inline functions in C++, you may be surprised at the way object destruction is handled. For example:
#define foo(a) ({int b = (a); b + 3; })
does not work the same way as:
inline int foo(int a) { int b = a; return b + 3; }
In particular, if the expression passed into foo involves the
creation of temporaries, the destructors for those temporaries will be
run earlier in the case of the macro than in the case of the function.
These considerations mean that it is probably a bad idea to use statement-expressions of this form in header files that are designed to work with C++. (Note that some versions of the GNU C Library contained header files using statement-expression that lead to precisely this bug.)
Each statement expression is a scope in which local labels can be
declared. A local label is simply an identifier; you can jump to it
with an ordinary goto statement, but only from within the
statement expression it belongs to.
A local label declaration looks like this:
__label__ label;
or
__label__ label1, label2, /* ... */;
Local label declarations must come at the beginning of the statement expression, right after the ({, before any ordinary declarations.
The label declaration defines the label name, but does not define
the label itself. You must do this in the usual way, with
label:, within the statements of the statement expression.
The local label feature is useful because statement expressions are
often used in macros. If the macro contains nested loops, a goto
can be useful for breaking out of them. However, an ordinary label
whose scope is the whole function cannot be used: if the macro can be
expanded several times in one function, the label will be multiply
defined in that function. A local label avoids this problem. For
example:
#define SEARCH(array, target) \
({ \
__label__ found; \
typeof (target) _SEARCH_target = (target); \
typeof (*(array)) *_SEARCH_array = (array); \
int i, j; \
int value; \
for (i = 0; i < max; i++) \
for (j = 0; j < max; j++) \
if (_SEARCH_array[i][j] == _SEARCH_target) \
{ value = i; goto found; } \
value = -1; \
found: \
value; \
})
You can get the address of a label defined in the current function
(or a containing function) with the unary operator &&. The
value has type void *. This value is a constant and can be used
wherever a constant of that type is valid. For example:
void *ptr;
/* ... */
ptr = &&foo;
To use these values, you need to be able to jump to one. This is done
with the computed goto statement3, goto *exp;. For example,
goto *ptr;
Any expression of type void * is allowed.
One way of using these constants is in initializing a static array that will serve as a jump table:
static void *array[] = { &&foo, &&bar, &&hack };
Then you can select a label with indexing, like this:
goto *array[i];
Note that this does not check whether the subscript is in bounds—array indexing in C never does that.
Such an array of label values serves a purpose much like that of the
switch statement. The switch statement is cleaner, so
use that rather than an array unless the problem does not fit a
switch statement very well.
Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching.
You may not use this mechanism to jump to code in a different function. If you do that, totally unpredictable things will happen. The best way to avoid this is to store the label address only in automatic variables and never pass it as an argument.
An alternate way to write the above example is
static const int array[] = { &&foo - &&foo, &&bar - &&foo,
&&hack - &&foo };
goto *(&&foo + array[i]);
This is more friendly to code living in shared libraries, as it reduces the number of dynamic relocations that are needed, and by consequence, allows the data to be read-only.
A nested function is a function defined inside another function.
(Nested functions are not supported for GNU C++.) The nested function's
name is local to the block where it is defined. For example, here we
define a nested function named square, and call it twice:
foo (double a, double b)
{
double square (double z) { return z * z; }
return square (a) + square (b);
}
The nested function can access all the variables of the containing
function that are visible at the point of its definition. This is
called lexical scoping. For example, here we show a nested
function which uses an inherited variable named offset:
bar (int *array, int offset, int size)
{
int access (int *array, int index)
{ return array[index + offset]; }
int i;
/* ... */
for (i = 0; i < size; i++)
/* ... */ access (array, i) /* ... */
}
Nested function definitions are permitted within functions in the places where variable definitions are allowed; that is, in any block, before the first statement in the block.
It is possible to call the nested function from outside the scope of its name by storing its address or passing the address to another function:
hack (int *array, int size)
{
void store (int index, int value)
{ array[index] = value; }
intermediate (store, size);
}
Here, the function intermediate receives the address of
store as an argument. If intermediate calls store,
the arguments given to store are used to store into array.
But this technique works only so long as the containing function
(hack, in this example) does not exit.
If you try to call the nested function through its address after the containing function has exited, all hell will break loose. If you try to call it after a containing scope level has exited, and if it refers to some of the variables that are no longer in scope, you may be lucky, but it's not wise to take the risk. If, however, the nested function does not refer to anything that has gone out of scope, you should be safe.
GCC implements taking the address of a nested function using a technique called trampolines. A paper describing them is available as
http://people.debian.org/~aaronl/Usenix88-lexic.pdf.
A nested function can jump to a label inherited from a containing
function, provided the label was explicitly declared in the containing
function (see Local Labels). Such a jump returns instantly to the
containing function, exiting the nested function which did the
goto and any intermediate functions as well. Here is an example:
bar (int *array, int offset, int size)
{
__label__ failure;
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
int i;
/* ... */
for (i = 0; i < size; i++)
/* ... */ access (array, i) /* ... */
/* ... */
return 0;
/* Control comes here from access
if it detects an error. */
failure:
return -1;
}
A nested function always has internal linkage. Declaring one with
extern is erroneous. If you need to declare the nested function
before its definition, use auto (which is otherwise meaningless
for function declarations).
bar (int *array, int offset, int size)
{
__label__ failure;
auto int access (int *, int);
/* ... */
int access (int *array, int index)
{
if (index > size)
goto failure;
return array[index + offset];
}
/* ... */
}
Using the built-in functions described below, you can record the arguments a function received, and call another function with the same arguments, without knowing the number or types of the arguments.
You can also record the return value of that function call, and later return that value, without knowing what data type the function tried to return (as long as your caller expects that data type).
This built-in function returns a pointer to data describing how to perform a call with the same arguments as were passed to the current function.
The function saves the arg pointer register, structure value address, and all registers that might be used to pass arguments to a function into a block of memory allocated on the stack. Then it returns the address of that block.
This built-in function invokes function with a copy of the parameters described by arguments and size.
The value of arguments should be the value returned by
__builtin_apply_args. The argument size specifies the size of the stack argument data, in bytes.This function returns a pointer to data describing how to return whatever value was returned by function. The data is saved in a block of memory allocated on the stack.
It is not always simple to compute the proper value for size. The value is used by
__builtin_applyto compute the amount of data that should be pushed on the stack and copied from the incoming argument area.
This built-in function returns the value described by result from the containing function. You should specify, for result, a value returned by
__builtin_apply.
typeof
Another way to refer to the type of an expression is with typeof.
The syntax of using of this keyword looks like sizeof, but the
construct acts semantically like a type name defined with typedef.
There are two ways of writing the argument to typeof: with an
expression or with a type. Here is an example with an expression:
typeof (x[0](1))
This assumes that x is an array of pointers to functions;
the type described is that of the values of the functions.
Here is an example with a typename as the argument:
typeof (int *)
Here the type described is that of pointers to int.
If you are writing a header file that must work when included in ISO C
programs, write __typeof__ instead of typeof.
See Alternate Keywords.
A typeof-construct can be used anywhere a typedef name could be
used. For example, you can use it in a declaration, in a cast, or inside
of sizeof or typeof.
typeof is often useful in conjunction with the
statements-within-expressions feature. Here is how the two together can
be used to define a safe “maximum” macro that operates on any
arithmetic type and evaluates each of its arguments exactly once:
#define max(a,b) \
({ typeof (a) _a = (a); \
typeof (b) _b = (b); \
_a > _b ? _a : _b; })
The reason for using names that start with underscores for the local
variables is to avoid conflicts with variable names that occur within the
expressions that are substituted for a and b. Eventually we
hope to design a new form of declaration syntax that allows you to declare
variables whose scopes start only after their initializers; this will be a
more reliable way to prevent such conflicts.
Some more examples of the use of typeof:
y with the type of what x points to.
typeof (*x) y;
y as an array of such values.
typeof (*x) y[4];
y as an array of pointers to characters:
typeof (typeof (char *)[4]) y;
It is equivalent to the following traditional C declaration:
char *y[4];
To see the meaning of the declaration using typeof, and why it
might be a useful way to write, let's rewrite it with these macros:
#define pointer(T) typeof(T *)
#define array(T, N) typeof(T [N])
Now the declaration can be rewritten this way:
array (pointer (char), 4) y;
Thus, array (pointer (char), 4) is the type of arrays of 4
pointers to char.
Compatibility Note: In addition to typeof, GCC 2 supported
a more limited extension which permitted one to write
typedef T = expr;
with the effect of declaring T to have the type of the expression
expr. This extension does not work with GCC 3 (versions between
3.0 and 3.2 will crash; 3.2.1 and later give an error). Code which
relies on it should be rewritten to use typeof:
typedef typeof(expr) T;
This will work with all versions of GCC.
Compound expressions, conditional expressions and casts are allowed as lvalues provided their operands are lvalues. This means that you can take their addresses or store values into them.
All these extensions are deprecated.
For example, a compound expression can be assigned, provided the last expression in the sequence is an lvalue. These two expressions are equivalent:
(a, b) += 5
a, (b += 5)
Similarly, the address of the compound expression can be taken. These two expressions are equivalent:
&(a, b)
a, &b
A conditional expression is a valid lvalue if its type is not void and the true and false branches are both valid lvalues. For example, these two expressions are equivalent:
(a ? b : c) = 5
(a ? b = 5 : (c = 5))
A cast is a valid lvalue if its operand is an lvalue. A simple
assignment whose left-hand side is a cast works by converting the
right-hand side first to the specified type, then to the type of the
inner left-hand side expression. After this is stored, the value is
converted back to the specified type to become the value of the
assignment. Thus, if a has type char *, the following two
expressions are equivalent:
(int)a = 5
(int)(a = (char *)(int)5)
An assignment-with-arithmetic operation such as += applied to a cast performs the arithmetic using the type resulting from the cast, and then continues as in the previous case. Therefore, these two expressions are equivalent:
(int)a += 5
(int)(a = (char *)(int) ((int)a + 5))
You cannot take the address of an lvalue cast, because the use of its
address would not work out coherently. Suppose that &(int)f were
permitted, where f has type float. Then the following
statement would try to store an integer bit-pattern where a floating
point number belongs:
*&(int)f = 1;
This is quite different from what (int)f = 1 would do—that
would convert 1 to floating point and store it. Rather than cause this
inconsistency, we think it is better to prohibit use of & on a cast.
If you really do want an int * pointer with the address of
f, you can simply write (int *)&f.
The middle operand in a conditional expression may be omitted. Then if the first operand is nonzero, its value is the value of the conditional expression.
Therefore, the expression
x ? : y
has the value of x if that is nonzero; otherwise, the value of
y.
This example is perfectly equivalent to
x ? x : y
In this simple case, the ability to omit the middle operand is not especially useful. When it becomes useful is when the first operand does, or may (if it is a macro argument), contain a side effect. Then repeating the operand in the middle would perform the side effect twice. Omitting the middle operand uses the value already computed without the undesirable effects of recomputing it.
ISO C99 supports data types for integers that are at least 64 bits wide,
and as an extension GCC supports them in C89 mode and in C++.
Simply write long long int for a signed integer, or
unsigned long long int for an unsigned integer. To make an
integer constant of type long long int, add the suffix LL
to the integer. To make an integer constant of type unsigned long
long int, add the suffix ULL to the integer.
You can use these types in arithmetic like any other integer types. Addition, subtraction, and bitwise boolean operations on these types are open-coded on all types of machines. Multiplication is open-coded if the machine supports fullword-to-doubleword a widening multiply instruction. Division and shifts are open-coded only on machines that provide special support. The operations that are not open-coded use special library routines that come with GCC.
There may be pitfalls when you use long long types for function
arguments, unless you declare function prototypes. If a function
expects type int for its argument, and you pass a value of type
long long int, confusion will result because the caller and the
subroutine will disagree about the number of bytes for the argument.
Likewise, if the function expects long long int and you pass
int. The best way to avoid such problems is to use prototypes.
ISO C99 supports complex floating data types, and as an extension GCC
supports them in C89 mode and in C++, and supports complex integer data
types which are not part of ISO C99. You can declare complex types
using the keyword _Complex. As an extension, the older GNU
keyword __complex__ is also supported.
For example, _Complex double x; declares x as a
variable whose real part and imaginary part are both of type
double. _Complex short int y; declares y to
have real and imaginary parts of type short int; this is not
likely to be useful, but it shows that the set of complex types is
complete.
To write a constant with a complex data type, use the suffix i or
j (either one; they are equivalent). For example, 2.5fi
has type _Complex float and 3i has type
_Complex int. Such a constant always has a pure imaginary
value, but you can form any complex value you like by adding one to a
real constant. This is a GNU extension; if you have an ISO C99
conforming C library (such as GNU libc), and want to construct complex
constants of floating type, you should include <complex.h> and
use the macros I or _Complex_I instead.
To extract the real part of a complex-valued expression exp, write
__real__ exp. Likewise, use __imag__ to
extract the imaginary part. This is a GNU extension; for values of
floating type, you should use the ISO C99 functions crealf,
creal, creall, cimagf, cimag and
cimagl, declared in <complex.h> and also provided as
built-in functions by GCC.
The operator ~ performs complex conjugation when used on a value
with a complex type. This is a GNU extension; for values of
floating type, you should use the ISO C99 functions conjf,
conj and conjl, declared in <complex.h> and also
provided as built-in functions by GCC.
GCC can allocate complex automatic variables in a noncontiguous
fashion; it's even possible for the real part to be in a register while
the imaginary part is on the stack (or vice-versa). Only the DWARF2
debug info format can represent this, so use of DWARF2 is recommended.
If you are using the stabs debug info format, GCC describes a noncontiguous
complex variable as if it were two separate variables of noncomplex type.
If the variable's actual name is foo, the two fictitious
variables are named foo$real and foo$imag. You can
examine and set these two fictitious variables with your debugger.
ISO C99 supports floating-point numbers written not only in the usual
decimal notation, such as 1.55e1, but also numbers such as
0x1.fp3 written in hexadecimal format. As a GNU extension, GCC
supports this in C89 mode (except in some cases when strictly
conforming) and in C++. In that format the
0x hex introducer and the p or P exponent field are
mandatory. The exponent is a decimal number that indicates the power of
2 by which the significant part will be multiplied. Thus 0x1.f is
1 15/16,
p3 multiplies it by 8, and the value of 0x1.fp3
is the same as 1.55e1.
Unlike for floating-point numbers in the decimal notation the exponent
is always required in the hexadecimal notation. Otherwise the compiler
would not be able to resolve the ambiguity of, e.g., 0x1.f. This
could mean 1.0f or 1.9375 since f is also the
extension for floating-point constants of type float.
Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure which is really a header for a variable-length object:
struct line {
int length;
char contents[0];
};
struct line *thisline = (struct line *)
malloc (sizeof (struct line) + this_length);
thisline->length = this_length;
In ISO C90, you would have to give contents a length of 1, which
means either you waste space or complicate the argument to malloc.
In ISO C99, you would use a flexible array member, which is slightly different in syntax and semantics:
contents[] without
the 0.
sizeof
operator may not be applied. As a quirk of the original implementation
of zero-length arrays, sizeof evaluates to zero.
struct that is otherwise non-empty.
GCC versions before 3.0 allowed zero-length arrays to be statically initialized, as if they were flexible arrays. In addition to those cases that were useful, it also allowed initializations in situations that would corrupt later data. Non-empty initialization of zero-length arrays is now treated like any case where there are more initializer elements than the array holds, in that a suitable warning about "excess elements in array" is given, and the excess elements (all of them, in this case) are ignored.
Instead GCC allows static initialization of flexible array members.
This is equivalent to defining a new structure containing the original
structure followed by an array of sufficient size to contain the data.
I.e. in the following, f1 is constructed as if it were declared
like f2.
struct f1 {
int x; int y[];
} f1 = { 1, { 2, 3, 4 } };
struct f2 {
struct f1 f1; int data[3];
} f2 = { { 1 }, { 2, 3, 4 } };
The convenience of this extension is that f1 has the desired
type, eliminating the need to consistently refer to f2.f1.
This has symmetry with normal static arrays, in that an array of
unknown size is also written with [].
Of course, this extension only makes sense if the extra data comes at the end of a top-level object, as otherwise we would be overwriting data at subsequent offsets. To avoid undue complication and confusion with initialization of deeply nested arrays, we simply disallow any non-empty initialization except when the structure is the top-level object. For example:
struct foo { int x; int y[]; };
struct bar { struct foo z; };
struct foo a = { 1, { 2, 3, 4 } }; // Valid.
struct bar b = { { 1, { 2, 3, 4 } } }; // Invalid.
struct bar c = { { 1, { } } }; // Valid.
struct foo d[1] = { { 1 { 2, 3, 4 } } }; // Invalid.
GCC permits a C structure to have no members:
struct empty {
};
The structure will have size zero. In C++, empty structures are part
of the language. G++ treats empty structures as if they had a single
member of type char.
Variable-length automatic arrays are allowed in ISO C99, and as an extension GCC accepts them in C89 mode and in C++. (However, GCC's implementation of variable-length arrays does not yet conform in detail to the ISO C99 standard.) These arrays are declared like any other automatic arrays, but with a length that is not a constant expression. The storage is allocated at the point of declaration and deallocated when the brace-level is exited. For example:
FILE *
concat_fopen (char *s1, char *s2, char *mode)
{
char str[strlen (s1) + strlen (s2) + 1];
strcpy (str, s1);
strcat (str, s2);
return fopen (str, mode);
}
Jumping or breaking out of the scope of the array name deallocates the storage. Jumping into the scope is not allowed; you get an error message for it.
You can use the function alloca to get an effect much like
variable-length arrays. The function alloca is available in
many other C implementations (but not in all). On the other hand,
variable-length arrays are more elegant.
There are other differences between these two methods. Space allocated
with alloca exists until the containing function returns.
The space for a variable-length array is deallocated as soon as the array
name's scope ends. (If you use both variable-length arrays and
alloca in the same function, deallocation of a variable-length array
will also deallocate anything more recently allocated with alloca.)
You can also use variable-length arrays as arguments to functions:
struct entry
tester (int len, char data[len][len])
{
/* ... */
}
The length of an array is computed once when the storage is allocated
and is remembered for the scope of the array in case you access it with
sizeof.
If you want to pass the array first and the length afterward, you can use a forward declaration in the parameter list—another GNU extension.
struct entry
tester (int len; char data[len][len], int len)
{
/* ... */
}
The int len before the semicolon is a parameter forward
declaration, and it serves the purpose of making the name len
known when the declaration of data is parsed.
You can write any number of such parameter forward declarations in the parameter list. They can be separated by commas or semicolons, but the last one must end with a semicolon, which is followed by the “real” parameter declarations. Each forward declaration must match a “real” declaration in parameter name and data type. ISO C99 does not support parameter forward declarations.
In the ISO C standard of 1999, a macro can be declared to accept a variable number of arguments much as a function can. The syntax for defining the macro is similar to that of a function. Here is an example:
#define debug(format, ...) fprintf (stderr, format, __VA_ARGS__)
Here ... is a variable argument. In the invocation of
such a macro, it represents the zero or more tokens until the closing
parenthesis that ends the invocation, including any commas. This set of
tokens replaces the identifier __VA_ARGS__ in the macro body
wherever it appears. See the CPP manual for more information.
GCC has long supported variadic macros, and used a different syntax that allowed you to give a name to the variable arguments just like any other argument. Here is an example:
#define debug(format, args...) fprintf (stderr, format, args)
This is in all ways equivalent to the ISO C example above, but arguably more readable and descriptive.
GNU CPP has two further variadic macro extensions, and permits them to be used with either of the above forms of macro definition.
In standard C, you are not allowed to leave the variable argument out entirely; but you are allowed to pass an empty argument. For example, this invocation is invalid in ISO C, because there is no comma after the string:
debug ("A message")
GNU CPP permits you to completely omit the variable arguments in this way. In the above examples, the compiler would complain, though since the expansion of the macro still has the extra comma after the format string.
To help solve this problem, CPP behaves specially for variable arguments used with the token paste operator, ##. If instead you write
#define debug(format, ...) fprintf (stderr, format, ## __VA_ARGS__)
and if the variable arguments are omitted or empty, the ## operator causes the preprocessor to remove the comma before it. If you do provide some variable arguments in your macro invocation, GNU CPP does not complain about the paste operation and instead places the variable arguments after the comma. Just like any other pasted macro argument, these arguments are not macro expanded.
Recently, the preprocessor has relaxed its treatment of escaped newlines. Previously, the newline had to immediately follow a backslash. The current implementation allows whitespace in the form of spaces, horizontal and vertical tabs, and form feeds between the backslash and the subsequent newline. The preprocessor issues a warning, but treats it as a valid escaped newline and combines the two lines to form a single logical line. This works within comments and tokens, as well as between tokens. Comments are not treated as whitespace for the purposes of this relaxation, since they have not yet been replaced with spaces.
In ISO C99, arrays that are not lvalues still decay to pointers, and may be subscripted, although they may not be modified or used after the next sequence point and the unary & operator may not be applied to them. As an extension, GCC allows such arrays to be subscripted in C89 mode, though otherwise they do not decay to pointers outside C99 mode. For example, this is valid in GNU C though not valid in C89:
struct foo {int a[4];};
struct foo f();
bar (int index)
{
return f().a[index];
}
void- and Function-Pointers
In GNU C, addition and subtraction operations are supported on pointers to
void and on pointers to functions. This is done by treating the
size of a void or of a function as 1.
A consequence of this is that sizeof is also allowed on void
and on function types, and returns 1.
The option -Wpointer-arith requests a warning if these extensions are used.
As in standard C++ and ISO C99, the elements of an aggregate initializer for an automatic variable are not required to be constant expressions in GNU C. Here is an example of an initializer with run-time varying elements:
foo (float f, float g)
{
float beat_freqs[2] = { f-g, f+g };
/* ... */
}
ISO C99 supports compound literals. A compound literal looks like a cast containing an initializer. Its value is an object of the type specified in the cast, containing the elements specified in the initializer; it is an lvalue. As an extension, GCC supports compound literals in C89 mode and in C++.
Usually, the specified type is a structure. Assume that
struct foo and structure are declared as shown:
struct foo {int a; char b[2];} structure;
Here is an example of constructing a struct foo with a compound literal:
structure = ((struct foo) {x + y, 'a', 0});
This is equivalent to writing the following:
{
struct foo temp = {x + y, 'a', 0};
structure = temp;
}
You can also construct an array. If all the elements of the compound literal are (made up of) simple constant expressions, suitable for use in initializers of objects of static storage duration, then the compound literal can be coerced to a pointer to its first element and used in such an initializer, as shown here:
char **foo = (char *[]) { "x", "y", "z" };
Compound literals for scalar types and union types are is also allowed, but then the compound literal is equivalent to a cast.
As a GNU extension, GCC allows initialization of objects with static storage duration by compound literals (which is not possible in ISO C99, because the initializer is not a constant). It is handled as if the object was initialized only with the bracket enclosed list if compound literal's and object types match. The initializer list of the compound literal must be constant. If the object being initialized has array type of unknown size, the size is determined by compound literal size.
static struct foo x = (struct foo) {1, 'a', 'b'};
static int y[] = (int []) {1, 2, 3};
static int z[] = (int [3]) {1};
The above lines are equivalent to the following:
static struct foo x = {1, 'a', 'b'};
static int y[] = {1, 2, 3};
static int z[] = {1, 0, 0};
Standard C89 requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.
In ISO C99 you can give the elements in any order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C89 mode as well. This extension is not implemented in GNU C++.
To specify an array index, write [index] = before the element value. For example,
int a[6] = { [4] = 29, [2] = 15 };
is equivalent to
int a[6] = { 0, 0, 15, 0, 29, 0 };
The index values must be constant expressions, even if the array being initialized is automatic.
An alternative syntax for this which has been obsolete since GCC 2.5 but GCC still accepts is to write [index] before the element value, with no =.
To initialize a range of elements to the same value, write [first ... last] = value. This is a GNU extension. For example,
int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };
If the value in it has side-effects, the side-effects will happen only once, not for each initialized field by the range initializer.
Note that the length of the array is the highest value specified plus one.
In a structure initializer, specify the name of a field to initialize with .fieldname = before the element value. For example, given the following structure,
struct point { int x, y; };
the following initialization
struct point p = { .y = yvalue, .x = xvalue };
is equivalent to
struct point p = { xvalue, yvalue };
Another syntax which has the same meaning, obsolete since GCC 2.5, is fieldname:, as shown here:
struct point p = { y: yvalue, x: xvalue };
The [index] or .fieldname is known as a designator. You can also use a designator (or the obsolete colon syntax) when initializing a union, to specify which element of the union should be used. For example,
union foo { int i; double d; };
union foo f = { .d = 4 };
will convert 4 to a double to store it in the union using
the second element. By contrast, casting 4 to type union foo
would store it into the union as the integer i, since it is
an integer. (See Cast to Union.)
You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a designator applies to the next consecutive element of the array or structure. For example,
int a[6] = { [1] = v1, v2, [4] = v4 };
is equivalent to
int a[6] = { 0, v1, v2, 0, v4, 0 };
Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an enum type.
For example:
int whitespace[256]
= { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };
You can also write a series of .fieldname and [index] designators before an = to specify a nested subobject to initialize; the list is taken relative to the subobject corresponding to the closest surrounding brace pair. For example, with the struct point declaration above:
struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };
If the same field is initialized multiple times, it will have value from the last initialization. If any such overridden initialization has side-effect, it is unspecified whether the side-effect happens or not. Currently, gcc will discard them and issue a warning.
You can specify a range of consecutive values in a single case label,
like this:
case low ... high:
This has the same effect as the proper number of individual case
labels, one for each integer value from low to high, inclusive.
This feature is especially useful for ranges of ASCII character codes:
case 'A' ... 'Z':
Be careful: Write spaces around the ..., for otherwise
it may be parsed wrong when you use it with integer values. For example,
write this:
case 1 ... 5:
rather than this:
case 1...5:
A cast to union type is similar to other casts, except that the type
specified is a union type. You can specify the type either with
union tag or with a typedef name. A cast to union is actually
a constructor though, not a cast, and hence does not yield an lvalue like
normal casts. (See Compound Literals.)
The types that may be cast to the union type are those of the members of the union. Thus, given the following union and variables:
union foo { int i; double d; };
int x;
double y;
both x and y can be cast to type union foo.
Using the cast as the right-hand side of an assignment to a variable of union type is equivalent to storing in a member of the union:
union foo u;
/* ... */
u = (union foo) x == u.i = x
u = (union foo) y == u.d = y
You can also use the union cast as a function argument:
void hack (union foo);
/* ... */
hack ((union foo) x);
ISO C99 and ISO C++ allow declarations and code to be freely mixed within compound statements. As an extension, GCC also allows this in C89 mode. For example, you could do:
int i;
/* ... */
i++;
int j = i + 2;
Each identifier is visible from where it is declared until the end of the enclosing block.
In GNU C, you declare certain things about functions called in your program which help the compiler optimize function calls and check your code more carefully.
The keyword __attribute__ allows you to specify special
attributes when making a declaration. This keyword is followed by an
attribute specification inside double parentheses. The following
attributes are currently defined for functions on all targets:
noreturn, noinline, always_inline,
pure, const, nothrow,
format, format_arg, no_instrument_function,
section, constructor, destructor, used,
unused, deprecated, weak, malloc,
alias, and nonnull. Several other attributes are defined
for functions on particular target systems. Other attributes, including
section are supported for variables declarations
(see Variable Attributes) and for types (see Type Attributes).
You may also specify attributes with __ preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use __noreturn__ instead of noreturn.
See Attribute Syntax, for details of the exact syntax for using attributes.
noreturnabort and exit,
cannot return. GCC knows this automatically. Some programs define
their own functions that never return. You can declare them
noreturn to tell the compiler this fact. For example,
void fatal () __attribute__ ((noreturn));
void
fatal (/* ... */)
{
/* ... */ /* Print error message. */ /* ... */
exit (1);
}
The noreturn keyword tells the compiler to assume that
fatal cannot return. It can then optimize without regard to what
would happen if fatal ever did return. This makes slightly
better code. More importantly, it helps avoid spurious warnings of
uninitialized variables.
Do not assume that registers saved by the calling function are
restored before calling the noreturn function.
It does not make sense for a noreturn function to have a return
type other than void.
The attribute noreturn is not implemented in GCC versions
earlier than 2.5. An alternative way to declare that a function does
not return, which works in the current version and in some older
versions, is as follows:
typedef void voidfn ();
volatile voidfn fatal;
noinlinealways_inlinepurepure. For example,
int square (int) __attribute__ ((pure));
says that the hypothetical function square is safe to call
fewer times than the program says.
Some of common examples of pure functions are strlen or memcmp.
Interesting non-pure functions are functions with infinite loops or those
depending on volatile memory or other system resource, that may change between
two consecutive calls (such as feof in a multithreading environment).
The attribute pure is not implemented in GCC versions earlier
than 2.96.
constpure attribute above, since function is not
allowed to read global memory.
Note that a function that has pointer arguments and examines the data
pointed to must not be declared const. Likewise, a
function that calls a non-const function usually must not be
const. It does not make sense for a const function to
return void.
The attribute const is not implemented in GCC versions earlier
than 2.5. An alternative way to declare that a function has no side
effects, which works in the current version and in some older versions,
is as follows:
typedef int intfn ();
extern const intfn square;
This approach does not work in GNU C++ from 2.6.0 on, since the language specifies that the const must be attached to the return value.
nothrownothrow attribute is used to inform the compiler that a
function cannot throw an exception. For example, most functions in
the standard C library can be guaranteed not to throw an exception
with the notable exceptions of qsort and bsearch that
take function pointer arguments. The nothrow attribute is not
implemented in GCC versions earlier than 3.2.
format (archetype, string-index, first-to-check)format attribute specifies that a function takes printf,
scanf, strftime or strfmon style arguments which
should be type-checked against a format string. For example, the
declaration:
extern int
my_printf (void *my_object, const char *my_format, ...)
__attribute__ ((format (printf, 2, 3)));
causes the compiler to check the arguments in calls to my_printf
for consistency with the printf style format string argument
my_format.
The parameter archetype determines how the format string is
interpreted, and should be printf, scanf, strftime
or strfmon. (You can also use __printf__,
__scanf__, __strftime__ or __strfmon__.) The
parameter string-index specifies which argument is the format
string argument (starting from 1), while first-to-check is the
number of the first argument to check against the format string. For
functions where the arguments are not available to be checked (such as
vprintf), specify the third parameter as zero. In this case the
compiler only checks the format string for consistency. For
strftime formats, the third parameter is required to be zero.
Since non-static C++ methods have an implicit this argument, the
arguments of such methods should be counted from two, not one, when
giving values for string-index and first-to-check.
In the example above, the format string (my_format) is the second
argument of the function my_print, and the arguments to check
start with the third argument, so the correct parameters for the format
attribute are 2 and 3.
The format attribute allows you to identify your own functions
which take format strings as arguments, so that GCC can check the
calls to these functions for errors. The compiler always (unless
-ffreestanding is used) checks formats
for the standard library functions printf, fprintf,
sprintf, scanf, fscanf, sscanf, strftime,
vprintf, vfprintf and vsprintf whenever such
warnings are requested (using -Wformat), so there is no need to
modify the header file stdio.h. In C99 mode, the functions
snprintf, vsnprintf, vscanf, vfscanf and
vsscanf are also checked. Except in strictly conforming C
standard modes, the X/Open function strfmon is also checked as
are printf_unlocked and fprintf_unlocked.
See Options Controlling C Dialect.
format_arg (string-index)format_arg attribute specifies that a function takes a format
string for a printf, scanf, strftime or
strfmon style function and modifies it (for example, to translate
it into another language), so the result can be passed to a
printf, scanf, strftime or strfmon style
function (with the remaining arguments to the format function the same
as they would have been for the unmodified string). For example, the
declaration:
extern char *
my_dgettext (char *my_domain, const char *my_format)
__attribute__ ((format_arg (2)));
causes the compiler to check the arguments in calls to a printf,
scanf, strftime or strfmon type function, whose
format string argument is a call to the my_dgettext function, for
consistency with the format string argument my_format. If the
format_arg attribute had not been specified, all the compiler
could tell in such calls to format functions would be that the format
string argument is not constant; this would generate a warning when
-Wformat-nonliteral is used, but the calls could not be checked
without the attribute.
The parameter string-index specifies which argument is the format
string argument (starting from one). Since non-static C++ methods have
an implicit this argument, the arguments of such methods should
be counted from two.
The format-arg attribute allows you to identify your own
functions which modify format strings, so that GCC can check the
calls to printf, scanf, strftime or strfmon
type function whose operands are a call to one of your own function.
The compiler always treats gettext, dgettext, and
dcgettext in this manner except when strict ISO C support is
requested by -ansi or an appropriate -std option, or
-ffreestanding is used. See Options Controlling C Dialect.
nonnull (arg-index, ...)nonnull attribute specifies that some function parameters should
be non-null pointers. For instance, the declaration:
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull (1, 2)));
causes the compiler to check that, in calls to my_memcpy,
arguments dest and src are non-null. If the compiler
determines that a null pointer is passed in an argument slot marked
as non-null, and the -Wnonnull option is enabled, a warning
is issued. The compiler may also choose to make optimizations based
on the knowledge that certain function arguments will not be null.
If no argument index list is given to the nonnull attribute,
all pointer arguments are marked as non-null. To illustrate, the
following declaration is equivalent to the previous example:
extern void *
my_memcpy (void *dest, const void *src, size_t len)
__attribute__((nonnull));
no_instrument_functionsection ("section-name")text section.
Sometimes, however, you need additional sections, or you need certain
particular functions to appear in special sections. The section
attribute specifies that a function lives in a particular section.
For example, the declaration:
extern void foobar (void) __attribute__ ((section ("bar")));
puts the function foobar in the bar section.
Some file formats do not support arbitrary sections so the section
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
constructordestructorconstructor attribute causes the function to be called
automatically before execution enters main (). Similarly, the
destructor attribute causes the function to be called
automatically after main () has completed or exit () has
been called. Functions with these attributes are useful for
initializing data that will be used implicitly during the execution of
the program.
These attributes are not currently implemented for Objective-C.
unuseduseddeprecateddeprecated attribute results in a warning if the function
is used anywhere in the source file. This is useful when identifying
functions that are expected to be removed in a future version of a
program. The warning also includes the location of the declaration
of the deprecated function, to enable users to easily find further
information about why the function is deprecated, or what they should
do instead. Note that the warnings only occurs for uses:
int old_fn () __attribute__ ((deprecated));
int old_fn ();
int (*fn_ptr)() = old_fn;
results in a warning on line 3 but not line 2.
The deprecated attribute can also be used for variables and
types (see Variable Attributes, see Type Attributes.)
weakweak attribute causes the declaration to be emitted as a weak
symbol rather than a global. This is primarily useful in defining
library functions which can be overridden in user code, though it can
also be used with non-function declarations. Weak symbols are supported
for ELF targets, and also for a.out targets when using the GNU assembler
and linker.
mallocmalloc attribute is used to tell the compiler that a function
may be treated as if it were the malloc function. The compiler assumes
that calls to malloc result in pointers that cannot alias anything.
This will often improve optimization.
alias ("target")alias attribute causes the declaration to be emitted as an
alias for another symbol, which must be specified. For instance,
void __f () { /* Do something. */; }
void f () __attribute__ ((weak, alias ("__f")));
declares f to be a weak alias for __f. In C++, the mangled name for the target must be used.
Not all target machines support this attribute.
visibility ("visibility_type")visibility attribute on ELF targets causes the declaration
to be emitted with default, hidden, protected or internal visibility.
void __attribute__ ((visibility ("protected")))
f () { /* Do something. */; }
int i __attribute__ ((visibility ("hidden")));
See the ELF gABI for complete details, but the short story is:
Not all ELF targets support this attribute.
regparm (number)regparm attribute causes the compiler to
pass up to number integer arguments in registers EAX,
EDX, and ECX instead of on the stack. Functions that take a
variable number of arguments will continue to be passed all of their
arguments on the stack.
Beware that on some ELF systems this attribute is unsuitable for
global functions in shared libraries with lazy binding (which is the
default). Lazy binding will send the first call via resolving code in
the loader, which might assume EAX, EDX and ECX can be clobbered, as
per the standard calling conventions. Solaris 8 is affected by this.
GNU systems with GLIBC 2.1 or higher, and FreeBSD, are believed to be
safe since the loaders there save all registers. (Lazy binding can be
disabled with the linker or the loader if desired, to avoid the
problem.)
stdcallstdcall attribute causes the compiler to
assume that the called function will pop off the stack space used to
pass arguments, unless it takes a variable number of arguments.
cdeclcdecl attribute causes the compiler to
assume that the calling function will pop off the stack space used to
pass arguments. This is
useful to override the effects of the -mrtd switch.
longcall/shortcalllongcall attribute causes the
compiler to always call this function via a pointer, just as it would if
the -mlongcall option had been specified. The shortcall
attribute causes the compiler not to do this. These attributes override
both the -mlongcall switch and the #pragma longcall
setting.
See RS/6000 and PowerPC Options, for more information on whether long
calls are necessary.
long_call/short_call#pragma long_calls settings. The
long_call attribute causes the compiler to always call the
function by first loading its address into a register and then using the
contents of that register. The short_call attribute always places
the offset to the function from the call site into the BL
instruction directly.
function_vectorYou must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.
interruptNote, interrupt handlers for the H8/300, H8/300H and SH processors can
be specified via the interrupt_handler attribute.
Note, on the AVR, interrupts will be enabled inside the function.
Note, for the ARM, you can specify the kind of interrupt to be handled by adding an optional parameter to the interrupt attribute like this:
void f () __attribute__ ((interrupt ("IRQ")));
Permissible values for this parameter are: IRQ, FIQ, SWI, ABORT and UNDEF.
interrupt_handlersp_switchinterrupt_handler
function should switch to an alternate stack. It expects a string
argument that names a global variable holding the address of the
alternate stack.
void *alt_stack;
void f () __attribute__ ((interrupt_handler,
sp_switch ("alt_stack")));
trap_exitinterrupt_handle to return using
trapa instead of rte. This attribute expects an integer
argument specifying the trap number to be used.
eightbit_dataYou must use GAS and GLD from GNU binutils version 2.7 or later for
this attribute to work correctly.
tiny_datasignalnakedmodel (model-name)small, medium,
or large, representing each of the code models.
Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the ld24 instruction), and are
callable with the bl instruction.
Medium model objects may live anywhere in the 32-bit address space (the
compiler will generate seth/add3 instructions to load their addresses),
and are callable with the bl instruction.
Large model objects may live anywhere in the 32-bit address space (the
compiler will generate seth/add3 instructions to load their addresses),
and may not be reachable with the bl instruction (the compiler will
generate the much slower seth/add3/jl instruction sequence).
farfar attribute causes the compiler to
use a calling convention that takes care of switching memory banks when
entering and leaving a function. This calling convention is also the
default when using the -mlong-calls option.
On 68HC12 the compiler will use the call and rtc instructions
to call and return from a function.
On 68HC11 the compiler will generate a sequence of instructions
to invoke a board-specific routine to switch the memory bank and call the
real function. The board-specific routine simulates a call.
At the end of a function, it will jump to a board-specific routine
instead of using rts. The board-specific return routine simulates
the rtc.
nearnear attribute causes the compiler to
use the normal calling convention based on jsr and rts.
This attribute can be used to cancel the effect of the -mlong-calls
option.
dllimportdllimport attribute causes the compiler
to reference a function or variable via a global pointer to a pointer
that is set up by the Windows dll library. The pointer name is formed by
combining _imp__ and the function or variable name. The attribute
implies extern storage.
Currently, the attribute is ignored for inlined functions. If the
attribute is applied to a symbol definition, an error is reported.
If a symbol previously declared dllimport is later defined, the
attribute is ignored in subsequent references, and a warning is emitted.
The attribute is also overridden by a subsequent declaration as
dllexport.
When applied to C++ classes, the attribute marks non-inlined member functions and static data members as imports. However, the attribute is ignored for virtual methods to allow creation of vtables using thunks.
On cygwin, mingw and arm-pe targets, __declspec(dllimport) is
recognized as a synonym for __attribute__ ((dllimport)) for
compatibility with other Windows compilers.
The use of the dllimport attribute on functions is not necessary,
but provides a small performance benefit by eliminating a thunk in the
dll. The use of the dllimport attribute on imported variables was
required on older versions of GNU ld, but can now be avoided by passing
the --enable-auto-import switch to ld. As with functions, using
the attribute for a variable eliminates a thunk in the dll.
One drawback to using this attribute is that a pointer to a function or
variable marked as dllimport cannot be used as a constant address. The
attribute can be disabled for functions by setting the
-mnop-fun-dllimport flag.
dllexportdllexport attribute causes the compiler to
provide a global pointer to a pointer in a dll, so that it can be
referenced with the dllimport attribute. The pointer name is
formed by combining _imp__ and the function or variable name.
Currently, the dllexportattribute is ignored for inlined
functions, but export can be forced by using the
-fkeep-inline-functions flag. The attribute is also ignored for
undefined symbols.
When applied to C++ classes. the attribute marks defined non-inlined member functions and static data members as exports. Static consts initialized in-class are not marked unless they are also defined out-of-class.
On cygwin, mingw and arm-pe targets, __declspec(dllexport) is
recognized as a synonym for __attribute__ ((dllexport)) for
compatibility with other Windows compilers.
Alternative methods for including the symbol in the dll's export table
are to use a .def file with an EXPORTS section or, with GNU ld,
using the --export-all linker flag.
You can specify multiple attributes in a declaration by separating them by commas within the double parentheses or by immediately following an attribute declaration with another attribute declaration.
Some people object to the __attribute__ feature, suggesting that
ISO C's #pragma should be used instead. At the time
__attribute__ was designed, there were two reasons for not doing
this.
#pragma commands from a macro.
#pragma might mean in another
compiler.
These two reasons applied to almost any application that might have been
proposed for #pragma. It was basically a mistake to use
#pragma for anything.
The ISO C99 standard includes _Pragma, which now allows pragmas
to be generated from macros. In addition, a #pragma GCC
namespace is now in use for GCC-specific pragmas. However, it has been
found convenient to use __attribute__ to achieve a natural
attachment of attributes to their corresponding declarations, whereas
#pragma GCC is of use for constructs that do not naturally form
part of the grammar. See Miscellaneous Preprocessing Directives (The GNU C Preprocessor).
This section describes the syntax with which __attribute__ may be
used, and the constructs to which attribute specifiers bind, for the C
language. Some details may vary for C++ and Objective-C. Because of
infelicities in the grammar for attributes, some forms described here
may not be successfully parsed in all cases.
There are some problems with the semantics of attributes in C++. For
example, there are no manglings for attributes, although they may affect
code generation, so problems may arise when attributed types are used in
conjunction with templates or overloading. Similarly, typeid
does not distinguish between types with different attributes. Support
for attributes in C++ may be restricted in future to attributes on
declarations only, but not on nested declarators.
See Function Attributes, for details of the semantics of attributes applying to functions. See Variable Attributes, for details of the semantics of attributes applying to variables. See Type Attributes, for details of the semantics of attributes applying to structure, union and enumerated types.
An attribute specifier is of the form
__attribute__ ((attribute-list)). An attribute list
is a possibly empty comma-separated sequence of attributes, where
each attribute is one of the following:
unused, or a reserved
word such as const).
mode attributes use this form.
format attributes use this form.
format_arg attributes use this form with the list being a single
integer constant expression, and alias attributes use this form
with the list being a single string constant.
An attribute specifier list is a sequence of one or more attribute specifiers, not separated by any other tokens.
An attribute specifier list may appear after the colon following a
label, other than a case or default label. The only
attribute it makes sense to use after a label is unused. This
feature is intended for code generated by programs which contains labels
that may be unused but which is compiled with -Wall. It would
not normally be appropriate to use in it human-written code, though it
could be useful in cases where the code that jumps to the label is
contained within an #ifdef conditional.
An attribute specifier list may appear as part of a struct,
union or enum specifier. It may go either immediately
after the struct, union or enum keyword, or after
the closing brace. It is ignored if the content of the structure, union
or enumerated type is not defined in the specifier in which the
attribute specifier list is used—that is, in usages such as
struct __attribute__((foo)) bar with no following opening brace.
Where attribute specifiers follow the closing brace, they are considered
to relate to the structure, union or enumerated type defined, not to any
enclosing declaration the type specifier appears in, and the type
defined is not complete until after the attribute specifiers.
Otherwise, an attribute specifier appears as part of a declaration, counting declarations of unnamed parameters and type names, and relates to that declaration (which may be nested in another declaration, for example in the case of a parameter declaration), or to a particular declarator within a declaration. Where an attribute specifier is applied to a parameter declared as a function or an array, it should apply to the function or array rather than the pointer to which the parameter is implicitly converted, but this is not yet correctly implemented.
Any list of specifiers and qualifiers at the start of a declaration may
contain attribute specifiers, whether or not such a list may in that
context contain storage class specifiers. (Some attributes, however,
are essentially in the nature of storage class specifiers, and only make
sense where storage class specifiers may be used; for example,
section.) There is one necessary limitation to this syntax: the
first old-style parameter declaration in a function definition cannot
begin with an attribute specifier, because such an attribute applies to
the function instead by syntax described below (which, however, is not
yet implemented in this case). In some other cases, attribute
specifiers are permitted by this grammar but not yet supported by the
compiler. All attribute specifiers in this place relate to the
declaration as a whole. In the obsolescent usage where a type of
int is implied by the absence of type specifiers, such a list of
specifiers and qualifiers may be an attribute specifier list with no
other specifiers or qualifiers.
An attribute specifier list may appear immediately before a declarator (other than the first) in a comma-separated list of declarators in a declaration of more than one identifier using a single list of specifiers and qualifiers. Such attribute specifiers apply only to the identifier before whose declarator they appear. For example, in
__attribute__((noreturn)) void d0 (void),
__attribute__((format(printf, 1, 2))) d1 (const char *, ...),
d2 (void)
the noreturn attribute applies to all the functions
declared; the format attribute only applies to d1.
An attribute specifier list may appear immediately before the comma,
= or semicolon terminating the declaration of an identifier other
than a function definition. At present, such attribute specifiers apply
to the declared object or function, but in future they may attach to the
outermost adjacent declarator. In simple cases there is no difference,
but, for example, in
void (****f)(void) __attribute__((noreturn));
at present the noreturn attribute applies to f, which
causes a warning since f is not a function, but in future it may
apply to the function ****f. The precise semantics of what
attributes in such cases will apply to are not yet specified. Where an
assembler name for an object or function is specified (see Asm Labels), at present the attribute must follow the asm
specification; in future, attributes before the asm specification
may apply to the adjacent declarator, and those after it to the declared
object or function.
An attribute specifier list may, in future, be permitted to appear after the declarator in a function definition (before any old-style parameter declarations or the function body).
Attribute specifiers may be mixed with type qualifiers appearing inside
the [] of a parameter array declarator, in the C99 construct by
which such qualifiers are applied to the pointer to which the array is
implicitly converted. Such attribute specifiers apply to the pointer,
not to the array, but at present this is not implemented and they are
ignored.
An attribute specifier list may appear at the start of a nested
declarator. At present, there are some limitations in this usage: the
attributes correctly apply to the declarator, but for most individual
attributes the semantics this implies are not implemented.
When attribute specifiers follow the * of a pointer
declarator, they may be mixed with any type qualifiers present.
The following describes the formal semantics of this syntax. It will make the
most sense if you are familiar with the formal specification of
declarators in the ISO C standard.
Consider (as in C99 subclause 6.7.5 paragraph 4) a declaration T
D1, where T contains declaration specifiers that specify a type
Type (such as int) and D1 is a declarator that
contains an identifier ident. The type specified for ident
for derived declarators whose type does not include an attribute
specifier is as in the ISO C standard.
If D1 has the form ( attribute-specifier-list D ),
and the declaration T D specifies the type
“derived-declarator-type-list Type” for ident, then
T D1 specifies the type “derived-declarator-type-list
attribute-specifier-list Type” for ident.
If D1 has the form *
type-qualifier-and-attribute-specifier-list D, and the
declaration T D specifies the type
“derived-declarator-type-list Type” for ident, then
T D1 specifies the type “derived-declarator-type-list
type-qualifier-and-attribute-specifier-list Type” for
ident.
For example,
void (__attribute__((noreturn)) ****f) (void);
specifies the type “pointer to pointer to pointer to pointer to
non-returning function returning void”. As another example,
char *__attribute__((aligned(8))) *f;
specifies the type “pointer to 8-byte-aligned pointer to char”.
Note again that this does not work with most attributes; for example,
the usage of aligned and noreturn attributes given above
is not yet supported.
For compatibility with existing code written for compiler versions that did not implement attributes on nested declarators, some laxity is allowed in the placing of attributes. If an attribute that only applies to types is applied to a declaration, it will be treated as applying to the type of that declaration. If an attribute that only applies to declarations is applied to the type of a declaration, it will be treated as applying to that declaration; and, for compatibility with code placing the attributes immediately before the identifier declared, such an attribute applied to a function return type will be treated as applying to the function type, and such an attribute applied to an array element type will be treated as applying to the array type. If an attribute that only applies to function types is applied to a pointer-to-function type, it will be treated as applying to the pointer target type; if such an attribute is applied to a function return type that is not a pointer-to-function type, it will be treated as applying to the function type.
GNU C extends ISO C to allow a function prototype to override a later old-style non-prototype definition. Consider the following example:
/* Use prototypes unless the compiler is old-fashioned. */ #ifdef __STDC__ #define P(x) x #else #define P(x) () #endif /* Prototype function declaration. */ int isroot P((uid_t)); /* Old-style function definition. */ int isroot (x) /* ??? lossage here ??? */ uid_t x; { return x == 0; }
Suppose the type uid_t happens to be short. ISO C does
not allow this example, because subword arguments in old-style
non-prototype definitions are promoted. Therefore in this example the
function definition's argument is really an int, which does not
match the prototype argument type of short.
This restriction of ISO C makes it hard to write code that is portable
to traditional C compilers, because the programmer does not know
whether the uid_t type is short, int, or
long. Therefore, in cases like these GNU C allows a prototype
to override a later old-style definition. More precisely, in GNU C, a
function prototype argument type overrides the argument type specified
by a later old-style definition if the former type is the same as the
latter type before promotion. Thus in GNU C the above example is
equivalent to the following:
int isroot (uid_t);
int
isroot (uid_t x)
{
return x == 0;
}
GNU C++ does not support old-style function definitions, so this extension is irrelevant.
In GNU C, you may use C++ style comments, which start with // and continue until the end of the line. Many other C implementations allow such comments, and they are included in the 1999 C standard. However, C++ style comments are not recognized if you specify an -std option specifying a version of ISO C before C99, or -ansi (equivalent to -std=c89).
In GNU C, you may normally use dollar signs in identifier names. This is because many traditional C implementations allow such identifiers. However, dollar signs in identifiers are not supported on a few target machines, typically because the target assembler does not allow them.
You can use the sequence \e in a string or character constant to stand for the ASCII character <ESC>.
The keyword __alignof__ allows you to inquire about how an object
is aligned, or the minimum alignment usually required by a type. Its
syntax is just like sizeof.
For example, if the target machine requires a double value to be
aligned on an 8-byte boundary, then __alignof__ (double) is 8.
This is true on many RISC machines. On more traditional machine
designs, __alignof__ (double) is 4 or even 2.
Some machines never actually require alignment; they allow reference to any
data type even at an odd address. For these machines, __alignof__
reports the recommended alignment of a type.
If the operand of __alignof__ is an lvalue rather than a type,
its value is the required alignment for its type, taking into account
any minimum alignment specified with GCC's __attribute__
extension (see Variable Attributes). For example, after this
declaration:
struct foo { int x; char y; } foo1;
the value of __alignof__ (foo1.y) is 1, even though its actual
alignment is probably 2 or 4, the same as __alignof__ (int).
It is an error to ask for the alignment of an incomplete type.
The keyword __attribute__ allows you to specify special
attributes of variables or structure fields. This keyword is followed
by an attribute specification inside double parentheses. Some
attributes are currently defined generically for variables.
Other attributes are defined for variables on particular target
systems. Other attributes are available for functions
(see Function Attributes) and for types (see Type Attributes).
Other front ends might define more attributes
(see Extensions to the C++ Language).
You may also specify attributes with __ preceding and following
each keyword. This allows you to use them in header files without
being concerned about a possible macro of the same name. For example,
you may use __aligned__ instead of aligned.
See Attribute Syntax, for details of the exact syntax for using attributes.
aligned (alignment) int x __attribute__ ((aligned (16))) = 0;
causes the compiler to allocate the global variable x on a
16-byte boundary. On a 68040, this could be used in conjunction with
an asm expression to access the move16 instruction which
requires 16-byte aligned operands.
You can also specify the alignment of structure fields. For example, to
create a double-word aligned int pair, you could write:
struct foo { int x[2] __attribute__ ((aligned (8))); };
This is an alternative to creating a union with a double member
that forces the union to be double-word aligned.
As in the preceding examples, you can explicitly specify the alignment (in bytes) that you wish the compiler to use for a given variable or structure field. Alternatively, you can leave out the alignment factor and just ask the compiler to align a variable or field to the maximum useful alignment for the target machine you are compiling for. For example, you could write:
short array[3] __attribute__ ((aligned));
Whenever you leave out the alignment factor in an aligned attribute
specification, the compiler automatically sets the alignment for the declared
variable or field to the largest alignment which is ever used for any data
type on the target machine you are compiling for. Doing this can often make
copy operations more efficient, because the compiler can use whatever
instructions copy the biggest chunks of memory when performing copies to
or from the variables or fields that you have aligned this way.
The aligned attribute can only increase the alignment; but you
can decrease it by specifying packed as well. See below.
Note that the effectiveness of aligned attributes may be limited
by inherent limitations in your linker. On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment. (For some linkers, the maximum supported alignment may
be very very small.) If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying aligned(16)
in an __attribute__ will still only provide you with 8 byte
alignment. See your linker documentation for further information.
cleanup (cleanup_function)cleanup attribute runs a function when the variable goes
out of scope. This attribute can only be applied to auto function
scope variables; it may not be applied to parameters or variables
with static storage duration. The function must take one parameter,
a pointer to a type compatible with the variable. The return value
of the function (if any) is ignored.
If -fexceptions is enabled, then cleanup_function
will be run during the stack unwinding that happens during the
processing of the exception. Note that the cleanup attribute
does not allow the exception to be caught, only to perform an action.
It is undefined what happens if cleanup_function does not
return normally.
commonnocommoncommon attribute requests GCC to place a variable in
“common” storage. The nocommon attribute requests the
opposite – to allocate space for it directly.
These attributes override the default chosen by the
-fno-common and -fcommon flags respectively.
deprecateddeprecated attribute results in a warning if the variable
is used anywhere in the source file. This is useful when identifying
variables that are expected to be removed in a future version of a
program. The warning also includes the location of the declaration
of the deprecated variable, to enable users to easily find further
information about why the variable is deprecated, or what they should
do instead. Note that the warning only occurs for uses:
extern int old_var __attribute__ ((deprecated));
extern int old_var;
int new_fn () { return old_var; }
results in a warning on line 3 but not line 2.
The deprecated attribute can also be used for functions and
types (see Function Attributes, see Type Attributes.)
mode (mode)You may also specify a mode of byte or __byte__ to
indicate the mode corresponding to a one-byte integer, word or
__word__ for the mode of a one-word integer, and pointer
or __pointer__ for the mode used to represent pointers.
packedpacked attribute specifies that a variable or structure field
should have the smallest possible alignment—one byte for a variable,
and one bit for a field, unless you specify a larger value with the
aligned attribute.
Here is a structure in which the field x is packed, so that it
immediately follows a:
struct foo
{
char a;
int x[2] __attribute__ ((packed));
};
section ("section-name")data and bss. Sometimes, however, you need additional sections,
or you need certain particular variables to appear in special sections,
for example to map to special hardware. The section
attribute specifies that a variable (or function) lives in a particular
section. For example, this small program uses several specific section names:
struct duart a __attribute__ ((section ("DUART_A"))) = { 0 };
struct duart b __attribute__ ((section ("DUART_B"))) = { 0 };
char stack[10000] __attribute__ ((section ("STACK"))) = { 0 };
int init_data __attribute__ ((section ("INITDATA"))) = 0;
main()
{
/* Initialize stack pointer */
init_sp (stack + sizeof (stack));
/* Initialize initialized data */
memcpy (&init_data, &data, &edata - &data);
/* Turn on the serial ports */
init_duart (&a);
init_duart (&b);
}
Use the section attribute with an initialized definition
of a global variable, as shown in the example. GCC issues
a warning and otherwise ignores the section attribute in
uninitialized variable declarations.
You may only use the section attribute with a fully initialized
global definition because of the way linkers work. The linker requires
each object be defined once, with the exception that uninitialized
variables tentatively go in the common (or bss) section
and can be multiply “defined”. You can force a variable to be
initialized with the -fno-common flag or the nocommon
attribute.
Some file formats do not support arbitrary sections so the section
attribute is not available on all platforms.
If you need to map the entire contents of a module to a particular
section, consider using the facilities of the linker instead.
sharedshared and marking the section
shareable:
int foo __attribute__((section ("shared"), shared)) = 0;
int
main()
{
/* Read and write foo. All running
copies see the same value. */
return 0;
}
You may only use the shared attribute along with section
attribute with a fully initialized global definition because of the way
linkers work. See section attribute for more information.
The shared attribute is only available on Windows.
tls_model ("tls_model")tls_model attribute sets thread-local storage model
(see Thread-Local) of a particular __thread variable,
overriding -ftls-model= command line switch on a per-variable
basis.
The tls_model argument should be one of global-dynamic,
local-dynamic, initial-exec or local-exec.
Not all targets support this attribute.
transparent_uniontypedef for a union data type; then it
applies to all function parameters with that type.
unusedvector_size (bytes) int foo __attribute__ ((vector_size (16)));
causes the compiler to set the mode for foo, to be 16 bytes,
divided into int sized units. Assuming a 32-bit int (a vector of
4 units of 4 bytes), the corresponding mode of foo will be V4SI.
This attribute is only applicable to integral and float scalars, although arrays, pointers, and function return values are allowed in conjunction with this construct.
Aggregates with this attribute are invalid, even if they are of the same size as a corresponding scalar. For example, the declaration:
struct S { int a; };
struct S __attribute__ ((vector_size (16))) foo;
is invalid even if the size of the structure is the same as the size of
the int.
weakweak attribute is described in See Function Attributes.
model (model-name)small, medium,
or large, representing each of the code models.
Small model objects live in the lower 16MB of memory (so that their
addresses can be loaded with the ld24 instruction).
Medium and large model objects may live anywhere in the 32-bit address space
(the compiler will generate seth/add3 instructions to load their
addresses).
dllimportdllimport attribute is described in See Function Attributes.
dlexportdllexport attribute is described in See Function Attributes.
To specify multiple attributes, separate them by commas within the double parentheses: for example, __attribute__ ((aligned (16), packed)).
The keyword __attribute__ allows you to specify special
attributes of struct and union types when you define such
types. This keyword is followed by an attribute specification inside
double parentheses. Six attributes are currently defined for types:
aligned, packed, transparent_union, unused,
deprecated and may_alias. Other attributes are defined for
functions (see Function Attributes) and for variables
(see Variable Attributes).
You may also specify any one of these attributes with __
preceding and following its keyword. This allows you to use these
attributes in header files without being concerned about a possible
macro of the same name. For example, you may use __aligned__
instead of aligned.
You may specify the aligned and transparent_union
attributes either in a typedef declaration or just past the
closing curly brace of a complete enum, struct or union type
definition and the packed attribute only past the closing
brace of a definition.
You may also specify attributes between the enum, struct or union tag and the name of the type rather than after the closing brace.
See Attribute Syntax, for details of the exact syntax for using attributes.
aligned (alignment) struct S { short f[3]; } __attribute__ ((aligned (8)));
typedef int more_aligned_int __attribute__ ((aligned (8)));
force the compiler to insure (as far as it can) that each variable whose
type is struct S or more_aligned_int will be allocated and
aligned at least on a 8-byte boundary. On a SPARC, having all
variables of type struct S aligned to 8-byte boundaries allows
the compiler to use the ldd and std (doubleword load and
store) instructions when copying one variable of type struct S to
another, thus improving run-time efficiency.
Note that the alignment of any given struct or union type
is required by the ISO C standard to be at least a perfect multiple of
the lowest common multiple of the alignments of all of the members of
the struct or union in question. This means that you can
effectively adjust the alignment of a struct or union
type by attaching an aligned attribute to any one of the members
of such a type, but the notation illustrated in the example above is a
more obvious, intuitive, and readable way to request the compiler to
adjust the alignment of an entire struct or union type.
As in the preceding example, you can explicitly specify the alignment
(in bytes) that you wish the compiler to use for a given struct
or union type. Alternatively, you can leave out the alignment factor
and just ask the compiler to align a type to the maximum
useful alignment for the target machine you are compiling for. For
example, you could write:
struct S { short f[3]; } __attribute__ ((aligned));
Whenever you leave out the alignment factor in an aligned
attribute specification, the compiler automatically sets the alignment
for the type to the largest alignment which is ever used for any data
type on the target machine you are compiling for. Doing this can often
make copy operations more efficient, because the compiler can use
whatever instructions copy the biggest chunks of memory when performing
copies to or from the variables which have types that you have aligned
this way.
In the example above, if the size of each short is 2 bytes, then
the size of the entire struct S type is 6 bytes. The smallest
power of two which is greater than or equal to that is 8, so the
compiler sets the alignment for the entire struct S type to 8
bytes.
Note that although you can ask the compiler to select a time-efficient alignment for a given type and then declare only individual stand-alone objects of that type, the compiler's ability to select a time-efficient alignment is primarily useful only when you plan to create arrays of variables having the relevant (efficiently aligned) type. If you declare or use arrays of variables of an efficiently-aligned type, then it is likely that your program will also be doing pointer arithmetic (or subscripting, which amounts to the same thing) on pointers to the relevant type, and the code that the compiler generates for these pointer arithmetic operations will often be more efficient for efficiently-aligned types than for other types.
The aligned attribute can only increase the alignment; but you
can decrease it by specifying packed as well. See below.
Note that the effectiveness of aligned attributes may be limited
by inherent limitations in your linker. On many systems, the linker is
only able to arrange for variables to be aligned up to a certain maximum
alignment. (For some linkers, the maximum supported alignment may
be very very small.) If your linker is only able to align variables
up to a maximum of 8 byte alignment, then specifying aligned(16)
in an __attribute__ will still only provide you with 8 byte
alignment. See your linker documentation for further information.
packedenum, struct, or
union type definition, specifies that the minimum required memory
be used to represent the type.
Specifying this attribute for struct and union types is
equivalent to specifying the packed attribute on each of the
structure or union members. Specifying the -fshort-enums
flag on the line is equivalent to specifying the packed
attribute on all enum definitions.
You may only specify this attribute after a closing curly brace on an
enum definition, not in a typedef declaration, unless that
declaration also contains the definition of the enum.
transparent_unionunion type definition, indicates
that any function parameter having that union type causes calls to that
function to be treated in a special way.
First, the argument corresponding to a transparent union type can be of
any type in the union; no cast is required. Also, if the union contains
a pointer type, the corresponding argument can be a null pointer
constant or a void pointer expression; and if the union contains a void
pointer type, the corresponding argument can be any pointer expression.
If the union member type is a pointer, qualifiers like const on
the referenced type must be respected, just as with normal pointer
conversions.
Second, the argument is passed to the function using the calling conventions of the first member of the transparent union, not the calling conventions of the union itself. All members of the union must have the same machine representation; this is necessary for this argument passing to work properly.
Transparent unions are designed for library functions that have multiple
interfaces for compatibility reasons. For example, suppose the
wait function must accept either a value of type int * to
comply with Posix, or a value of type union wait * to comply with
the 4.1BSD interface. If wait's parameter were void *,
wait would accept both kinds of arguments, but it would also
accept any other pointer type and this would make argument type checking
less useful. Instead, <sys/wait.h> might define the interface
as follows:
typedef union
{
int *__ip;
union wait *__up;
} wait_status_ptr_t __attribute__ ((__transparent_union__));
pid_t wait (wait_status_ptr_t);
This interface allows either int * or union wait *
arguments to be passed, using the int * calling convention.
The program can call wait with arguments of either type:
int w1 () { int w; return wait (&w); }
int w2 () { union wait w; return wait (&w); }
With this interface, wait's implementation might look like this:
pid_t wait (wait_status_ptr_t p)
{
return waitpid (-1, p.__ip, 0);
}
unusedunion or a struct),
this attribute means that variables of that type are meant to appear
possibly unused. GCC will not produce a warning for any variables of
that type, even if the variable appears to do nothing. This is often
the case with lock or thread classes, which are usually defined and then
not referenced, but contain constructors and destructors that have
nontrivial bookkeeping functions.
deprecateddeprecated attribute results in a warning if the type
is used anywhere in the source file. This is useful when identifying
types that are expected to be removed in a future version of a program.
If possible, the warning also includes the location of the declaration
of the deprecated type, to enable users to easily find further
information about why the type is deprecated, or what they should do
instead. Note that the warnings only occur for uses and then only
if the type is being applied to an identifier that itself is not being
declared as deprecated.
typedef int T1 __attribute__ ((deprecated));
T1 x;
typedef T1 T2;
T2 y;
typedef T1 T3 __attribute__ ((deprecated));
T3 z __attribute__ ((deprecated));
results in a warning on line 2 and 3 but not lines 4, 5, or 6. No warning is issued for line 4 because T2 is not explicitly deprecated. Line 5 has no warning because T3 is explicitly deprecated. Similarly for line 6.
The deprecated attribute can also be used for functions and
variables (see Function Attributes, see Variable Attributes.)
may_aliaschar type. See
-fstrict-aliasing for more information on aliasing issues.
Example of use:
typedef short __attribute__((__may_alias__)) short_a;
int
main (void)
{
int a = 0x12345678;
short_a *b = (short_a *) &a;
b[1] = 0;
if (a == 0x12345678)
abort();
exit(0);
}
If you replaced short_a with short in the variable
declaration, the above program would abort when compiled with
-fstrict-aliasing, which is on by default at -O2 or
above in recent GCC versions.
To specify multiple attributes, separate them by commas within the double parentheses: for example, __attribute__ ((aligned (16), packed)).
By declaring a function inline, you can direct GCC to
integrate that function's code into the code for its callers. This
makes execution faster by eliminating the function-call overhead; in
addition, if any of the actual argument values are constant, their known
values may permit simplifications at compile time so that not all of the
inline function's code needs to be included. The effect on code size is
less predictable; object code may be larger or smaller with function
inlining, depending on the particular case. Inlining of functions is an
optimization and it really “works” only in optimizing compilation. If
you don't use -O, no function is really inline.
Inline functions are included in the ISO C99 standard, but there are currently substantial differences between what GCC implements and what the ISO C99 standard requires.
To declare a function inline, use the inline keyword in its
declaration, like this:
inline int
inc (int *a)
{
(*a)++;
}
(If you are writing a header file to be included in ISO C programs, write
__inline__ instead of inline. See Alternate Keywords.)
You can also make all “simple enough” functions inline with the option
-finline-functions.
Note that certain usages in a function definition can make it unsuitable
for inline substitution. Among these usages are: use of varargs, use of
alloca, use of variable sized data types (see Variable Length),
use of computed goto (see Labels as Values), use of nonlocal goto,
and nested functions (see Nested Functions). Using -Winline
will warn when a function marked inline could not be substituted,
and will give the reason for the failure.
Note that in C and Objective-C, unlike C++, the inline keyword
does not affect the linkage of the function.
GCC automatically inlines member functions defined within the class
body of C++ programs even if they are not explicitly declared
inline. (You can override this with -fno-default-inline;
see Options Controlling C++ Dialect.)
When a function is both inline and static, if all calls to the
function are integrated into the caller, and the function's address is
never used, then the function's own assembler code is never referenced.
In this case, GCC does not actually output assembler code for the
function, unless you specify the option -fkeep-inline-functions.
Some calls cannot be integrated for various reasons (in particular,
calls that precede the function's definition cannot be integrated, and
neither can recursive calls within the definition). If there is a
nonintegrated call, then the function is compiled to assembler code as
usual. The function must also be compiled as usual if the program
refers to its address, because that can't be inlined.
When an inline function is not static, then the compiler must assume
that there may be calls from other source files; since a global symbol can
be defined only once in any program, the function must not be defined in
the other source files, so the calls therein cannot be integrated.
Therefore, a non-static inline function is always compiled on its
own in the usual fashion.
If you specify both inline and extern in the function
definition, then the definition is used only for inlining. In no case
is the function compiled on its own, not even if you refer to its
address explicitly. Such an address becomes an external reference, as
if you had only declared the function, and had not defined it.
This combination of inline and extern has almost the
effect of a macro. The way to use it is to put a function definition in
a header file with these keywords, and put another copy of the
definition (lacking inline and extern) in a library file.
The definition in the header file will cause most calls to the function
to be inlined. If any uses of the function remain, they will refer to
the single copy in the library.
Since GCC eventually will implement ISO C99 semantics for
inline functions, it is best to use static inline only
to guarantee compatibility. (The
existing semantics will remain available when -std=gnu89 is
specified, but eventually the default will be -std=gnu99 and
that will implement the C99 semantics, though it does not do so yet.)
GCC does not inline any functions when not optimizing unless you specify the always_inline attribute for the function, like this:
/* Prototype. */
inline void foo (const char) __attribute__((always_inline));
In an assembler instruction using asm, you can specify the
operands of the instruction using C expressions. This means you need not
guess which registers or memory locations will contain the data you want
to use.
You must specify an assembler instruction template much like what appears in a machine description, plus an operand constraint string for each operand.
For example, here is how to use the 68881's fsinx instruction:
asm ("fsinx %1,%0" : "=f" (result) : "f" (angle));
Here angle is the C expression for the input operand while
result is that of the output operand. Each has "f" as its
operand constraint, saying that a floating point register is required.
The = in =f indicates that the operand is an output; all
output operands' constraints must use =. The constraints use the
same language used in the machine description (see Constraints).
Each operand is described by an operand-constraint string followed by the C expression in parentheses. A colon separates the assembler template from the first output operand and another separates the last output operand from the first input, if any. Commas separate the operands within each group. The total number of operands is currently limited to 30; this limitation may be lifted in some future version of GCC.
If there are no output operands but there are input operands, you must place two consecutive colons surrounding the place where the output operands would go.
As of GCC version 3.1, it is also possible to specify input and output
operands using symbolic names which can be referenced within the
assembler code. These names are specified inside square brackets
preceding the constraint string, and can be referenced inside the
assembler code using %[name] instead of a percentage sign
followed by the operand number. Using named operands the above example
could look like:
asm ("fsinx %[angle],%[output]"
: [output] "=f" (result)
: [angle] "f" (angle));
Note that the symbolic operand names have no relation whatsoever to other C identifiers. You may use any name you like, even those of existing C symbols, but you must ensure that no two operands within the same assembler construct use the same symbolic name.
Output operand expressions must be lvalues; the compiler can check this.
The input operands need not be lvalues. The compiler cannot check
whether the operands have data types that are reasonable for the
instruction being executed. It does not parse the assembler instruction
template and does not know what it means or even whether it is valid
assembler input. The extended asm feature is most often used for
machine instructions the compiler itself does not know exist. If
the output expression cannot be directly addressed (for example, it is a
bit-field), your constraint must allow a register. In that case, GCC
will use the register as the output of the asm, and then store
that register into the output.
The ordinary output operands must be write-only; GCC will assume that the values in these operands before the instruction are dead and need not be generated. Extended asm supports input-output or read-write operands. Use the constraint character + to indicate such an operand and list it with the output operands.
When the constraints for the read-write operand (or the operand in which
only some of the bits are to be changed) allows a register, you may, as
an alternative, logically split its function into two separate operands,
one input operand and one write-only output operand. The connection
between them is expressed by constraints which say they need to be in
the same location when the instruction executes. You can use the same C
expression for both operands, or different expressions. For example,
here we write the (fictitious) combine instruction with
bar as its read-only source operand and foo as its
read-write destination:
asm ("combine %2,%0" : "=r" (foo) : "0" (foo), "g" (bar));
The constraint "0" for operand 1 says that it must occupy the same location as operand 0. A number in constraint is allowed only in an input operand and it must refer to an output operand.
Only a number in the constraint can guarantee that one operand will be in
the same place as another. The mere fact that foo is the value
of both operands is not enough to guarantee that they will be in the
same place in the generated assembler code. The following would not
work reliably:
asm ("combine %2,%0" : "=r" (foo) : "r" (foo), "g" (bar));
Various optimizations or reloading could cause operands 0 and 1 to be in
different registers; GCC knows no reason not to do so. For example, the
compiler might find a copy of the value of foo in one register and
use it for operand 1, but generate the output operand 0 in a different
register (copying it afterward to foo's own address). Of course,
since the register for operand 1 is not even mentioned in the assembler
code, the result will not work, but GCC can't tell that.
As of GCC version 3.1, one may write [name] instead of
the operand number for a matching constraint. For example:
asm ("cmoveq %1,%2,%[result]"
: [result] "=r"(result)
: "r" (test), "r"(new), "[result]"(old));
Some instructions clobber specific hard registers. To describe this, write a third colon after the input operands, followed by the names of the clobbered hard registers (given as strings). Here is a realistic example for the VAX:
asm volatile ("movc3 %0,%1,%2"
: /* no outputs */
: "g" (from), "g" (to), "g" (count)
: "r0", "r1", "r2", "r3", "r4", "r5");
You may not write a clobber description in a way that overlaps with an
input or output operand. For example, you may not have an operand
describing a register class with one member if you mention that register
in the clobber list. Variables declared to live in specific registers
(see Explicit Reg Vars), and used as asm input or output operands must
have no part mentioned in the clobber description.
There is no way for you to specify that an input
operand is modified without also specifying it as an output
operand. Note that if all the output operands you specify are for this
purpose (and hence unused), you will then also need to specify
volatile for the asm construct, as described below, to
prevent GCC from deleting the asm statement as unused.
If you refer to a particular hardware register from the assembler code, you will probably have to list the register after the third colon to tell the compiler the register's value is modified. In some assemblers, the register names begin with %; to produce one % in the assembler code, you must write %% in the input.
If your assembler instruction can alter the condition code register, add cc to the list of clobbered registers. GCC on some machines represents the condition codes as a specific hardware register; cc serves to name this register. On other machines, the condition code is handled differently, and specifying cc has no effect. But it is valid no matter what the machine.
If your assembler instructions access memory in an unpredictable
fashion, add memory to the list of clobbered registers. This
will cause GCC to not keep memory values cached in registers across the
assembler instruction and not optimize stores or loads to that memory.
You will also want to add the volatile keyword if the memory
affected is not listed in the inputs or outputs of the asm, as
the memory clobber does not count as a side-effect of the
asm. If you know how large the accessed memory is, you can add
it as input or output but if this is not known, you should add
memory. As an example, if you access ten bytes of a string, you
can use a memory input like:
{"m"( ({ struct { char x[10]; } *p = (void *)ptr ; *p; }) )}.
Note that in the following example the memory input is necessary,
otherwise GCC might optimize the store to x away:
int foo ()
{
int x = 42;
int *y = &x;
int result;
asm ("magic stuff accessing an 'int' pointed to by '%1'"
"=&d" (r) : "a" (y), "m" (*y));
return result;
}
You can put multiple assembler instructions together in a single
asm template, separated by the characters normally used in assembly
code for the system. A combination that works in most places is a newline
to break the line, plus a tab character to move to the instruction field
(written as \n\t). Sometimes semicolons can be used, if the
assembler allows semicolons as a line-breaking character. Note that some
assembler dialects use semicolons to start a comment.
The input operands are guaranteed not to use any of the clobbered
registers, and neither will the output operands' addresses, so you can
read and write the clobbered registers as many times as you like. Here
is an example of multiple instructions in a template; it assumes the
subroutine _foo accepts arguments in registers 9 and 10:
asm ("movl %0,r9\n\tmovl %1,r10\n\tcall _foo"
: /* no outputs */
: "g" (from), "g" (to)
: "r9", "r10");
Unless an output operand has the & constraint modifier, GCC may allocate it in the same register as an unrelated input operand, on the assumption the inputs are consumed before the outputs are produced. This assumption may be false if the assembler code actually consists of more than one instruction. In such a case, use & for each output operand that may not overlap an input. See Modifiers.
If you want to test the condition code produced by an assembler
instruction, you must include a branch and a label in the asm
construct, as follows:
asm ("clr %0\n\tfrob %1\n\tbeq 0f\n\tmov #1,%0\n0:"
: "g" (result)
: "g" (input));
This assumes your assembler supports local labels, as the GNU assembler and most Unix assemblers do.
Speaking of labels, jumps from one asm to another are not
supported. The compiler's optimizers do not know about these jumps, and
therefore they cannot take account of them when deciding how to
optimize.
Usually the most convenient way to use these asm instructions is to
encapsulate them in macros that look like functions. For example,
#define sin(x) \
({ double __value, __arg = (x); \
asm ("fsinx %1,%0": "=f" (__value): "f" (__arg)); \
__value; })
Here the variable __arg is used to make sure that the instruction
operates on a proper double value, and to accept only those
arguments x which can convert automatically to a double.
Another way to make sure the instruction operates on the correct data
type is to use a cast in the asm. This is different from using a
variable __arg in that it converts more different types. For
example, if the desired type were int, casting the argument to
int would accept a pointer with no complaint, while assigning the
argument to an int variable named __arg would warn about
using a pointer unless the caller explicitly casts it.
If an asm has output operands, GCC assumes for optimization
purposes the instruction has no side effects except to change the output
operands. This does not mean instructions with a side effect cannot be
used, but you must be careful, because the compiler may eliminate them
if the output operands aren't used, or move them out of loops, or
replace two with one if they constitute a common subexpression. Also,
if your instruction does have a side effect on a variable that otherwise
appears not to change, the old value of the variable may be reused later
if it happens to be found in a register.
You can prevent an asm instruction from being deleted, moved
significantly, or combined, by writing the keyword volatile after
the asm. For example:
#define get_and_set_priority(new) \
({ int __old; \
asm volatile ("get_and_set_priority %0, %1" \
: "=g" (__old) : "g" (new)); \
__old; })
If you write an asm instruction with no outputs, GCC will know
the instruction has side-effects and will not delete the instruction or
move it outside of loops.
The volatile keyword indicates that the instruction has
important side-effects. GCC will not delete a volatile asm if
it is reachable. (The instruction can still be deleted if GCC can
prove that control-flow will never reach the location of the
instruction.) In addition, GCC will not reschedule instructions
across a volatile asm instruction. For example:
*(volatile int *)addr = foo;
asm volatile ("eieio" : : );
Assume addr contains the address of a memory mapped device
register. The PowerPC eieio instruction (Enforce In-order
Execution of I/O) tells the CPU to make sure that the store to that
device register happens before it issues any other I/O.
Note that even a volatile asm instruction can be moved in ways
that appear insignificant to the compiler, such as across jump
instructions. You can't expect a sequence of volatile asm
instructions to remain perfectly consecutive. If you want consecutive
output, use a single asm. Also, GCC will perform some
optimizations across a volatile asm instruction; GCC does not
“forget everything” when it encounters a volatile asm
instruction the way some other compilers do.
An asm instruction without any operands or clobbers (an “old
style” asm) will be treated identically to a volatile
asm instruction.
It is a natural idea to look for a way to give access to the condition code left by the assembler instruction. However, when we attempted to implement this, we found no way to make it work reliably. The problem is that output operands might need reloading, which would result in additional following “store” instructions. On most machines, these instructions would alter the condition code before there was time to test it. This problem doesn't arise for ordinary “test” and “compare” instructions because they don't have any output operands.
For reasons similar to those described above, it is not possible to give an assembler instruction access to the condition code left by previous instructions.
If you are writing a header file that should be includable in ISO C
programs, write __asm__ instead of asm. See Alternate Keywords.
There are several rules on the usage of stack-like regs in asm_operands insns. These rules apply only to the operands that are stack-like regs:
An input reg that is implicitly popped by the asm must be explicitly clobbered, unless it is constrained to match an output operand.
All implicitly popped input regs must be closer to the top of the reg-stack than any input that is not implicitly popped.
It is possible that if an input dies in an insn, reload might use the input reg for an output reload. Consider this example:
asm ("foo" : "=t" (a) : "f" (b));
This asm says that input B is not popped by the asm, and that the asm pushes a result onto the reg-stack, i.e., the stack is one deeper after the asm than it was before. But, it is possible that reload will think that it can use the same reg for both the input and the output, if input B dies in this insn.
If any input operand uses the f constraint, all output reg
constraints must use the & earlyclobber.
The asm above would be written as
asm ("foo" : "=&t" (a) : "f" (b));
Output operands must specifically indicate which reg an output
appears in after an asm. =f is not allowed: the operand
constraints must select a class with a single reg.
Output operands must start at the top of the reg-stack: output operands may not “skip” a reg.
Here are a couple of reasonable asms to want to write. This asm takes one input, which is internally popped, and produces two outputs.
asm ("fsincos" : "=t" (cos), "=u" (sin) : "0" (inp));
This asm takes two inputs, which are popped by the fyl2xp1 opcode,
and replaces them with one output. The user must code the st(1)
clobber for reg-stack.c to know that fyl2xp1 pops both inputs.
asm ("fyl2xp1" : "=t" (result) : "0" (x), "u" (y) : "st(1)");
asm Operands
Here are specific details on what constraint letters you can use with
asm operands.
Constraints can say whether
an operand may be in a register, and which kinds of register; whether the
operand can be a memory reference, and which kinds of address; whether the
operand may be an immediate constant, and which possible values it may
have. Constraints can also require two operands to match.
The simplest kind of constraint is a string full of letters, each of which describes one kind of operand that is permitted. Here are the letters that are allowed:
For example, an address which is constant is offsettable; so is an address that is the sum of a register and a constant (as long as a slightly larger constant is also within the range of address-offsets supported by the machine); but an autoincrement or autodecrement address is not offsettable. More complicated indirect/indexed addresses may or may not be offsettable depending on the other addressing modes that the machine supports.
Note that in an output operand which can be matched by another operand, the constraint letter o is valid only when accompanied by both < (if the target machine has predecrement addressing) and > (if the target machine has preincrement addressing).
const_double) is
allowed, but only if the target floating point format is the same as
that of the host machine (on which the compiler is running).
const_double or
const_vector) is allowed.
This might appear strange; if an insn allows a constant operand with a value not known at compile time, it certainly must allow any known value. So why use s instead of i? Sometimes it allows better code to be generated.
For example, on the 68000 in a fullword instruction it is possible to use an immediate operand; but if the immediate value is between −128 and 127, better code results from loading the value into a register and using the register. This is because the load into the register can be done with a moveq instruction. We arrange for this to happen by defining the letter K to mean “any integer outside the range −128 to 127”, and then specifying Ks in the operand constraints.
This number is allowed to be more than a single digit. If multiple digits are encountered consecutively, they are interpreted as a single decimal integer. There is scant chance for ambiguity, since to-date it has never been desirable that 10 be interpreted as matching either operand 1 or operand 0. Should this be desired, one can use multiple alternatives instead.
This is called a matching constraint and what it really means is
that the assembler has only a single operand that fills two roles
which asm distinguishes. For example, an add instruction uses
two input operands and an output operand, but on most CISC
machines an add instruction really has only two operands, one of them an
input-output operand:
addl #35,r12
Matching constraints are used in these circumstances. More precisely, the two operands that match must include one input-only operand and one output-only operand. Moreover, the digit must be a smaller number than the number of the operand that uses it in the constraint.
p in the constraint must be accompanied by address_operand
as the predicate in the match_operand. This predicate interprets
the mode specified in the match_operand as the mode of the memory
reference for which the address would be valid.
Sometimes a single instruction has multiple alternative sets of possible operands. For example, on the 68000, a logical-or instruction can combine register or an immediate value into memory, or it can combine any kind of operand into a register; but it cannot combine one memory location into another.
These constraints are represented as multiple alternatives. An alternative can be described by a series of letters for each operand. The overall constraint for an operand is made from the letters for this operand from the first alternative, a comma, the letters for this operand from the second alternative, a comma, and so on until the last alternative.
If all the operands fit any one alternative, the instruction is valid. Otherwise, for each alternative, the compiler counts how many instructions must be added to copy the operands so that that alternative applies. The alternative requiring the least copying is chosen. If two alternatives need the same amount of copying, the one that comes first is chosen. These choices can be altered with the ? and ! characters:
?!Here are constraint modifier characters.
When the compiler fixes up the operands to satisfy the constraints, it needs to know which operands are inputs to the instruction and which are outputs from it. = identifies an output; + identifies an operand that is both input and output; all other operands are assumed to be input only.
If you specify = or + in a constraint, you put it in the first character of the constraint string.
& applies only to the alternative in which it is written. In constraints with multiple alternatives, sometimes one alternative requires & while others do not. See, for example, the movdf insn of the 68000.
An input operand can be tied to an earlyclobber operand if its only use as an input occurs before the early result is written. Adding alternatives of this form often allows GCC to produce better code when only some of the inputs can be affected by the earlyclobber. See, for example, the mulsi3 insn of the ARM.
& does not obviate the need to write =.
Whenever possible, you should use the general-purpose constraint letters
in asm arguments, since they will convey meaning more readily to
people reading your code. Failing that, use the constraint letters
that usually have very similar meanings across architectures. The most
commonly used constraints are m and r (for memory and
general-purpose registers respectively; see Simple Constraints), and
I, usually the letter indicating the most common
immediate-constant format.
For each machine architecture, the
config/machine/machine.h file defines additional
constraints. These constraints are used by the compiler itself for
instruction generation, as well as for asm statements; therefore,
some of the constraints are not particularly interesting for asm.
The constraints are defined through these macros:
REG_CLASS_FROM_LETTERCONST_OK_FOR_LETTER_PCONST_DOUBLE_OK_FOR_LETTER_PEXTRA_CONSTRAINTInspecting these macro definitions in the compiler source for your machine is the best way to be certain you have the right constraints. However, here is a summary of the machine-dependent constraints available on some particular machines.
fFGIJKLMQasm statements)
RSladwebqtxyzIJKLMNOPGbfhqclxyzIJSImode constants)
KLMNOPGQasm statements)
RSTUqb, c, or d register for the i386.
For x86-64 it is equivalent to r class. (for 8-bit instructions that
do not use upper halves)
Qb, c, or d register. (for 8-bit instructions,
that do use upper halves)
Rr class in i386 mode.
(for non-8-bit registers used together with 8-bit upper halves in a single
instruction)
AftuabcCdDSxyIJKLMlea instruction)
Nout instruction)
Z0xffffffff or symbolic reference known to fit specified range.
(for using immediates in zero extending 32-bit to 64-bit x86-64 instructions)
eGffp0 to fp3)
lr0 to r15)
bg0 to g15)
dIJKGHar0 to r3 for addl instruction
bcdefmGIJKLMNOPdep instruction
QRshladd instruction
SaACC_REGS (acc0 to acc7).
bEVEN_ACC_REGS (acc0 to acc7).
cCC_REGS (fcc0 to fcc3 and
icc0 to icc3).
dGPR_REGS (gr0 to gr63).
eEVEN_REGS (gr0 to gr63).
Odd registers are excluded not in the class but through the use of a machine
mode larger than 4 bytes.
fFPR_REGS (fr0 to fr63).
hFEVEN_REGS (fr0 to fr63).
Odd registers are excluded not in the class but through the use of a machine
mode larger than 4 bytes.
lLR_REG (the lr register).
qQUAD_REGS (gr2 to gr63).
Register numbers not divisible by 4 are excluded not in the class but through
the use of a machine mode larger than 8 bytes.
tICC_REGS (icc0 to icc3).
uFCC_REGS (fcc0 to fcc3).
vICR_REGS (cc4 to cc7).
wFCR_REGS (cc0 to cc3).
xQUAD_FPR_REGS (fr0 to fr63).
Register numbers not divisible by 4 are excluded not in the class but through
the use of a machine mode larger than 8 bytes.
zSPR_REGS (lcr and lr).
AQUAD_ACC_REGS (acc0 to acc7).
BACCG_REGS (accg0 to accg7).
CCR_REGS (cc0 to cc7).
GIJLMNOPafjkbyzqcduRQImode, since we
can't access extra bytes
STIJKLMNOPdfhlxyzIJKLlui)
MNOPGQasm statements)
Rasm statements)
Sasm statements)
adfxyIJKLMGHabdqtuwxyzABDLMNOPfecdbhIJKsethi instruction)
Lmovcc instructions
Mmovrcc instructions
NSImode
OGHQRSTUWabcfkqtuvxyzGHIJKLMNOQRSTUadfIJKLQSlarl instruction
abcdetyzIJKLMNOPQRSTUabAIJKL
You can specify the name to be used in the assembler code for a C
function or variable by writing the asm (or __asm__)
keyword after the declarator as follows:
int foo asm ("myfoo") = 2;
This specifies that the name to be used for the variable foo in
the assembler code should be myfoo rather than the usual
_foo.
On systems where an underscore is normally prepended to the name of a C function or variable, this feature allows you to define names for the linker that do not start with an underscore.
It does not make sense to use this feature with a non-static local variable since such variables do not have assembler names. If you are trying to put the variable in a particular register, see Explicit Reg Vars. GCC presently accepts such code with a warning, but will probably be changed to issue an error, rather than a warning, in the future.
You cannot use asm in this way in a function definition; but
you can get the same effect by writing a declaration for the function
before its definition and putting asm there, like this:
extern func () asm ("FUNC");
func (x, y)
int x, y;
/* ... */
It is up to you to make sure that the assembler names you choose do not conflict with any other assembler symbols. Also, you must not use a register name; that would produce completely invalid assembler code. GCC does not as yet have the ability to store static variables in registers. Perhaps that will be added.
GNU C allows you to put a few global variables into specified hardware registers. You can also specify the register in which an ordinary register variable should be allocated.
These local variables are sometimes convenient for use with the extended
asm feature (see Extended Asm), if you want to write one
output of the assembler instruction directly into a particular register.
(This will work provided the register you specify fits the constraints
specified for that operand in the asm.)
You can define a global register variable in GNU C like this:
register int *foo asm ("a5");
Here a5 is the name of the register which should be used. Choose a
register which is normally saved and restored by function calls on your
machine, so that library routines will not clobber it.
Naturally the register name is cpu-dependent, so you would need to
conditionalize your program according to cpu type. The register
a5 would be a good choice on a 68000 for a variable of pointer
type. On machines with register windows, be sure to choose a “global”
register that is not affected magically by the function call mechanism.
In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals. For
example, some 68000 operating systems call this register %a5.
Eventually there may be a way of asking the compiler to choose a register automatically, but first we need to figure out how it should choose and how to enable you to guide the choice. No solution is evident.
Defining a global register variable in a certain register reserves that register entirely for this use, at least within the current compilation. The register will not be allocated for any other purpose in the functions in the current compilation. The register will not be saved and restored by these functions. Stores into this register are never deleted even if they would appear to be dead, but references may be deleted or moved or simplified.
It is not safe to access the global register variables from signal handlers, or from more than one thread of control, because the system library routines may temporarily use the register for other things (unless you recompile them specially for the task at hand).
It is not safe for one function that uses a global register variable to
call another such function foo by way of a third function
lose that was compiled without knowledge of this variable (i.e. in a
different source file in which the variable wasn't declared). This is
because lose might save the register and put some other value there.
For example, you can't expect a global register variable to be available in
the comparison-function that you pass to qsort, since qsort
might have put something else in that register. (If you are prepared to
recompile qsort with the same global register variable, you can
solve this problem.)
If you want to recompile qsort or other source files which do not
actually use your global register variable, so that they will not use that
register for any other purpose, then it suffices to specify the compiler
option -ffixed-reg. You need not actually add a global
register declaration to their source code.
A function which can alter the value of a global register variable cannot safely be called from a function compiled without this variable, because it could clobber the value the caller expects to find there on return. Therefore, the function which is the entry point into the part of the program that uses the global register variable must explicitly save and restore the value which belongs to its caller.
On most machines, longjmp will restore to each global register
variable the value it had at the time of the setjmp. On some
machines, however, longjmp will not change the value of global
register variables. To be portable, the function that called setjmp
should make other arrangements to save the values of the global register
variables, and to restore them in a longjmp. This way, the same
thing will happen regardless of what longjmp does.
All global register variable declarations must precede all function definitions. If such a declaration could appear after function definitions, the declaration would be too late to prevent the register from being used for other purposes in the preceding functions.
Global register variables may not have initial values, because an executable file has no means to supply initial contents for a register.
On the SPARC, there are reports that g3 ... g7 are suitable
registers, but certain library functions, such as getwd, as well
as the subroutines for division and remainder, modify g3 and g4. g1 and
g2 are local temporaries.
On the 68000, a2 ... a5 should be suitable, as should d2 ... d7. Of course, it will not do to use more than a few of those.
You can define a local register variable with a specified register like this:
register int *foo asm ("a5");
Here a5 is the name of the register which should be used. Note
that this is the same syntax used for defining global register
variables, but for a local variable it would appear within a function.
Naturally the register name is cpu-dependent, but this is not a problem, since specific registers are most often useful with explicit assembler instructions (see Extended Asm). Both of these things generally require that you conditionalize your program according to cpu type.
In addition, operating systems on one type of cpu may differ in how they
name the registers; then you would need additional conditionals. For
example, some 68000 operating systems call this register %a5.
Defining such a register variable does not reserve the register; it remains available for other uses in places where flow control determines the variable's value is not live. However, these registers are made unavailable for use in the reload pass; excessive use of this feature leaves the compiler too few available registers to compile certain functions.
This option does not guarantee that GCC will generate code that has
this variable in the register you specify at all times. You may not
code an explicit reference to this register in an asm statement
and assume it will always refer to this variable.
Stores into local register variables may be deleted when they appear to be dead according to dataflow analysis. References to local register variables may be deleted or moved or simplified.
-ansi and the various -std options disable certain
keywords. This causes trouble when you want to use GNU C extensions, or
a general-purpose header file that should be usable by all programs,
including ISO C programs. The keywords asm, typeof and
inline are not available in programs compiled with
-ansi or -std (although inline can be used in a
program compiled with -std=c99). The ISO C99 keyword
restrict is only available when -std=gnu99 (which will
eventually be the default) or -std=c99 (or the equivalent
-std=iso9899:1999) is used.
The way to solve these problems is to put __ at the beginning and
end of each problematical keyword. For example, use __asm__
instead of asm, and __inline__ instead of inline.
Other C compilers won't accept these alternative keywords; if you want to compile with another compiler, you can define the alternate keywords as macros to replace them with the customary keywords. It looks like this:
#ifndef __GNUC__
#define __asm__ asm
#endif
-pedantic and other options cause warnings for many GNU C extensions.
You can
prevent such warnings within one expression by writing
__extension__ before the expression. __extension__ has no
effect aside from this.
enum TypesYou can define an enum tag without specifying its possible values.
This results in an incomplete type, much like what you get if you write
struct foo without describing the elements. A later declaration
which does specify the possible values completes the type.
You can't allocate variables or storage using the type while it is incomplete. However, you can work with pointers to that type.
This extension may not be very useful, but it makes the handling of
enum more consistent with the way struct and union
are handled.
This extension is not supported by GNU C++.
GCC predefines two magic identifiers to hold the name of the current
function. The identifier __FUNCTION__ holds the name of the function
as it appears in the source. The identifier __PRETTY_FUNCTION__
holds the name of the function pretty printed in a language specific
fashion.
These names are always the same in a C function, but in a C++ function they may be different. For example, this program:
extern "C" {
extern int printf (char *, ...);
}
class a {
public:
sub (int i)
{
printf ("__FUNCTION__ = %s\n", __FUNCTION__);
printf ("__PRETTY_FUNCTION__ = %s\n", __PRETTY_FUNCTION__);
}
};
int
main (void)
{
a ax;
ax.sub (0);
return 0;
}
gives this output:
__FUNCTION__ = sub
__PRETTY_FUNCTION__ = int a::sub (int)
The compiler automagically replaces the identifiers with a string
literal containing the appropriate name. Thus, they are neither
preprocessor macros, like __FILE__ and __LINE__, nor
variables. This means that they catenate with other string literals, and
that they can be used to initialize char arrays. For example
char here[] = "Function " __FUNCTION__ " in " __FILE__;
On the other hand, #ifdef __FUNCTION__ does not have any special
meaning inside a function, since the preprocessor does not do anything
special with the identifier __FUNCTION__.
Note that these semantics are deprecated, and that GCC 3.2 will handle
__FUNCTION__ and __PRETTY_FUNCTION__ the same way as
__func__. __func__ is defined by the ISO standard C99:
The identifier__func__is implicitly declared by the translator as if, immediately following the opening brace of each function definition, the declarationstatic const char __func__[] = "function-name";appeared, where function-name is the name of the lexically-enclosing function. This name is the unadorned name of the function.
By this definition, __func__ is a variable, not a string literal.
In particular, __func__ does not catenate with other string
literals.
In C++, __FUNCTION__ and __PRETTY_FUNCTION__ are
variables, declared in the same way as __func__.
These functions may be used to get information about the callers of a function.
This function returns the return address of the current function, or of one of its callers. The level argument is number of frames to scan up the call stack. A value of
0yields the return address of the current function, a value of1yields the return address of the caller of the current function, and so forth. When inlining the expected behavior is that the function will return the address of the function that will be returned to. To work around this behavior use thenoinlinefunction attribute.The level argument must be a constant integer.
On some machines it may be impossible to determine the return address of any function other than the current one; in such cases, or when the top of the stack has been reached, this function will return
0or a random value. In addition,__builtin_frame_addressmay be used to determine if the top of the stack has been reached.This function should only be used with a nonzero argument for debugging purposes.
This function is similar to
__builtin_return_address, but it returns the address of the function frame rather than the return address of the function. Calling__builtin_frame_addresswith a value of0yields the frame address of the current function, a value of1yields the frame address of the caller of the current function, and so forth.The frame is the area on the stack which holds local variables and saved registers. The frame address is normally the address of the first word pushed on to the stack by the function. However, the exact definition depends upon the processor and the calling convention. If the processor has a dedicated frame pointer register, and the function has a frame, then
__builtin_frame_addresswill return the value of the frame pointer register.On some machines it may be impossible to determine the frame address of any function other than the current one; in such cases, or when the top of the stack has been reached, this function will return
0if the first frame pointer is properly initialized by the startup code.This function should only be used with a nonzero argument for debugging purposes.
On some targets, the instruction set contains SIMD vector instructions that operate on multiple values contained in one large register at the same time. For example, on the i386 the MMX, 3Dnow! and SSE extensions can be used this way.
The first step in using these extensions is to provide the necessary data
types. This should be done using an appropriate typedef:
typedef int v4si __attribute__ ((mode(V4SI)));
The base type int is effectively ignored by the compiler, the
actual properties of the new type v4si are defined by the
__attribute__. It defines the machine mode to be used; for vector
types these have the form VnB; n should be the
number of elements in the vector, and B should be the base mode of the
individual elements. The following can be used as base modes:
QIHISIDISFDFSpecifying a combination that is not valid for the current architecture
will cause gcc to synthesize the instructions using a narrower mode.
For example, if you specify a variable of type V4SI and your
architecture does not allow for this specific SIMD type, gcc will
produce code that uses 4 SIs.
The types defined in this manner can be used with a subset of normal C
operations. Currently, gcc will allow using the following operators on
these types: +, -, *, /, unary minus.
The operations behave like C++ valarrays. Addition is defined as
the addition of the corresponding elements of the operands. For
example, in the code below, each of the 4 elements in a will be
added to the corresponding 4 elements in b and the resulting
vector will be stored in c.
typedef int v4si __attribute__ ((mode(V4SI)));
v4si a, b, c;
c = a + b;
Subtraction, multiplication, and division operate in a similar manner. Likewise, the result of using the unary minus operator on a vector type is a vector whose elements are the negative value of the corresponding elements in the operand.
You can declare variables and use them in function calls and returns, as well as in assignments and some casts. You can specify a vector type as a return type for a function. Vector types can also be used as function arguments. It is possible to cast from one vector type to another, provided they are of the same size (in fact, you can also cast vectors to and from other datatypes of the same size).
You cannot operate between vectors of different lengths or different signedness without a cast.
A port that supports hardware vector operations, usually provides a set of built-in functions that can be used to operate on vectors. For example, a function to add two vectors and multiply the result by a third could look like this:
v4si f (v4si a, v4si b, v4si c)
{
v4si tmp = __builtin_addv4si (a, b);
return __builtin_mulv4si (tmp, c);
}
GCC provides a large number of built-in functions other than the ones mentioned above. Some of these are for internal use in the processing of exceptions or variable-length argument lists and will not be documented here because they may change from time to time; we do not recommend general use of these functions.
The remaining functions are provided for optimization purposes.
GCC includes built-in versions of many of the functions in the standard
C library. The versions prefixed with __builtin_ will always be
treated as having the same meaning as the C library function even if you
specify the -fno-builtin option. (see C Dialect Options)
Many of these functions are only optimized in certain cases; if they are
not optimized in a particular case, a call to the library function will
be emitted.
The functions abort, exit, _Exit and _exit
are recognized and presumed not to return, but otherwise are not built
in. _exit is not recognized in strict ISO C mode (-ansi,
-std=c89 or -std=c99). _Exit is not recognized in
strict C89 mode (-ansi or -std=c89). All these functions
have corresponding versions prefixed with __builtin_, which may be
used even in strict C89 mode.
Outside strict ISO C mode, the functions alloca, bcmp,
bzero, index, rindex, ffs, fputs_unlocked,
printf_unlocked and fprintf_unlocked may be handled as
built-in functions. All these functions have corresponding versions
prefixed with __builtin_, which may be used even in strict C89
mode.
The ISO C99 functions conj, conjf, conjl,
creal, crealf, creall, cimag, cimagf,
cimagl, imaxabs, llabs, snprintf,
vscanf, vsnprintf and vsscanf are handled as built-in
functions except in strict ISO C90 mode. There are also built-in
versions of the ISO C99 functions cosf, cosl,
expf, expl, fabsf, fabsl,
logf, logl, sinf, sinl, sqrtf, and
sqrtl, that are recognized in any mode since ISO C90 reserves
these names for the purpose to which ISO C99 puts them. All these
functions have corresponding versions prefixed with __builtin_.
The ISO C90 functions abs, cos, exp, fabs,
fprintf, fputs, labs, log,
memcmp, memcpy,
memset, printf, putchar, puts, scanf,
sin, snprintf, sprintf, sqrt, sscanf,
strcat,
strchr, strcmp, strcpy, strcspn,
strlen, strncat, strncmp, strncpy,
strpbrk, strrchr, strspn, strstr,
vprintf and vsprintf are all
recognized as built-in functions unless -fno-builtin is
specified (or -fno-builtin-function is specified for an
individual function). All of these functions have corresponding
versions prefixed with __builtin_.
GCC provides built-in versions of the ISO C99 floating point comparison
macros that avoid raising exceptions for unordered operands. They have
the same names as the standard macros ( isgreater,
isgreaterequal, isless, islessequal,
islessgreater, and isunordered) , with __builtin_
prefixed. We intend for a library implementor to be able to simply
#define each standard macro to its built-in equivalent.
You can use the built-in function
__builtin_types_compatible_pto determine whether two types are the same.This built-in function returns 1 if the unqualified versions of the types type1 and type2 (which are types, not expressions) are compatible, 0 otherwise. The result of this built-in function can be used in integer constant expressions.
This built-in function ignores top level qualifiers (e.g.,
const,volatile). For example,intis equivalent toconst int.The type
int[]andint[5]are compatible. On the other hand,intandchar *are not compatible, even if the size of their types, on the particular architecture are the same. Also, the amount of pointer indirection is taken into account when determining similarity. Consequently,short *is not similar toshort **. Furthermore, two types that are typedefed are considered compatible if their underlying types are compatible.An
enumtype is considered to be compatible with anotherenumtype. For example,enum {foo, bar}is similar toenum {hot, dog}.You would typically use this function in code whose execution varies depending on the arguments' types. For example:
#define foo(x) \ ({ \ typeof (x) tmp; \ if (__builtin_types_compatible_p (typeof (x), long double)) \ tmp = foo_long_double (tmp); \ else if (__builtin_types_compatible_p (typeof (x), double)) \ tmp = foo_double (tmp); \ else if (__builtin_types_compatible_p (typeof (x), float)) \ tmp = foo_float (tmp); \ else \ abort (); \ tmp; \ })Note: This construct is only available for C.
You can use the built-in function
__builtin_choose_exprto evaluate code depending on the value of a constant expression. This built-in function returns exp1 if const_exp, which is a constant expression that must be able to be determined at compile time, is nonzero. Otherwise it returns 0.This built-in function is analogous to the ? : operator in C, except that the expression returned has its type unaltered by promotion rules. Also, the built-in function does not evaluate the expression that was not chosen. For example, if const_exp evaluates to true, exp2 is not evaluated even if it has side-effects.
This built-in function can return an lvalue if the chosen argument is an lvalue.
If exp1 is returned, the return type is the same as exp1's type. Similarly, if exp2 is returned, its return type is the same as exp2.
Example:
#define foo(x) \ __builtin_choose_expr ( \ __builtin_types_compatible_p (typeof (x), double), \ foo_double (x), \ __builtin_choose_expr ( \ __builtin_types_compatible_p (typeof (x), float), \ foo_float (x), \ /* The void expression results in a compile-time error \ when assigning the result to something. */ \ (void)0))Note: This construct is only available for C. Furthermore, the unused expression (exp1 or exp2 depending on the value of const_exp) may still generate syntax errors. This may change in future revisions.
You can use the built-in function
__builtin_constant_pto determine if a value is known to be constant at compile-time and hence that GCC can perform constant-folding on expressions involving that value. The argument of the function is the value to test. The function returns the integer 1 if the argument is known to be a compile-time constant and 0 if it is not known to be a compile-time constant. A return of 0 does not indicate that the value is not a constant, but merely that GCC cannot prove it is a constant with the specified value of the -O option.You would typically use this function in an embedded application where memory was a critical resource. If you have some complex calculation, you may want it to be folded if it involves constants, but need to call a function if it does not. For example:
#define Scale_Value(X) \ (__builtin_constant_p (X) \ ? ((X) * SCALE + OFFSET) : Scale (X))You may use this built-in function in either a macro or an inline function. However, if you use it in an inlined function and pass an argument of the function as the argument to the built-in, GCC will never return 1 when you call the inline function with a string constant or compound literal (see Compound Literals) and will not return 1 when you pass a constant numeric value to the inline function unless you specify the -O option.
You may also use
__builtin_constant_pin initializers for static data. For instance, you can writestatic const int table[] = { __builtin_constant_p (EXPRESSION) ? (EXPRESSION) : -1, /* ... */ };This is an acceptable initializer even if EXPRESSION is not a constant expression. GCC must be more conservative about evaluating the built-in in this case, because it has no opportunity to perform optimization.
Previous versions of GCC did not accept this built-in in data initializers. The earliest version where it is completely safe is 3.0.1.
You may use
__builtin_expectto provide the compiler with branch prediction information. In general, you should prefer to use actual profile feedback for this (-fprofile-arcs), as programmers are notoriously bad at predicting how their programs actually perform. However, there are applications in which this data is hard to collect.The return value is the value of exp, which should be an integral expression. The value of c must be a compile-time constant. The semantics of the built-in are that it is expected that exp == c. For example:
if (__builtin_expect (x, 0)) foo ();would indicate that we do not expect to call
foo, since we expectxto be zero. Since you are limited to integral expressions for exp, you should use constructions such asif (__builtin_expect (ptr != NULL, 1)) error ();when testing pointer or floating-point values.
This function is used to minimize cache-miss latency by moving data into a cache before it is accessed. You can insert calls to
__builtin_prefetchinto code for which you know addresses of data in memory that is likely to be accessed soon. If the target supports them, data prefetch instructions will be generated. If the prefetch is done early enough before the access then the data will be in the cache by the time it is accessed.The value of addr is the address of the memory to prefetch. There are two optional arguments, rw and locality. The value of rw is a compile-time constant one or zero; one means that the prefetch is preparing for a write to the memory address and zero, the default, means that the prefetch is preparing for a read. The value locality must be a compile-time constant integer between zero and three. A value of zero means that the data has no temporal locality, so it need not be left in the cache after the access. A value of three means that the data has a high degree of temporal locality and should be left in all levels of cache possible. Values of one and two mean, respectively, a low or moderate degree of temporal locality. The default is three.
for (i = 0; i < n; i++) { a[i] = a[i] + b[i]; __builtin_prefetch (&a[i+j], 1, 1); __builtin_prefetch (&b[i+j], 0, 1); /* ... */ }Data prefetch does not generate faults if addr is invalid, but the address expression itself must be valid. For example, a prefetch of
p->nextwill not fault ifp->nextis not a valid address, but evaluation will fault ifpis not a valid address.If the target does not support data prefetch, the address expression is evaluated if it includes side effects but no other code is generated and GCC does not issue a warning.
Returns a positive infinity, if supported by the floating-point format, else
DBL_MAX. This function is suitable for implementing the ISO C macroHUGE_VAL.
Similar to
__builtin_huge_val, except the return type isfloat.
Similar to
__builtin_huge_val, except the return type islong double.
Similar to
__builtin_huge_val, except a warning is generated if the target floating-point format does not support infinities. This function is suitable for implementing the ISO C99 macroINFINITY.
Similar to
__builtin_inf, except the return type isfloat.
Similar to
__builtin_inf, except the return type islong double.
This is an implementation of the ISO C99 function
nan.Since ISO C99 defines this function in terms of
strtod, which we do not implement, a description of the parsing is in order. The string is parsed as bystrtol; that is, the base is recognized by leading 0 or 0x prefixes. The number parsed is placed in the significand such that the least significant bit of the number is at the least significant bit of the significand. The number is truncated to fit the significand field provided. The significand is forced to be a quiet NaN.This function, if given a string literal, is evaluated early enough that it is considered a compile-time constant.
Similar to
__builtin_nan, except the return type isfloat.
Similar to
__builtin_nan, except the return type islong double.
Similar to
__builtin_nan, except the significand is forced to be a signaling NaN. Thenansfunction is proposed by WG14 N965.
Similar to
__builtin_nans, except the return type isfloat.
Similar to
__builtin_nans, except the return type islong double.
On some target machines, GCC supports many built-in functions specific to those machines. Generally these generate calls to specific machine instructions, but allow the compiler to schedule those calls.
These built-in functions are available for the Alpha family of processors, depending on the command-line switches used.
The following built-in functions are always available. They all generate the machine instruction that is part of the name.
long __builtin_alpha_implver (void)
long __builtin_alpha_rpcc (void)
long __builtin_alpha_amask (long)
long __builtin_alpha_cmpbge (long, long)
long __builtin_alpha_extbl (long, long)
long __builtin_alpha_extwl (long, long)
long __builtin_alpha_extll (long, long)
long __builtin_alpha_extql (long, long)
long __builtin_alpha_extwh (long, long)
long __builtin_alpha_extlh (long, long)
long __builtin_alpha_extqh (long, long)
long __builtin_alpha_insbl (long, long)
long __builtin_alpha_inswl (long, long)
long __builtin_alpha_insll (long, long)
long __builtin_alpha_insql (long, long)
long __builtin_alpha_inswh (long, long)
long __builtin_alpha_inslh (long, long)
long __builtin_alpha_insqh (long, long)
long __builtin_alpha_mskbl (long, long)
long __builtin_alpha_mskwl (long, long)
long __builtin_alpha_mskll (long, long)
long __builtin_alpha_mskql (long, long)
long __builtin_alpha_mskwh (long, long)
long __builtin_alpha_msklh (long, long)
long __builtin_alpha_mskqh (long, long)
long __builtin_alpha_umulh (long, long)
long __builtin_alpha_zap (long, long)
long __builtin_alpha_zapnot (long, long)
The following built-in functions are always with -mmax
or -mcpu=cpu where cpu is pca56 or
later. They all generate the machine instruction that is part
of the name.
long __builtin_alpha_pklb (long)
long __builtin_alpha_pkwb (long)
long __builtin_alpha_unpkbl (long)
long __builtin_alpha_unpkbw (long)
long __builtin_alpha_minub8 (long, long)
long __builtin_alpha_minsb8 (long, long)
long __builtin_alpha_minuw4 (long, long)
long __builtin_alpha_minsw4 (long, long)
long __builtin_alpha_maxub8 (long, long)
long __builtin_alpha_maxsb8 (long, long)
long __builtin_alpha_maxuw4 (long, long)
long __builtin_alpha_maxsw4 (long, long)
long __builtin_alpha_perr (long, long)
The following built-in functions are always with -mcix
or -mcpu=cpu where cpu is ev67 or
later. They all generate the machine instruction that is part
of the name.
long __builtin_alpha_cttz (long)
long __builtin_alpha_ctlz (long)
long __builtin_alpha_ctpop (long)
The following builtins are available on systems that use the OSF/1
PALcode. Normally they invoke the rduniq and wruniq
PAL calls, but when invoked with -mtls-kernel, they invoke
rdval and wrval.
void *__builtin_thread_pointer (void)
void __builtin_set_thread_pointer (void *)
These built-in functions are available for the i386 and x86-64 family of computers, depending on the command-line switches used.
The following machine modes are available for use with MMX built-in functions
(see Vector Extensions): V2SI for a vector of two 32-bit integers,
V4HI for a vector of four 16-bit integers, and V8QI for a
vector of eight 8-bit integers. Some of the built-in functions operate on
MMX registers as a whole 64-bit entity, these use DI as their mode.
If 3Dnow extensions are enabled, V2SF is used as a mode for a vector
of two 32-bit floating point values.
If SSE extensions are enabled, V4SF is used for a vector of four 32-bit
floating point values. Some instructions use a vector of four 32-bit
integers, these use V4SI. Finally, some instructions operate on an
entire vector register, interpreting it as a 128-bit integer, these use mode
TI.
The following built-in functions are made available by -mmmx. All of them generate the machine instruction that is part of the name.
v8qi __builtin_ia32_paddb (v8qi, v8qi)
v4hi __builtin_ia32_paddw (v4hi, v4hi)
v2si __builtin_ia32_paddd (v2si, v2si)
v8qi __builtin_ia32_psubb (v8qi, v8qi)
v4hi __builtin_ia32_psubw (v4hi, v4hi)
v2si __builtin_ia32_psubd (v2si, v2si)
v8qi __builtin_ia32_paddsb (v8qi, v8qi)
v4hi __builtin_ia32_paddsw (v4hi, v4hi)
v8qi __builtin_ia32_psubsb (v8qi, v8qi)
v4hi __builtin_ia32_psubsw (v4hi, v4hi)
v8qi __builtin_ia32_paddusb (v8qi, v8qi)
v4hi __builtin_ia32_paddusw (v4hi, v4hi)
v8qi __builtin_ia32_psubusb (v8qi, v8qi)
v4hi __builtin_ia32_psubusw (v4hi, v4hi)
v4hi __builtin_ia32_pmullw (v4hi, v4hi)
v4hi __builtin_ia32_pmulhw (v4hi, v4hi)
di __builtin_ia32_pand (di, di)
di __builtin_ia32_pandn (di,di)
di __builtin_ia32_por (di, di)
di __builtin_ia32_pxor (di, di)
v8qi __builtin_ia32_pcmpeqb (v8qi, v8qi)
v4hi __builtin_ia32_pcmpeqw (v4hi, v4hi)
v2si __builtin_ia32_pcmpeqd (v2si, v2si)
v8qi __builtin_ia32_pcmpgtb (v8qi, v8qi)
v4hi __builtin_ia32_pcmpgtw (v4hi, v4hi)
v2si __builtin_ia32_pcmpgtd (v2si, v2si)
v8qi __builtin_ia32_punpckhbw (v8qi, v8qi)
v4hi __builtin_ia32_punpckhwd (v4hi, v4hi)
v2si __builtin_ia32_punpckhdq (v2si, v2si)
v8qi __builtin_ia32_punpcklbw (v8qi, v8qi)
v4hi __builtin_ia32_punpcklwd (v4hi, v4hi)
v2si __builtin_ia32_punpckldq (v2si, v2si)
v8qi __builtin_ia32_packsswb (v4hi, v4hi)
v4hi __builtin_ia32_packssdw (v2si, v2si)
v8qi __builtin_ia32_packuswb (v4hi, v4hi)
The following built-in functions are made available either with -msse, or with a combination of -m3dnow and -march=athlon. All of them generate the machine instruction that is part of the name.
v4hi __builtin_ia32_pmulhuw (v4hi, v4hi)
v8qi __builtin_ia32_pavgb (v8qi, v8qi)
v4hi __builtin_ia32_pavgw (v4hi, v4hi)
v4hi __builtin_ia32_psadbw (v8qi, v8qi)
v8qi __builtin_ia32_pmaxub (v8qi, v8qi)
v4hi __builtin_ia32_pmaxsw (v4hi, v4hi)
v8qi __builtin_ia32_pminub (v8qi, v8qi)
v4hi __builtin_ia32_pminsw (v4hi, v4hi)
int __builtin_ia32_pextrw (v4hi, int)
v4hi __builtin_ia32_pinsrw (v4hi, int, int)
int __builtin_ia32_pmovmskb (v8qi)
void __builtin_ia32_maskmovq (v8qi, v8qi, char *)
void __builtin_ia32_movntq (di *, di)
void __builtin_ia32_sfence (void)
The following built-in functions are available when -msse is used. All of them generate the machine instruction that is part of the name.
int __builtin_ia32_comieq (v4sf, v4sf)
int __builtin_ia32_comineq (v4sf, v4sf)
int __builtin_ia32_comilt (v4sf, v4sf)
int __builtin_ia32_comile (v4sf, v4sf)
int __builtin_ia32_comigt (v4sf, v4sf)
int __builtin_ia32_comige (v4sf, v4sf)
int __builtin_ia32_ucomieq (v4sf, v4sf)
int __builtin_ia32_ucomineq (v4sf, v4sf)
int __builtin_ia32_ucomilt (v4sf, v4sf)
int __builtin_ia32_ucomile (v4sf, v4sf)
int __builtin_ia32_ucomigt (v4sf, v4sf)
int __builtin_ia32_ucomige (v4sf, v4sf)
v4sf __builtin_ia32_addps (v4sf, v4sf)
v4sf __builtin_ia32_subps (v4sf, v4sf)
v4sf __builtin_ia32_mulps (v4sf, v4sf)
v4sf __builtin_ia32_divps (v4sf, v4sf)
v4sf __builtin_ia32_addss (v4sf, v4sf)
v4sf __builtin_ia32_subss (v4sf, v4sf)
v4sf __builtin_ia32_mulss (v4sf, v4sf)
v4sf __builtin_ia32_divss (v4sf, v4sf)
v4si __builtin_ia32_cmpeqps (v4sf, v4sf)
v4si __builtin_ia32_cmpltps (v4sf, v4sf)
v4si __builtin_ia32_cmpleps (v4sf, v4sf)
v4si __builtin_ia32_cmpgtps (v4sf, v4sf)
v4si __builtin_ia32_cmpgeps (v4sf, v4sf)
v4si __builtin_ia32_cmpunordps (v4sf, v4sf)
v4si __builtin_ia32_cmpneqps (v4sf, v4sf)
v4si __builtin_ia32_cmpnltps (v4sf, v4sf)
v4si __builtin_ia32_cmpnleps (v4sf, v4sf)
v4si __builtin_ia32_cmpngtps (v4sf, v4sf)
v4si __builtin_ia32_cmpngeps (v4sf, v4sf)
v4si __builtin_ia32_cmpordps (v4sf, v4sf)
v4si __builtin_ia32_cmpeqss (v4sf, v4sf)
v4si __builtin_ia32_cmpltss (v4sf, v4sf)
v4si __builtin_ia32_cmpless (v4sf, v4sf)
v4si __builtin_ia32_cmpunordss (v4sf, v4sf)
v4si __builtin_ia32_cmpneqss (v4sf, v4sf)
v4si __builtin_ia32_cmpnlts (v4sf, v4sf)
v4si __builtin_ia32_cmpnless (v4sf, v4sf)
v4si __builtin_ia32_cmpordss (v4sf, v4sf)
v4sf __builtin_ia32_maxps (v4sf, v4sf)
v4sf __builtin_ia32_maxss (v4sf, v4sf)
v4sf __builtin_ia32_minps (v4sf, v4sf)
v4sf __builtin_ia32_minss (v4sf, v4sf)
v4sf __builtin_ia32_andps (v4sf, v4sf)
v4sf __builtin_ia32_andnps (v4sf, v4sf)
v4sf __builtin_ia32_orps (v4sf, v4sf)
v4sf __builtin_ia32_xorps (v4sf, v4sf)
v4sf __builtin_ia32_movss (v4sf, v4sf)
v4sf __builtin_ia32_movhlps (v4sf, v4sf)
v4sf __builtin_ia32_movlhps (v4sf, v4sf)
v4sf __builtin_ia32_unpckhps (v4sf, v4sf)
v4sf __builtin_ia32_unpcklps (v4sf, v4sf)
v4sf __builtin_ia32_cvtpi2ps (v4sf, v2si)
v4sf __builtin_ia32_cvtsi2ss (v4sf, int)
v2si __builtin_ia32_cvtps2pi (v4sf)
int __builtin_ia32_cvtss2si (v4sf)
v2si __builtin_ia32_cvttps2pi (v4sf)
int __builtin_ia32_cvttss2si (v4sf)
v4sf __builtin_ia32_rcpps (v4sf)
v4sf __builtin_ia32_rsqrtps (v4sf)
v4sf __builtin_ia32_sqrtps (v4sf)
v4sf __builtin_ia32_rcpss (v4sf)
v4sf __builtin_ia32_rsqrtss (v4sf)
v4sf __builtin_ia32_sqrtss (v4sf)
v4sf __builtin_ia32_shufps (v4sf, v4sf, int)
void __builtin_ia32_movntps (float *, v4sf)
int __builtin_ia32_movmskps (v4sf)
The following built-in functions are available when -msse is used.
v4sf __builtin_ia32_loadaps (float *)movaps machine instruction as a load from memory.
void __builtin_ia32_storeaps (float *, v4sf)movaps machine instruction as a store to memory.
v4sf __builtin_ia32_loadups (float *)movups machine instruction as a load from memory.
void __builtin_ia32_storeups (float *, v4sf)movups machine instruction as a store to memory.
v4sf __builtin_ia32_loadsss (float *)movss machine instruction as a load from memory.
void __builtin_ia32_storess (float *, v4sf)movss machine instruction as a store to memory.
v4sf __builtin_ia32_loadhps (v4sf, v2si *)movhps machine instruction as a load from memory.
v4sf __builtin_ia32_loadlps (v4sf, v2si *)movlps machine instruction as a load from memory
void __builtin_ia32_storehps (v4sf, v2si *)movhps machine instruction as a store to memory.
void __builtin_ia32_storelps (v4sf, v2si *)movlps machine instruction as a store to memory.
The following built-in functions are available when -msse3 is used. All of them generate the machine instruction that is part of the name.
v2df __builtin_ia32_addsubpd (v2df, v2df)
v2df __builtin_ia32_addsubps (v2df, v2df)
v2df __builtin_ia32_haddpd (v2df, v2df)
v2df __builtin_ia32_haddps (v2df, v2df)
v2df __builtin_ia32_hsubpd (v2df, v2df)
v2df __builtin_ia32_hsubps (v2df, v2df)
v16qi __builtin_ia32_lddqu (char const *)
void __builtin_ia32_monitor (void *, unsigned int, unsigned int)
v2df __builtin_ia32_movddup (v2df)
v4sf __builtin_ia32_movshdup (v4sf)
v4sf __builtin_ia32_movsldup (v4sf)
void __builtin_ia32_mwait (unsigned int, unsigned int)
The following built-in functions are available when -msse3 is used.
v2df __builtin_ia32_loadddup (double const *)movddup machine instruction as a load from memory.
The following built-in functions are available when -m3dnow is used. All of them generate the machine instruction that is part of the name.
void __builtin_ia32_femms (void)
v8qi __builtin_ia32_pavgusb (v8qi, v8qi)
v2si __builtin_ia32_pf2id (v2sf)
v2sf __builtin_ia32_pfacc (v2sf, v2sf)
v2sf __builtin_ia32_pfadd (v2sf, v2sf)
v2si __builtin_ia32_pfcmpeq (v2sf, v2sf)
v2si __builtin_ia32_pfcmpge (v2sf, v2sf)
v2si __builtin_ia32_pfcmpgt (v2sf, v2sf)
v2sf __builtin_ia32_pfmax (v2sf, v2sf)
v2sf __builtin_ia32_pfmin (v2sf, v2sf)
v2sf __builtin_ia32_pfmul (v2sf, v2sf)
v2sf __builtin_ia32_pfrcp (v2sf)
v2sf __builtin_ia32_pfrcpit1 (v2sf, v2sf)
v2sf __builtin_ia32_pfrcpit2 (v2sf, v2sf)
v2sf __builtin_ia32_pfrsqrt (v2sf)
v2sf __builtin_ia32_pfrsqrtit1 (v2sf, v2sf)
v2sf __builtin_ia32_pfsub (v2sf, v2sf)
v2sf __builtin_ia32_pfsubr (v2sf, v2sf)
v2sf __builtin_ia32_pi2fd (v2si)
v4hi __builtin_ia32_pmulhrw (v4hi, v4hi)
The following built-in functions are available when both -m3dnow and -march=athlon are used. All of them generate the machine instruction that is part of the name.
v2si __builtin_ia32_pf2iw (v2sf)
v2sf __builtin_ia32_pfnacc (v2sf, v2sf)
v2sf __builtin_ia32_pfpnacc (v2sf, v2sf)
v2sf __builtin_ia32_pi2fw (v2si)
v2sf __builtin_ia32_pswapdsf (v2sf)
v2si __builtin_ia32_pswapdsi (v2si)
These built-in functions are available for the PowerPC family of computers, depending on the command-line switches used.
The following machine modes are available for use with AltiVec built-in
functions (see Vector Extensions): V4SI for a vector of four
32-bit integers, V4SF for a vector of four 32-bit floating point
numbers, V8HI for a vector of eight 16-bit integers, and
V16QI for a vector of sixteen 8-bit integers.
The following functions are made available by including
<altivec.h> and using -maltivec and
-mabi=altivec. The functions implement the functionality
described in Motorola's AltiVec Programming Interface Manual.
There are a few differences from Motorola's documentation and GCC's
implementation. Vector constants are done with curly braces (not
parentheses). Vector initializers require no casts if the vector
constant is of the same type as the variable it is initializing. The
vector bool type is deprecated and will be discontinued in
further revisions. Use vector signed instead. If signed
or unsigned is omitted, the vector type will default to
signed. Lastly, all overloaded functions are implemented with macros
for the C implementation. So code the following example will not work:
vec_add ((vector signed int){1, 2, 3, 4}, foo);
Since vec_add is a macro, the vector constant in the above example will be treated as four different arguments. Wrap the entire argument in parentheses for this to work. The C++ implementation does not use macros.
Note: Only the <altivec.h> interface is supported.
Internally, GCC uses built-in functions to achieve the functionality in
the aforementioned header file, but they are not supported and are
subject to change without notice.
vector signed char vec_abs (vector signed char, vector signed char);
vector signed short vec_abs (vector signed short, vector signed short);
vector signed int vec_abs (vector signed int, vector signed int);
vector signed float vec_abs (vector signed float, vector signed float);
vector signed char vec_abss (vector signed char, vector signed char);
vector signed short vec_abss (vector signed short, vector signed short);
vector signed char vec_add (vector signed char, vector signed char);
vector unsigned char vec_add (vector signed char, vector unsigned char);
vector unsigned char vec_add (vector unsigned char, vector signed char);
vector unsigned char vec_add (vector unsigned char,
vector unsigned char);
vector signed short vec_add (vector signed short, vector signed short);
vector unsigned short vec_add (vector signed short,
vector unsigned short);
vector unsigned short vec_add (vector unsigned short,
vector signed short);
vector unsigned short vec_add (vector unsigned short,
vector unsigned short);
vector signed int vec_add (vector signed int, vector signed int);
vector unsigned int vec_add (vector signed int, vector unsigned int);
vector unsigned int vec_add (vector unsigned int, vector signed int);
vector unsigned int vec_add (vector unsigned int, vector unsigned int);
vector float vec_add (vector float, vector float);
vector unsigned int vec_addc (vector unsigned int, vector unsigned int);
vector unsigned char vec_adds (vector signed char,
vector unsigned char);
vector unsigned char vec_adds (vector unsigned char,
vector signed char);
vector unsigned char vec_adds (vector unsigned char,
vector unsigned char);
vector signed char vec_adds (vector signed char, vector signed char);
vector unsigned short vec_adds (vector signed short,
vector unsigned short);
vector unsigned short vec_adds (vector unsigned short,
vector signed short);
vector unsigned short vec_adds (vector unsigned short,
vector unsigned short);
vector signed short vec_adds (vector signed short, vector signed short);
vector unsigned int vec_adds (vector signed int, vector unsigned int);
vector unsigned int vec_adds (vector unsigned int, vector signed int);
vector unsigned int vec_adds (vector unsigned int, vector unsigned int);
vector signed int vec_adds (vector signed int, vector signed int);
vector float vec_and (vector float, vector float);
vector float vec_and (vector float, vector signed int);
vector float vec_and (vector signed int, vector float);
vector signed int vec_and (vector signed int, vector signed int);
vector unsigned int vec_and (vector signed int, vector unsigned int);
vector unsigned int vec_and (vector unsigned int, vector signed int);
vector unsigned int vec_and (vector unsigned int, vector unsigned int);
vector signed short vec_and (vector signed short, vector signed short);
vector unsigned short vec_and (vector signed short,
vector unsigned short);
vector unsigned short vec_and (vector unsigned short,
vector signed short);
vector unsigned short vec_and (vector unsigned short,
vector unsigned short);
vector signed char vec_and (vector signed char, vector signed char);
vector unsigned char vec_and (vector signed char, vector unsigned char);
vector unsigned char vec_and (vector unsigned char, vector signed char);
vector unsigned char vec_and (vector unsigned char,
vector unsigned char);
vector float vec_andc (vector float, vector float);
vector float vec_andc (vector float, vector signed int);
vector float vec_andc (vector signed int, vector float);
vector signed int vec_andc (vector signed int, vector signed int);
vector unsigned int vec_andc (vector signed int, vector unsigned int);
vector unsigned int vec_andc (vector unsigned int, vector signed int);
vector unsigned int vec_andc (vector unsigned int, vector unsigned int);
vector signed short vec_andc (vector signed short, vector signed short);
vector unsigned short vec_andc (vector signed short,
vector unsigned short);
vector unsigned short vec_andc (vector unsigned short,
vector signed short);
vector unsigned short vec_andc (vector unsigned short,
vector unsigned short);
vector signed char vec_andc (vector signed char, vector signed char);
vector unsigned char vec_andc (vector signed char,
vector unsigned char);
vector unsigned char vec_andc (vector unsigned char,
vector signed char);
vector unsigned char vec_andc (vector unsigned char,
vector unsigned char);
vector unsigned char vec_avg (vector unsigned char,
vector unsigned char);
vector signed char vec_avg (vector signed char, vector signed char);
vector unsigned short vec_avg (vector unsigned short,
vector unsigned short);
vector signed short vec_avg (vector signed short, vector signed short);
vector unsigned int vec_avg (vector unsigned int, vector unsigned int);
vector signed int vec_avg (vector signed int, vector signed int);
vector float vec_ceil (vector float);
vector signed int vec_cmpb (vector float, vector float);
vector signed char vec_cmpeq (vector signed char, vector signed char);
vector signed char vec_cmpeq (vector unsigned char,
vector unsigned char);
vector signed short vec_cmpeq (vector signed short,
vector signed short);
vector signed short vec_cmpeq (vector unsigned short,
vector unsigned short);
vector signed int vec_cmpeq (vector signed int, vector signed int);
vector signed int vec_cmpeq (vector unsigned int, vector unsigned int);
vector signed int vec_cmpeq (vector float, vector float);
vector signed int vec_cmpge (vector float, vector float);
vector signed char vec_cmpgt (vector unsigned char,
vector unsigned char);
vector signed char vec_cmpgt (vector signed char, vector signed char);
vector signed short vec_cmpgt (vector unsigned short,
vector unsigned short);
vector signed short vec_cmpgt (vector signed short,
vector signed short);
vector signed int vec_cmpgt (vector unsigned int, vector unsigned int);
vector signed int vec_cmpgt (vector signed int, vector signed int);
vector signed int vec_cmpgt (vector float, vector float);
vector signed int vec_cmple (vector float, vector float);
vector signed char vec_cmplt (vector unsigned char,
vector unsigned char);
vector signed char vec_cmplt (vector signed char, vector signed char);
vector signed short vec_cmplt (vector unsigned short,
vector unsigned short);
vector signed short vec_cmplt (vector signed short,
vector signed short);
vector signed int vec_cmplt (vector unsigned int, vector unsigned int);
vector signed int vec_cmplt (vector signed int, vector signed int);
vector signed int vec_cmplt (vector float, vector float);
vector float vec_ctf (vector unsigned int, const char);
vector float vec_ctf (vector signed int, const char);
vector signed int vec_cts (vector float, const char);
vector unsigned int vec_ctu (vector float, const char);
void vec_dss (const char);
void vec_dssall (void);
void vec_dst (void *, int, const char);
void vec_dstst (void *, int, const char);
void vec_dststt (void *, int, const char);
void vec_dstt (void *, int, const char);
vector float vec_expte (vector float, vector float);
vector float vec_floor (vector float, vector float);
vector float vec_ld (int, vector float *);
vector float vec_ld (int, float *):
vector signed int vec_ld (int, int *);
vector signed int vec_ld (int, vector signed int *);
vector unsigned int vec_ld (int, vector unsigned int *);
vector unsigned int vec_ld (int, unsigned int *);
vector signed short vec_ld (int, short *, vector signed short *);
vector unsigned short vec_ld (int, unsigned short *,
vector unsigned short *);
vector signed char vec_ld (int, signed char *);
vector signed char vec_ld (int, vector signed char *);
vector unsigned char vec_ld (int, unsigned char *);
vector unsigned char vec_ld (int, vector unsigned char *);
vector signed char vec_lde (int, signed char *);
vector unsigned char vec_lde (int, unsigned char *);
vector signed short vec_lde (int, short *);
vector unsigned short vec_lde (int, unsigned short *);
vector float vec_lde (int, float *);
vector signed int vec_lde (int, int *);
vector unsigned int vec_lde (int, unsigned int *);
void float vec_ldl (int, float *);
void float vec_ldl (int, vector float *);
void signed int vec_ldl (int, vector signed int *);
void signed int vec_ldl (int, int *);
void unsigned int vec_ldl (int, unsigned int *);
void unsigned int vec_ldl (int, vector unsigned int *);
void signed short vec_ldl (int, vector signed short *);
void signed short vec_ldl (int, short *);
void unsigned short vec_ldl (int, vector unsigned short *);
void unsigned short vec_ldl (int, unsigned short *);
void signed char vec_ldl (int, vector signed char *);
void signed char vec_ldl (int, signed char *);
void unsigned char vec_ldl (int, vector unsigned char *);
void unsigned char vec_ldl (int, unsigned char *);
vector float vec_loge (vector float);
vector unsigned char vec_lvsl (int, void *, int *);
vector unsigned char vec_lvsr (int, void *, int *);
vector float vec_madd (vector float, vector float, vector float);
vector signed short vec_madds (vector signed short, vector signed short,
vector signed short);
vector unsigned char vec_max (vector signed char, vector unsigned char);
vector unsigned char vec_max (vector unsigned char, vector signed char);
vector unsigned char vec_max (vector unsigned char,
vector unsigned char);
vector signed char vec_max (vector signed char, vector signed char);
vector unsigned short vec_max (vector signed short,
vector unsigned short);
vector unsigned short vec_max (vector unsigned short,
vector signed short);
vector unsigned short vec_max (vector unsigned short,
vector unsigned short);
vector signed short vec_max (vector signed short, vector signed short);
vector unsigned int vec_max (vector signed int, vector unsigned int);
vector unsigned int vec_max (vector unsigned int, vector signed int);
vector unsigned int vec_max (vector unsigned int, vector unsigned int);
vector signed int vec_max (vector signed int, vector signed int);
vector float vec_max (vector float, vector float);
vector signed char vec_mergeh (vector signed char, vector signed char);
vector unsigned char vec_mergeh (vector unsigned char,
vector unsigned char);
vector signed short vec_mergeh (vector signed short,
vector signed short);
vector unsigned short vec_mergeh (vector unsigned short,
vector unsigned short);
vector float vec_mergeh (vector float, vector float);
vector signed int vec_mergeh (vector signed int, vector signed int);
vector unsigned int vec_mergeh (vector unsigned int,
vector unsigned int);
vector signed char vec_mergel (vector signed char, vector signed char);
vector unsigned char vec_mergel (vector unsigned char,
vector unsigned char);
vector signed short vec_mergel (vector signed short,
vector signed short);
vector unsigned short vec_mergel (vector unsigned short,
vector unsigned short);
vector float vec_mergel (vector float, vector float);
vector signed int vec_mergel (vector signed int, vector signed int);
vector unsigned int vec_mergel (vector unsigned int,
vector unsigned int);
vector unsigned short vec_mfvscr (void);
vector unsigned char vec_min (vector signed char, vector unsigned char);
vector unsigned char vec_min (vector unsigned char, vector signed char);
vector unsigned char vec_min (vector unsigned char,
vector unsigned char);
vector signed char vec_min (vector signed char, vector signed char);
vector unsigned short vec_min (vector signed short,
vector unsigned short);
vector unsigned short vec_min (vector unsigned short,
vector signed short);
vector unsigned short vec_min (vector unsigned short,
vector unsigned short);
vector signed short vec_min (vector signed short, vector signed short);
vector unsigned int vec_min (vector signed int, vector unsigned int);
vector unsigned int vec_min (vector unsigned int, vector signed int);
vector unsigned int vec_min (vector unsigned int, vector unsigned int);
vector signed int vec_min (vector signed int, vector signed int);
vector float vec_min (vector float, vector float);
vector signed short vec_mladd (vector signed short, vector signed short,
vector signed short);
vector signed short vec_mladd (vector signed short,
vector unsigned short,
vector unsigned short);
vector signed short vec_mladd (vector unsigned short,
vector signed short,
vector signed short);
vector unsigned short vec_mladd (vector unsigned short,
vector unsigned short,
vector unsigned short);
vector signed short vec_mradds (vector signed short,
vector signed short,
vector signed short);
vector unsigned int vec_msum (vector unsigned char,
vector unsigned char,
vector unsigned int);
vector signed int vec_msum (vector signed char, vector unsigned char,
vector signed int);
vector unsigned int vec_msum (vector unsigned short,
vector unsigned short,
vector unsigned int);
vector signed int vec_msum (vector signed short, vector signed short,
vector signed int);
vector unsigned int vec_msums (vector unsigned short,
vector unsigned short,
vector unsigned int);
vector signed int vec_msums (vector signed short, vector signed short,
vector signed int);
void vec_mtvscr (vector signed int);
void vec_mtvscr (vector unsigned int);
void vec_mtvscr (vector signed short);
void vec_mtvscr (vector unsigned short);
void vec_mtvscr (vector signed char);
void vec_mtvscr (vector unsigned char);
vector unsigned short vec_mule (vector unsigned char,
vector unsigned char);
vector signed short vec_mule (vector signed char, vector signed char);
vector unsigned int vec_mule (vector unsigned short,
vector unsigned short);
vector signed int vec_mule (vector signed short, vector signed short);
vector unsigned short vec_mulo (vector unsigned char,
vector unsigned char);
vector signed short vec_mulo (vector signed char, vector signed char);
vector unsigned int vec_mulo (vector unsigned short,
vector unsigned short);
vector signed int vec_mulo (vector signed short, vector signed short);
vector float vec_nmsub (vector float, vector float, vector float);
vector float vec_nor (vector float, vector float);
vector signed int vec_nor (vector signed int, vector signed int);
vector unsigned int vec_nor (vector unsigned int, vector unsigned int);
vector signed short vec_nor (vector signed short, vector signed short);
vector unsigned short vec_nor (vector unsigned short,
vector unsigned short);
vector signed char vec_nor (vector signed char, vector signed char);
vector unsigned char vec_nor (vector unsigned char,
vector unsigned char);
vector float vec_or (vector float, vector float);
vector float vec_or (vector float, vector signed int);
vector float vec_or (vector signed int, vector float);
vector signed int vec_or (vector signed int, vector signed int);
vector unsigned int vec_or (vector signed int, vector unsigned int);
vector unsigned int vec_or (vector unsigned int, vector signed int);
vector unsigned int vec_or (vector unsigned int, vector unsigned int);
vector signed short vec_or (vector signed short, vector signed short);
vector unsigned short vec_or (vector signed short,
vector unsigned short);
vector unsigned short vec_or (vector unsigned short,
vector signed short);
vector unsigned short vec_or (vector unsigned short,
vector unsigned short);
vector signed char vec_or (vector signed char, vector signed char);
vector unsigned char vec_or (vector signed char, vector unsigned char);
vector unsigned char vec_or (vector unsigned char, vector signed char);
vector unsigned char vec_or (vector unsigned char,
vector unsigned char);
vector signed char vec_pack (vector signed short, vector signed short);
vector unsigned char vec_pack (vector unsigned short,
vector unsigned short);
vector signed short vec_pack (vector signed int, vector signed int);
vector unsigned short vec_pack (vector unsigned int,
vector unsigned int);
vector signed short vec_packpx (vector unsigned int,
vector unsigned int);
vector unsigned char vec_packs (vector unsigned short,
vector unsigned short);
vector signed char vec_packs (vector signed short, vector signed short);
vector unsigned short vec_packs (vector unsigned int,
vector unsigned int);
vector signed short vec_packs (vector signed int, vector signed int);
vector unsigned char vec_packsu (vector unsigned short,
vector unsigned short);
vector unsigned char vec_packsu (vector signed short,
vector signed short);
vector unsigned short vec_packsu (vector unsigned int,
vector unsigned int);
vector unsigned short vec_packsu (vector signed int, vector signed int);
vector float vec_perm (vector float, vector float,
vector unsigned char);
vector signed int vec_perm (vector signed int, vector signed int,
vector unsigned char);
vector unsigned int vec_perm (vector unsigned int, vector unsigned int,
vector unsigned char);
vector signed short vec_perm (vector signed short, vector signed short,
vector unsigned char);
vector unsigned short vec_perm (vector unsigned short,
vector unsigned short,
vector unsigned char);
vector signed char vec_perm (vector signed char, vector signed char,
vector unsigned char);
vector unsigned char vec_perm (vector unsigned char,
vector unsigned char,
vector unsigned char);
vector float vec_re (vector float);
vector signed char vec_rl (vector signed char, vector unsigned char);
vector unsigned char vec_rl (vector unsigned char,
vector unsigned char);
vector signed short vec_rl (vector signed short, vector unsigned short);
vector unsigned short vec_rl (vector unsigned short,
vector unsigned short);
vector signed int vec_rl (vector signed int, vector unsigned int);
vector unsigned int vec_rl (vector unsigned int, vector unsigned int);
vector float vec_round (vector float);
vector float vec_rsqrte (vector float);
vector float vec_sel (vector float, vector float, vector signed int);
vector float vec_sel (vector float, vector float, vector unsigned int);
vector signed int vec_sel (vector signed int, vector signed int,
vector signed int);
vector signed int vec_sel (vector signed int, vector signed int,
vector unsigned int);
vector unsigned int vec_sel (vector unsigned int, vector unsigned int,
vector signed int);
vector unsigned int vec_sel (vector unsigned int, vector unsigned int,
vector unsigned int);
vector signed short vec_sel (vector signed short, vector signed short,
vector signed short);
vector signed short vec_sel (vector signed short, vector signed short,
vector unsigned short);
vector unsigned short vec_sel (vector unsigned short,
vector unsigned short,
vector signed short);
vector unsigned short vec_sel (vector unsigned short,
vector unsigned short,
vector unsigned short);
vector signed char vec_sel (vector signed char, vector signed char,
vector signed char);
vector signed char vec_sel (vector signed char, vector signed char,
vector unsigned char);
vector unsigned char vec_sel (vector unsigned char,
vector unsigned char,
vector signed char);
vector unsigned char vec_sel (vector unsigned char,
vector unsigned char,
vector unsigned char);
vector signed char vec_sl (vector signed char, vector unsigned char);
vector unsigned char vec_sl (vector unsigned char,
vector unsigned char);
vector signed short vec_sl (vector signed short, vector unsigned short);
vector unsigned short vec_sl (vector unsigned short,
vector unsigned short);
vector signed int vec_sl (vector signed int, vector unsigned int);
vector unsigned int vec_sl (vector unsigned int, vector unsigned int);
vector float vec_sld (vector float, vector float, const char);
vector signed int vec_sld (vector signed int, vector signed int,
const char);
vector unsigned int vec_sld (vector unsigned int, vector unsigned int,
const char);
vector signed short vec_sld (vector signed short, vector signed short,
const char);
vector unsigned short vec_sld (vector unsigned short,
vector unsigned short, const char);
vector signed char vec_sld (vector signed char, vector signed char,
const char);
vector unsigned char vec_sld (vector unsigned char,
vector unsigned char,
const char);
vector signed int vec_sll (vector signed int, vector unsigned int);
vector signed int vec_sll (vector signed int, vector unsigned short);
vector signed int vec_sll (vector signed int, vector unsigned char);
vector unsigned int vec_sll (vector unsigned int, vector unsigned int);
vector unsigned int vec_sll (vector unsigned int,
vector unsigned short);
vector unsigned int vec_sll (vector unsigned int, vector unsigned char);
vector signed short vec_sll (vector signed short, vector unsigned int);
vector signed short vec_sll (vector signed short,
vector unsigned short);
vector signed short vec_sll (vector signed short, vector unsigned char);
vector unsigned short vec_sll (vector unsigned short,
vector unsigned int);
vector unsigned short vec_sll (vector unsigned short,
vector unsigned short);
vector unsigned short vec_sll (vector unsigned short,
vector unsigned char);
vector signed char vec_sll (vector signed char, vector unsigned int);
vector signed char vec_sll (vector signed char, vector unsigned short);
vector signed char vec_sll (vector signed char, vector unsigned char);
vector unsigned char vec_sll (vector unsigned char,
vector unsigned int);
vector unsigned char vec_sll (vector unsigned char,
vector unsigned short);
vector unsigned char vec_sll (vector unsigned char,
vector unsigned char);
vector float vec_slo (vector float, vector signed char);
vector float vec_slo (vector float, vector unsigned char);
vector signed int vec_slo (vector signed int, vector signed char);
vector signed int vec_slo (vector signed int, vector unsigned char);
vector unsigned int vec_slo (vector unsigned int, vector signed char);
vector unsigned int vec_slo (vector unsigned int, vector unsigned char);
vector signed short vec_slo (vector signed short, vector signed char);
vector signed short vec_slo (vector signed short, vector unsigned char);
vector unsigned short vec_slo (vector unsigned short,
vector signed char);
vector unsigned short vec_slo (vector unsigned short,
vector unsigned char);
vector signed char vec_slo (vector signed char, vector signed char);
vector signed char vec_slo (vector signed char, vector unsigned char);
vector unsigned char vec_slo (vector unsigned char, vector signed char);
vector unsigned char vec_slo (vector unsigned char,
vector unsigned char);
vector signed char vec_splat (vector signed char, const char);
vector unsigned char vec_splat (vector unsigned char, const char);
vector signed short vec_splat (vector signed short, const char);
vector unsigned short vec_splat (vector unsigned short, const char);
vector float vec_splat (vector float, const char);
vector signed int vec_splat (vector signed int, const char);
vector unsigned int vec_splat (vector unsigned int, const char);
vector signed char vec_splat_s8 (const char);
vector signed short vec_splat_s16 (const char);
vector signed int vec_splat_s32 (const char);
vector unsigned char vec_splat_u8 (const char);
vector unsigned short vec_splat_u16 (const char);
vector unsigned int vec_splat_u32 (const char);
vector signed char vec_sr (vector signed char, vector unsigned char);
vector unsigned char vec_sr (vector unsigned char,
vector unsigned char);
vector signed short vec_sr (vector signed short, vector unsigned short);
vector unsigned short vec_sr (vector unsigned short,
vector unsigned short);
vector signed int vec_sr (vector signed int, vector unsigned int);
vector unsigned int vec_sr (vector unsigned int, vector unsigned int);
vector signed char vec_sra (vector signed char, vector unsigned char);
vector unsigned char vec_sra (vector unsigned char,
vector unsigned char);
vector signed short vec_sra (vector signed short,
vector unsigned short);
vector unsigned short vec_sra (vector unsigned short,
vector unsigned short);
vector signed int vec_sra (vector signed int, vector unsigned int);
vector unsigned int vec_sra (vector unsigned int, vector unsigned int);
vector signed int vec_srl (vector signed int, vector unsigned int);
vector signed int vec_srl (vector signed int, vector unsigned short);
vector signed int vec_srl (vector signed int, vector unsigned char);
vector unsigned int vec_srl (vector unsigned int, vector unsigned int);
vector unsigned int vec_srl (vector unsigned int,
vector unsigned short);
vector unsigned int vec_srl (vector unsigned int, vector unsigned char);
vector signed short vec_srl (vector signed short, vector unsigned int);
vector signed short vec_srl (vector signed short,
vector unsigned short);
vector signed short vec_srl (vector signed short, vector unsigned char);
vector unsigned short vec_srl (vector unsigned short,
vector unsigned int);
vector unsigned short vec_srl (vector unsigned short,
vector unsigned short);
vector unsigned short vec_srl (vector unsigned short,
vector unsigned char);
vector signed char vec_srl (vector signed char, vector unsigned int);
vector signed char vec_srl (vector signed char, vector unsigned short);
vector signed char vec_srl (vector signed char, vector unsigned char);
vector unsigned char vec_srl (vector unsigned char,
vector unsigned int);
vector unsigned char vec_srl (vector unsigned char,
vector unsigned short);
vector unsigned char vec_srl (vector unsigned char,
vector unsigned char);
vector float vec_sro (vector float, vector signed char);
vector float vec_sro (vector float, vector unsigned char);
vector signed int vec_sro (vector signed int, vector signed char);
vector signed int vec_sro (vector signed int, vector unsigned char);
vector unsigned int vec_sro (vector unsigned int, vector signed char);
vector unsigned int vec_sro (vector unsigned int, vector unsigned char);
vector signed short vec_sro (vector signed short, vector signed char);
vector signed short vec_sro (vector signed short, vector unsigned char);
vector unsigned short vec_sro (vector unsigned short,
vector signed char);
vector unsigned short vec_sro (vector unsigned short,
vector unsigned char);
vector signed char vec_sro (vector signed char, vector signed char);
vector signed char vec_sro (vector signed char, vector unsigned char);
vector unsigned char vec_sro (vector unsigned char, vector signed char);
vector unsigned char vec_sro (vector unsigned char,
vector unsigned char);
void vec_st (vector float, int, float *);
void vec_st (vector float, int, vector float *);
void vec_st (vector signed int, int, int *);
void vec_st (vector signed int, int, unsigned int *);
void vec_st (vector unsigned int, int, unsigned int *);
void vec_st (vector unsigned int, int, vector unsigned int *);
void vec_st (vector signed short, int, short *);
void vec_st (vector signed short, int, vector unsigned short *);
void vec_st (vector signed short, int, vector signed short *);
void vec_st (vector unsigned short, int, unsigned short *);
void vec_st (vector unsigned short, int, vector unsigned short *);
void vec_st (vector signed char, int, signed char *);
void vec_st (vector signed char, int, unsigned char *);
void vec_st (vector signed char, int, vector signed char *);
void vec_st (vector unsigned char, int, unsigned char *);
void vec_st (vector unsigned char, int, vector unsigned char *);
void vec_ste (vector signed char, int, unsigned char *);
void vec_ste (vector signed char, int, signed char *);
void vec_ste (vector unsigned char, int, unsigned char *);
void vec_ste (vector signed short, int, short *);
void vec_ste (vector signed short, int, unsigned short *);
void vec_ste (vector unsigned short, int, void *);
void vec_ste (vector signed int, int, unsigned int *);
void vec_ste (vector signed int, int, int *);
void vec_ste (vector unsigned int, int, unsigned int *);
void vec_ste (vector float, int, float *);
void vec_stl (vector float, int, vector float *);
void vec_stl (vector float, int, float *);
void vec_stl (vector signed int, int, vector signed int *);
void vec_stl (vector signed int, int, int *);
void vec_stl (vector signed int, int, unsigned int *);
void vec_stl (vector unsigned int, int, vector unsigned int *);
void vec_stl (vector unsigned int, int, unsigned int *);
void vec_stl (vector signed short, int, short *);
void vec_stl (vector signed short, int, unsigned short *);
void vec_stl (vector signed short, int, vector signed short *);
void vec_stl (vector unsigned short, int, unsigned short *);
void vec_stl (vector unsigned short, int, vector signed short *);
void vec_stl (vector signed char, int, signed char *);
void vec_stl (vector signed char, int, unsigned char *);
void vec_stl (vector signed char, int, vector signed char *);
void vec_stl (vector unsigned char, int, unsigned char *);
void vec_stl (vector unsigned char, int, vector unsigned char *);
vector signed char vec_sub (vector signed char, vector signed char);
vector unsigned char vec_sub (vector signed char, vector unsigned char);
vector unsigned char vec_sub (vector unsigned char, vector signed char);
vector unsigned char vec_sub (vector unsigned char,
vector unsigned char);
vector signed short vec_sub (vector signed short, vector signed short);
vector unsigned short vec_sub (vector signed short,
vector unsigned short);
vector unsigned short vec_sub (vector unsigned short,
vector signed short);
vector unsigned short vec_sub (vector unsigned short,
vector unsigned short);
vector signed int vec_sub (vector signed int, vector signed int);
vector unsigned int vec_sub (vector signed int, vector unsigned int);
vector unsigned int vec_sub (vector unsigned int, vector signed int);
vector unsigned int vec_sub (vector unsigned int, vector unsigned int);
vector float vec_sub (vector float, vector float);
vector unsigned int vec_subc (vector unsigned int, vector unsigned int);
vector unsigned char vec_subs (vector signed char,
vector unsigned char);
vector unsigned char vec_subs (vector unsigned char,
vector signed char);
vector unsigned char vec_subs (vector unsigned char,
vector unsigned char);
vector signed char vec_subs (vector signed char, vector signed char);
vector unsigned short vec_subs (vector signed short,
vector unsigned short);
vector unsigned short vec_subs (vector unsigned short,
vector signed short);
vector unsigned short vec_subs (vector unsigned short,
vector unsigned short);
vector signed short vec_subs (vector signed short, vector signed short);
vector unsigned int vec_subs (vector signed int, vector unsigned int);
vector unsigned int vec_subs (vector unsigned int, vector signed int);
vector unsigned int vec_subs (vector unsigned int, vector unsigned int);
vector signed int vec_subs (vector signed int, vector signed int);
vector unsigned int vec_sum4s (vector unsigned char,
vector unsigned int);
vector signed int vec_sum4s (vector signed char, vector signed int);
vector signed int vec_sum4s (vector signed short, vector signed int);
vector signed int vec_sum2s (vector signed int, vector signed int);
vector signed int vec_sums (vector signed int, vector signed int);
vector float vec_trunc (vector float);
vector signed short vec_unpackh (vector signed char);
vector unsigned int vec_unpackh (vector signed short);
vector signed int vec_unpackh (vector signed short);
vector signed short vec_unpackl (vector signed char);
vector unsigned int vec_unpackl (vector signed short);
vector signed int vec_unpackl (vector signed short);
vector float vec_xor (vector float, vector float);
vector float vec_xor (vector float, vector signed int);
vector float vec_xor (vector signed int, vector float);
vector signed int vec_xor (vector signed int, vector signed int);
vector unsigned int vec_xor (vector signed int, vector unsigned int);
vector unsigned int vec_xor (vector unsigned int, vector signed int);
vector unsigned int vec_xor (vector unsigned int, vector unsigned int);
vector signed short vec_xor (vector signed short, vector signed short);
vector unsigned short vec_xor (vector signed short,
vector unsigned short);
vector unsigned short vec_xor (vector unsigned short,
vector signed short);
vector unsigned short vec_xor (vector unsigned short,
vector unsigned short);
vector signed char vec_xor (vector signed char, vector signed char);
vector unsigned char vec_xor (vector signed char, vector unsigned char);
vector unsigned char vec_xor (vector unsigned char, vector signed char);
vector unsigned char vec_xor (vector unsigned char,
vector unsigned char);
vector signed int vec_all_eq (vector signed char, vector unsigned char);
vector signed int vec_all_eq (vector signed char, vector signed char);
vector signed int vec_all_eq (vector unsigned char, vector signed char);
vector signed int vec_all_eq (vector unsigned char,
vector unsigned char);
vector signed int vec_all_eq (vector signed short,
vector unsigned short);
vector signed int vec_all_eq (vector signed short, vector signed short);
vector signed int vec_all_eq (vector unsigned short,
vector signed short);
vector signed int vec_all_eq (vector unsigned short,
vector unsigned short);
vector signed int vec_all_eq (vector signed int, vector unsigned int);
vector signed int vec_all_eq (vector signed int, vector signed int);
vector signed int vec_all_eq (vector unsigned int, vector signed int);
vector signed int vec_all_eq (vector unsigned int, vector unsigned int);
vector signed int vec_all_eq (vector float, vector float);
vector signed int vec_all_ge (vector signed char, vector unsigned char);
vector signed int vec_all_ge (vector unsigned char, vector signed char);
vector signed int vec_all_ge (vector unsigned char,
vector unsigned char);
vector signed int vec_all_ge (vector signed char, vector signed char);
vector signed int vec_all_ge (vector signed short,
vector unsigned short);
vector signed int vec_all_ge (vector unsigned short,
vector signed short);
vector signed int vec_all_ge (vector unsigned short,
vector unsigned short);
vector signed int vec_all_ge (vector signed short, vector signed short);
vector signed int vec_all_ge (vector signed int, vector unsigned int);
vector signed int vec_all_ge (vector unsigned int, vector signed int);
vector signed int vec_all_ge (vector unsigned int, vector unsigned int);
vector signed int vec_all_ge (vector signed int, vector signed int);
vector signed int vec_all_ge (vector float, vector float);
vector signed int vec_all_gt (vector signed char, vector unsigned char);
vector signed int vec_all_gt (vector unsigned char, vector signed char);
vector signed int vec_all_gt (vector unsigned char,
vector unsigned char);
vector signed int vec_all_gt (vector signed char, vector signed char);
vector signed int vec_all_gt (vector signed short,
vector unsigned short);
vector signed int vec_all_gt (vector unsigned short,
vector signed short);
vector signed int vec_all_gt (vector unsigned short,
vector unsigned short);
vector signed int vec_all_gt (vector signed short, vector signed short);
vector signed int vec_all_gt (vector signed int, vector unsigned int);
vector signed int vec_all_gt (vector unsigned int, vector signed int);
vector signed int vec_all_gt (vector unsigned int, vector unsigned int);
vector signed int vec_all_gt (vector signed int, vector signed int);
vector signed int vec_all_gt (vector float, vector float);
vector signed int vec_all_in (vector float, vector float);
vector signed int vec_all_le (vector signed char, vector unsigned char);
vector signed int vec_all_le (vector unsigned char, vector signed char);
vector signed int vec_all_le (vector unsigned char,
vector unsigned char);
vector signed int vec_all_le (vector signed char, vector signed char);
vector signed int vec_all_le (vector signed short,
vector unsigned short);
vector signed int vec_all_le (vector unsigned short,
vector signed short);
vector signed int vec_all_le (vector unsigned short,
vector unsigned short);
vector signed int vec_all_le (vector signed short, vector signed short);
vector signed int vec_all_le (vector signed int, vector unsigned int);
vector signed int vec_all_le (vector unsigned int, vector signed int);
vector signed int vec_all_le (vector unsigned int, vector unsigned int);
vector signed int vec_all_le (vector signed int, vector signed int);
vector signed int vec_all_le (vector float, vector float);
vector signed int vec_all_lt (vector signed char, vector unsigned char);
vector signed int vec_all_lt (vector unsigned char, vector signed char);
vector signed int vec_all_lt (vector unsigned char,
vector unsigned char);
vector signed int vec_all_lt (vector signed char, vector signed char);
vector signed int vec_all_lt (vector signed short,
vector unsigned short);
vector signed int vec_all_lt (vector unsigned short,
vector signed short);
vector signed int vec_all_lt (vector unsigned short,
vector unsigned short);
vector signed int vec_all_lt (vector signed short, vector signed short);
vector signed int vec_all_lt (vector signed int, vector unsigned int);
vector signed int vec_all_lt (vector unsigned int, vector signed int);
vector signed int vec_all_lt (vector unsigned int, vector unsigned int);
vector signed int vec_all_lt (vector signed int, vector signed int);
vector signed int vec_all_lt (vector float, vector float);
vector signed int vec_all_nan (vector float);
vector signed int vec_all_ne (vector signed char, vector unsigned char);
vector signed int vec_all_ne (vector signed char, vector signed char);
vector signed int vec_all_ne (vector unsigned char, vector signed char);
vector signed int vec_all_ne (vector unsigned char,
vector unsigned char);
vector signed int vec_all_ne (vector signed short,
vector unsigned short);
vector signed int vec_all_ne (vector signed short, vector signed short);
vector signed int vec_all_ne (vector unsigned short,
vector signed short);
vector signed int vec_all_ne (vector unsigned short,
vector unsigned short);
vector signed int vec_all_ne (vector signed int, vector unsigned int);
vector signed int vec_all_ne (vector signed int, vector signed int);
vector signed int vec_all_ne (vector unsigned int, vector signed int);
vector signed int vec_all_ne (vector unsigned int, vector unsigned int);
vector signed int vec_all_ne (vector float, vector float);
vector signed int vec_all_nge (vector float, vector float);
vector signed int vec_all_ngt (vector float, vector float);
vector signed int vec_all_nle (vector float, vector float);
vector signed int vec_all_nlt (vector float, vector float);
vector signed int vec_all_numeric (vector float);
vector signed int vec_any_eq (vector signed char, vector unsigned char);
vector signed int vec_any_eq (vector signed char, vector signed char);
vector signed int vec_any_eq (vector unsigned char, vector signed char);
vector signed int vec_any_eq (vector unsigned char,
vector unsigned char);
vector signed int vec_any_eq (vector signed short,
vector unsigned short);
vector signed int vec_any_eq (vector signed short, vector signed short);
vector signed int vec_any_eq (vector unsigned short,
vector signed short);
vector signed int vec_any_eq (vector unsigned short,
vector unsigned short);
vector signed int vec_any_eq (vector signed int, vector unsigned int);
vector signed int vec_any_eq (vector signed int, vector signed int);
vector signed int vec_any_eq (vector unsigned int, vector signed int);
vector signed int vec_any_eq (vector unsigned int, vector unsigned int);
vector signed int vec_any_eq (vector float, vector float);
vector signed int vec_any_ge (vector signed char, vector unsigned char);
vector signed int vec_any_ge (vector unsigned char, vector signed char);
vector signed int vec_any_ge (vector unsigned char,
vector unsigned char);
vector signed int vec_any_ge (vector signed char, vector signed char);
vector signed int vec_any_ge (vector signed short,
vector unsigned short);
vector signed int vec_any_ge (vector unsigned short,
vector signed short);
vector signed int vec_any_ge (vector unsigned short,
vector unsigned short);
vector signed int vec_any_ge (vector signed short, vector signed short);
vector signed int vec_any_ge (vector signed int, vector unsigned int);
vector signed int vec_any_ge (vector unsigned int, vector signed int);
vector signed int vec_any_ge (vector unsigned int, vector unsigned int);
vector signed int vec_any_ge (vector signed int, vector signed int);
vector signed int vec_any_ge (vector float, vector float);
vector signed int vec_any_gt (vector signed char, vector unsigned char);
vector signed int vec_any_gt (vector unsigned char, vector signed char);
vector signed int vec_any_gt (vector unsigned char,
vector unsigned char);
vector signed int vec_any_gt (vector signed char, vector signed char);
vector signed int vec_any_gt (vector signed short,
vector unsigned short);
vector signed int vec_any_gt (vector unsigned short,
vector signed short);
vector signed int vec_any_gt (vector unsigned short,
vector unsigned short);
vector signed int vec_any_gt (vector signed short, vector signed short);
vector signed int vec_any_gt (vector signed int, vector unsigned int);
vector signed int vec_any_gt (vector unsigned int, vector signed int);
vector signed int vec_any_gt (vector unsigned int, vector unsigned int);
vector signed int vec_any_gt (vector signed int, vector signed int);
vector signed int vec_any_gt (vector float, vector float);
vector signed int vec_any_le (vector signed char, vector unsigned char);
vector signed int vec_any_le (vector unsigned char, vector signed char);
vector signed int vec_any_le (vector unsigned char,
vector unsigned char);
vector signed int vec_any_le (vector signed char, vector signed char);
vector signed int vec_any_le (vector signed short,
vector unsigned short);
vector signed int vec_any_le (vector unsigned short,
vector signed short);
vector signed int vec_any_le (vector unsigned short,
vector unsigned short);
vector signed int vec_any_le (vector signed short, vector signed short);
vector signed int vec_any_le (vector signed int, vector unsigned int);
vector signed int vec_any_le (vector unsigned int, vector signed int);
vector signed int vec_any_le (vector unsigned int, vector unsigned int);
vector signed int vec_any_le (vector signed int, vector signed int);
vector signed int vec_any_le (vector float, vector float);
vector signed int vec_any_lt (vector signed char, vector unsigned char);
vector signed int vec_any_lt (vector unsigned char, vector signed char);
vector signed int vec_any_lt (vector unsigned char,
vector unsigned char);
vector signed int vec_any_lt (vector signed char, vector signed char);
vector signed int vec_any_lt (vector signed short,
vector unsigned short);
vector signed int vec_any_lt (vector unsigned short,
vector signed short);
vector signed int vec_any_lt (vector unsigned short,
vector unsigned short);
vector signed int vec_any_lt (vector signed short, vector signed short);
vector signed int vec_any_lt (vector signed int, vector unsigned int);
vector signed int vec_any_lt (vector unsigned int, vector signed int);
vector signed int vec_any_lt (vector unsigned int, vector unsigned int);
vector signed int vec_any_lt (vector signed int, vector signed int);
vector signed int vec_any_lt (vector float, vector float);
vector signed int vec_any_nan (vector float);
vector signed int vec_any_ne (vector signed char, vector unsigned char);
vector signed int vec_any_ne (vector signed char, vector signed char);
vector signed int vec_any_ne (vector unsigned char, vector signed char);
vector signed int vec_any_ne (vector unsigned char,
vector unsigned char);
vector signed int vec_any_ne (vector signed short,
vector unsigned short);
vector signed int vec_any_ne (vector signed short, vector signed short);
vector signed int vec_any_ne (vector unsigned short,
vector signed short);
vector signed int vec_any_ne (vector unsigned short,
vector unsigned short);
vector signed int vec_any_ne (vector signed int, vector unsigned int);
vector signed int vec_any_ne (vector signed int, vector signed int);
vector signed int vec_any_ne (vector unsigned int, vector signed int);
vector signed int vec_any_ne (vector unsigned int, vector unsigned int);
vector signed int vec_any_ne (vector float, vector float);
vector signed int vec_any_nge (vector float, vector float);
vector signed int vec_any_ngt (vector float, vector float);
vector signed int vec_any_nle (vector float, vector float);
vector signed int vec_any_nlt (vector float, vector float);
vector signed int vec_any_numeric (vector float);
vector signed int vec_any_out (vector float, vector float);
GCC supports several types of pragmas, primarily in order to compile code originally written for other compilers. Note that in general we do not recommend the use of pragmas; See Function Attributes, for further explanation.
The ARM target defines pragmas for controlling the default addition of
long_call and short_call attributes to functions.
See Function Attributes, for information about the effects of these
attributes.
long_callslong_call attribute.
no_long_callsshort_call attribute.
long_calls_offlong_call or short_call attributes of
subsequent functions.
The RS/6000 and PowerPC targets define one pragma for controlling
whether or not the longcall attribute is added to function
declarations by default. This pragma overrides the -mlongcall
option, but not the longcall and shortcall attributes.
See RS/6000 and PowerPC Options, for more information about when long
calls are and are not necessary.
longcall (1)longcall attribute to all subsequent function
declarations.
longcall (0)longcall attribute to subsequent function
declarations.
The following pragmas are available for all architectures running the Darwin operating system. These are useful for compatibility with other Mac OS compilers.
mark tokens...options align=alignmentmac68k, to emulate m68k alignment, or
power, to emulate PowerPC alignment. Uses of this pragma nest
properly; to restore the previous setting, use reset for the
alignment.
segment tokens...unused (var [, var]...)unused attribute, except that this pragma may appear
anywhere within the variables' scopes.
For compatibility with the SunPRO compiler, the following pragma is supported.
redefine_extname oldname newname__PRAGMA_REDEFINE_EXTNAME
if the pragma is available.
For compatibility with the Compaq C compiler, the following pragma is supported.
extern_prefix stringextern_prefix pragma with the
empty string.
This pragma is similar in intent to to the asm labels extension
(see Asm Labels) in that the system programmer wants to change
the assembly-level ABI without changing the source-level API. The
preprocessor defines __PRAGMA_EXTERN_PREFIX if the pragma is
available.
For compatibility with other compilers, GCC allows you to define a structure or union that contains, as fields, structures and unions without names. For example:
struct {
int a;
union {
int b;
float c;
};
int d;
} foo;
In this example, the user would be able to access members of the unnamed
union with code like foo.b. Note that only unnamed structs and
unions are allowed, you may not have, for example, an unnamed
int.
You must never create such structures that cause ambiguous field definitions. For example, this structure:
struct {
int a;
struct {
int a;
};
} foo;
It is ambiguous which a is being referred to with foo.a.
Such constructs are not supported and must be avoided. In the future,
such constructs may be detected and treated as compilation errors.
Thread-local storage (TLS) is a mechanism by which variables are allocated such that there is one instance of the variable per extant thread. The run-time model GCC uses to implement this originates in the IA-64 processor-specific ABI, but has since been migrated to other processors as well. It requires significant support from the linker (ld), dynamic linker (ld.so), and system libraries (libc.so and libpthread.so), so it is not available everywhere.
At the user level, the extension is visible with a new storage
class keyword: __thread. For example:
__thread int i;
extern __thread struct state s;
static __thread char *p;
The __thread specifier may be used alone, with the extern
or static specifiers, but with no other storage class specifier.
When used with extern or static, __thread must appear
immediately after the other storage class specifier.
The __thread specifier may be applied to any global, file-scoped
static, function-scoped static, or static data member of a class. It may
not be applied to block-scoped automatic or non-static data member.
When the address-of operator is applied to a thread-local variable, it is evaluated at run-time and returns the address of the current thread's instance of that variable. An address so obtained may be used by any thread. When a thread terminates, any pointers to thread-local variables in that thread become invalid.
No static initialization may refer to the address of a thread-local variable.
In C++, if an initializer is present for a thread-local variable, it must be a constant-expression, as defined in 5.19.2 of the ANSI/ISO C++ standard.
See ELF Handling For Thread-Local Storage for a detailed explanation of the four thread-local storage addressing models, and how the run-time is expected to function.
The following are a set of changes to ISO/IEC 9899:1999 (aka C99) that document the exact semantics of the language extension.
Add new text after paragraph 1
Within either execution environment, a thread is a flow of control within a program. It is implementation defined whether or not there may be more than one thread associated with a program. It is implementation defined how threads beyond the first are created, the name and type of the function called at thread startup, and how threads may be terminated. However, objects with thread storage duration shall be initialized before thread startup.
Add new text before paragraph 3
An object whose identifier is declared with the storage-class
specifier __thread has thread storage duration.
Its lifetime is the entire execution of the thread, and its
stored value is initialized only once, prior to thread startup.
Add __thread.
Add __thread to the list of storage class specifiers in
paragraph 1.
Change paragraph 2 to
With the exception of__thread, at most one storage-class specifier may be given [...]. The__threadspecifier may be used alone, or immediately followingexternorstatic.
Add new text after paragraph 6
The declaration of an identifier for a variable that has block scope that specifies__threadshall also specify eitherexternorstatic.The
__threadspecifier shall be used only with variables.
The following are a set of changes to ISO/IEC 14882:1998 (aka C++98) that document the exact semantics of the language extension.
New text after paragraph 4
A thread is a flow of control within the abstract machine. It is implementation defined whether or not there may be more than one thread.
New text after paragraph 7
It is unspecified whether additional action must be taken to ensure when and whether side effects are visible to other threads.
Add __thread.
Add after paragraph 5
The thread that begins execution at themainfunction is called the main thread. It is implementation defined how functions beginning threads other than the main thread are designated or typed. A function so designated, as well as themainfunction, is called a thread startup function. It is implementation defined what happens if a thread startup function returns. It is implementation defined what happens to other threads when any thread callsexit.
Add after paragraph 4
The storage for an object of thread storage duration shall be statically initialized before the first statement of the thread startup function. An object of thread storage duration shall not require dynamic initialization.
Add after paragraph 3
The type of an object with thread storage duration shall not have a non-trivial destructor, nor shall it be an array type whose elements (directly or indirectly) have non-trivial destructors.
Add “thread storage duration” to the list in paragraph 1.
Change paragraph 2
Thread, static, and automatic storage durations are associated with objects introduced by declarations [...].
Add __thread to the list of specifiers in paragraph 3.
New section before [basic.stc.static]
The keyword__threadapplied to a non-local object gives the object thread storage duration.A local variable or class data member declared both
staticand__threadgives the variable or member thread storage duration.
Change paragraph 1
All objects which have neither thread storage duration, dynamic storage duration nor are local [...].
Add __thread to the list in paragraph 1.
Change paragraph 1
With the exception of__thread, at most one storage-class-specifier shall appear in a given decl-specifier-seq. The__threadspecifier may be used alone, or immediately following theexternorstaticspecifiers. [...]
Add after paragraph 5
The __thread specifier can be applied only to the names of objects
and to anonymous unions.
Add after paragraph 6
Non-staticmembers shall not be__thread.
The GNU compiler provides these extensions to the C++ language (and you
can also use most of the C language extensions in your C++ programs). If you
want to write code that checks whether these features are available, you can
test for the GNU compiler the same way as for C programs: check for a
predefined macro __GNUC__. You can also use __GNUG__ to
test specifically for GNU C++ (see Predefined Macros (The GNU C Preprocessor)).
It is very convenient to have operators which return the “minimum” or the “maximum” of two arguments. In GNU C++ (but not in GNU C),
<? b >? bThese operations are not primitive in ordinary C++, since you can use a macro to return the minimum of two things in C++, as in the following example.
#define MIN(X,Y) ((X) < (Y) ? : (X) : (Y))
You might then use int min = MIN (i, j); to set min to the minimum value of variables i and j.
However, side effects in X or Y may cause unintended
behavior. For example, MIN (i++, j++) will fail, incrementing
the smaller counter twice. The GNU C typeof extension allows you
to write safe macros that avoid this kind of problem (see Typeof).
However, writing MIN and MAX as macros also forces you to
use function-call notation for a fundamental arithmetic operation.
Using GNU C++ extensions, you can write int min = i <? j;
instead.
Since <? and >? are built into the compiler, they properly
handle expressions with side-effects; int min = i++ <? j++;
works correctly.
Both the C and C++ standard have the concept of volatile objects. These are normally accessed by pointers and used for accessing hardware. The standards encourage compilers to refrain from optimizations concerning accesses to volatile objects that it might perform on non-volatile objects. The C standard leaves it implementation defined as to what constitutes a volatile access. The C++ standard omits to specify this, except to say that C++ should behave in a similar manner to C with respect to volatiles, where possible. The minimum either standard specifies is that at a sequence point all previous accesses to volatile objects have stabilized and no subsequent accesses have occurred. Thus an implementation is free to reorder and combine volatile accesses which occur between sequence points, but cannot do so for accesses across a sequence point. The use of volatiles does not allow you to violate the restriction on updating objects multiple times within a sequence point.
In most expressions, it is intuitively obvious what is a read and what is a write. For instance
volatile int *dst = somevalue;
volatile int *src = someothervalue;
*dst = *src;
will cause a read of the volatile object pointed to by src and stores the
value into the volatile object pointed to by dst. There is no
guarantee that these reads and writes are atomic, especially for objects
larger than int.
Less obvious expressions are where something which looks like an access is used in a void context. An example would be,
volatile int *src = somevalue;
*src;
With C, such expressions are rvalues, and as rvalues cause a read of the object, GCC interprets this as a read of the volatile being pointed to. The C++ standard specifies that such expressions do not undergo lvalue to rvalue conversion, and that the type of the dereferenced object may be incomplete. The C++ standard does not specify explicitly that it is this lvalue to rvalue conversion which is responsible for causing an access. However, there is reason to believe that it is, because otherwise certain simple expressions become undefined. However, because it would surprise most programmers, G++ treats dereferencing a pointer to volatile object of complete type in a void context as a read of the object. When the object has incomplete type, G++ issues a warning.
struct S;
struct T {int m;};
volatile S *ptr1 = somevalue;
volatile T *ptr2 = somevalue;
*ptr1;
*ptr2;
In this example, a warning is issued for *ptr1, and *ptr2
causes a read of the object pointed to. If you wish to force an error on
the first case, you must force a conversion to rvalue with, for instance
a static cast, static_cast<S>(*ptr1).
When using a reference to volatile, G++ does not treat equivalent expressions as accesses to volatiles, but instead issues a warning that no volatile is accessed. The rationale for this is that otherwise it becomes difficult to determine where volatile access occur, and not possible to ignore the return value from functions returning volatile references. Again, if you wish to force a read, cast the reference to an rvalue.
As with gcc, g++ understands the C99 feature of restricted pointers,
specified with the __restrict__, or __restrict type
qualifier. Because you cannot compile C++ by specifying the -std=c99
language flag, restrict is not a keyword in C++.
In addition to allowing restricted pointers, you can specify restricted references, which indicate that the reference is not aliased in the local context.
void fn (int *__restrict__ rptr, int &__restrict__ rref)
{
/* ... */
}
In the body of fn, rptr points to an unaliased integer and
rref refers to a (different) unaliased integer.
You may also specify whether a member function's this pointer is
unaliased by using __restrict__ as a member function qualifier.
void T::fn () __restrict__
{
/* ... */
}
Within the body of T::fn, this will have the effective
definition T *__restrict__ const this. Notice that the
interpretation of a __restrict__ member function qualifier is
different to that of const or volatile qualifier, in that it
is applied to the pointer rather than the object. This is consistent with
other compilers which implement restricted pointers.
As with all outermost parameter qualifiers, __restrict__ is
ignored in function definition matching. This means you only need to
specify __restrict__ in a function definition, rather than
in a function prototype as well.
There are several constructs in C++ which require space in the object file but are not clearly tied to a single translation unit. We say that these constructs have “vague linkage”. Typically such constructs are emitted wherever they are needed, though sometimes we can be more clever.
Local static variables and string constants used in an inline function
are also considered to have vague linkage, since they must be shared
between all inlined and out-of-line instances of the function.
Note: If the chosen key method is later defined as inline, the
vtable will still be emitted in every translation unit which defines it.
Make sure that any inline virtuals are declared inline in the class
body, even if they are not defined there.
When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU or Solaris 2, or on Microsoft Windows, duplicate copies of these constructs will be discarded at link time. This is known as COMDAT support.
On targets that don't support COMDAT, but do support weak symbols, GCC will use them. This way one copy will override all the others, but the unused copies will still take up space in the executable.
For targets which do not support either COMDAT or weak symbols, most entities with vague linkage will be emitted as local symbols to avoid duplicate definition errors from the linker. This will not happen for local statics in inlines, however, as having multiple copies will almost certainly break things.
See Declarations and Definitions in One Header, for another way to control placement of these constructs.
C++ object definitions can be quite complex. In principle, your source code will need two kinds of things for each object that you use across more than one source file. First, you need an interface specification, describing its structure with type declarations and function prototypes. Second, you need the implementation itself. It can be tedious to maintain a separate interface description in a header file, in parallel to the actual implementation. It is also dangerous, since separate interface and implementation definitions may not remain parallel.
With GNU C++, you can use a single header file for both purposes.
Warning: The mechanism to specify this is in transition. For the nonce, you must use one of two#pragmacommands; in a future release of GNU C++, an alternative mechanism will make these#pragmacommands unnecessary.
The header file contains the full definitions, but is marked with
#pragma interface in the source code. This allows the compiler
to use the header file only as an interface specification when ordinary
source files incorporate it with #include. In the single source
file where the full implementation belongs, you can use either a naming
convention or #pragma implementation to indicate this alternate
use of the header file.
#pragma interface#pragma interface "subdir/objects.h"The second form of this directive is useful for the case where you have
multiple headers with the same name in different directories. If you
use this form, you must specify the same string to #pragma
implementation.
#pragma implementation#pragma implementation "objects.h"If you use #pragma implementation with no argument, it applies to an include file with the same basename4 as your source file. For example, in allclass.cc, giving just #pragma implementation by itself is equivalent to #pragma implementation "allclass.h".
In versions of GNU C++ prior to 2.6.0 allclass.h was treated as an implementation file whenever you would include it from allclass.cc even if you never specified #pragma implementation. This was deemed to be more trouble than it was worth, however, and disabled.
If you use an explicit #pragma implementation, it must appear in your source file before you include the affected header files.
Use the string argument if you want a single implementation file to include code from multiple header files. (You must also use #include to include the header file; #pragma implementation only specifies how to use the file—it doesn't actually include it.)
There is no way to split up the contents of a single header file into multiple implementation files.
#pragma implementation and #pragma interface also have an effect on function inlining.
If you define a class in a header file marked with #pragma
interface, the effect on a function defined in that class is similar to
an explicit extern declaration—the compiler emits no code at
all to define an independent version of the function. Its definition
is used only for inlining with its callers.
Conversely, when you include the same header file in a main source file that declares it as #pragma implementation, the compiler emits code for the function itself; this defines a version of the function that can be found via pointers (or by callers compiled without inlining). If all calls to the function can be inlined, you can avoid emitting the function by compiling with -fno-implement-inlines. If any calls were not inlined, you will get linker errors.
C++ templates are the first language feature to require more intelligence from the environment than one usually finds on a UNIX system. Somehow the compiler and linker have to make sure that each template instance occurs exactly once in the executable if it is needed, and not at all otherwise. There are two basic approaches to this problem, which I will refer to as the Borland model and the Cfront model.
When used with GNU ld version 2.8 or later on an ELF system such as Linux/GNU or Solaris 2, or on Microsoft Windows, g++ supports the Borland model. On other systems, g++ implements neither automatic model.
A future version of g++ will support a hybrid model whereby the compiler will emit any instantiations for which the template definition is included in the compile, and store template definitions and instantiation context information into the object file for the rest. The link wrapper will extract that information as necessary and invoke the compiler to produce the remaining instantiations. The linker will then combine duplicate instantiations.
In the mean time, you have the following options for dealing with template instantiations:
This is your best option for application code written for the Borland
model, as it will just work. Code written for the Cfront model will
need to be modified so that the template definitions are available at
one or more points of instantiation; usually this is as simple as adding
#include <tmethods.cc> to the end of each template header.
For library code, if you want the library to provide all of the template instantiations it needs, just try to link all of its object files together; the link will fail, but cause the instantiations to be generated as a side effect. Be warned, however, that this may cause conflicts if multiple libraries try to provide the same instantiations. For greater control, use explicit instantiation as described in the next option.
#include "Foo.h"
#include "Foo.cc"
template class Foo<int>;
template ostream& operator <<
(ostream&, const Foo<int>&);
for each of the instances you need, and create a template instantiation library from those.
If you are using Cfront-model code, you can probably get away with not using -fno-implicit-templates when compiling files that don't #include the member template definitions.
If you use one big file to do the instantiations, you may want to compile it without -fno-implicit-templates so you get all of the instances required by your explicit instantiations (but not by any other files) without having to specify them as well.
g++ has extended the template instantiation syntax given in the ISO
standard to allow forward declaration of explicit instantiations
(with extern), instantiation of the compiler support data for a
template class (i.e. the vtable) without instantiating any of its
members (with inline), and instantiation of only the static data
members of a template class, without the support data or member
functions (with (static):
extern template int max (int, int);
inline template class Foo<int>;
static template class Foo<int>;
See Declarations and Definitions in One Header, for more discussion of these pragmas.
In C++, pointer to member functions (PMFs) are implemented using a wide pointer of sorts to handle all the possible call mechanisms; the PMF needs to store information about how to adjust the this pointer, and if the function pointed to is virtual, where to find the vtable, and where in the vtable to look for the member function. If you are using PMFs in an inner loop, you should really reconsider that decision. If that is not an option, you can extract the pointer to the function that would be called for a given object/PMF pair and call it directly inside the inner loop, to save a bit of time.
Note that you will still be paying the penalty for the call through a function pointer; on most modern architectures, such a call defeats the branch prediction features of the CPU. This is also true of normal virtual function calls.
The syntax for this extension is
extern A a;
extern int (A::*fp)();
typedef int (*fptr)(A *);
fptr p = (fptr)(a.*fp);
For PMF constants (i.e. expressions of the form &Klasse::Member), no object is needed to obtain the address of the function. They can be converted to function pointers directly:
fptr p1 = (fptr)(&A::foo);
You must specify -Wno-pmf-conversions to use this extension.
Some attributes only make sense for C++ programs.
init_priority (priority)In Standard C++, objects defined at namespace scope are guaranteed to be
initialized in an order in strict accordance with that of their definitions
in a given translation unit. No guarantee is made for initializations
across translation units. However, GNU C++ allows users to control the
order of initialization of objects defined at namespace scope with the
init_priority attribute by specifying a relative priority,
a constant integral expression currently bounded between 101 and 65535
inclusive. Lower numbers indicate a higher priority.
In the following example, A would normally be created before
B, but the init_priority attribute has reversed that order:
Some_Class A __attribute__ ((init_priority (2000)));
Some_Class B __attribute__ ((init_priority (543)));
Note that the particular values of priority do not matter; only their
relative ordering.
java_interfaceextern "Java" block.
Calls to methods declared in this interface will be dispatched using GCJ's
interface table mechanism, instead of regular virtual table dispatch.
The Java language uses a slightly different exception handling model from C++. Normally, GNU C++ will automatically detect when you are writing C++ code that uses Java exceptions, and handle them appropriately. However, if C++ code only needs to execute destructors when Java exceptions are thrown through it, GCC will guess incorrectly. Sample problematic code is:
struct S { ~S(); };
extern void bar(); // is written in Java, and may throw exceptions
void foo()
{
S s;
bar();
}
The usual effect of an incorrect guess is a link failure, complaining of a missing routine called __gxx_personality_v0.
You can inform the compiler that Java exceptions are to be used in a translation unit, irrespective of what it might think, by writing #pragma GCC java_exceptions at the head of the file. This #pragma must appear before any functions that throw or catch exceptions, or run destructors when exceptions are thrown through them.
You cannot mix Java and C++ exceptions in the same translation unit. It is believed to be safe to throw a C++ exception from one file through another file compiled for the Java exception model, or vice versa, but there may be bugs in this area.
In the past, the GNU C++ compiler was extended to experiment with new features, at a time when the C++ language was still evolving. Now that the C++ standard is complete, some of those features are superseded by superior alternatives. Using the old features might cause a warning in some cases that the feature will be dropped in the future. In other cases, the feature might be gone already.
While the list below is not exhaustive, it documents some of the options that are now deprecated:
-fexternal-templates-falt-external-templates-fstrict-prototype-fno-strict-prototypeThe named return value extension has been deprecated, and is now removed from g++.
The use of initializer lists with new expressions has been deprecated, and is now removed from g++.
Floating and complex non-type template parameters have been deprecated, and are now removed from g++.
The implicit typename extension has been deprecated and will be removed
from g++ at some point. In some cases g++ determines that a dependent
type such as TPL<T>::X is a type without needing a
typename keyword, contrary to the standard.
Now that there is a definitive ISO standard C++, G++ has a specification to adhere to. The C++ language evolved over time, and features that used to be acceptable in previous drafts of the standard, such as the ARM [Annotated C++ Reference Manual], are no longer accepted. In order to allow compilation of C++ written to such drafts, G++ contains some backwards compatibilities. All such backwards compatibility features are liable to disappear in future versions of G++. They should be considered deprecated See Deprecated Features.
For scopeImplicit C languageextern "C" {...}
scope to set the language. On such systems, all header files are
implicitly scoped inside a C language scope. Also, an empty prototype
() will be treated as an unspecified number of arguments, rather
than no arguments, as C++ demands.
This document is meant to describe some of the GNU Objective-C runtime features. It is not intended to teach you Objective-C, there are several resources on the Internet that present the language. Questions and comments about this document to Ovidiu Predescu ovidiu@cup.hp.com.
+load: Executing code before mainThe GNU Objective-C runtime provides a way that allows you to execute
code before the execution of the program enters the main
function. The code is executed on a per-class and a per-category basis,
through a special class method +load.
This facility is very useful if you want to initialize global variables
which can be accessed by the program directly, without sending a message
to the class first. The usual way to initialize global variables, in the
+initialize method, might not be useful because
+initialize is only called when the first message is sent to a
class object, which in some cases could be too late.
Suppose for example you have a FileStream class that declares
Stdin, Stdout and Stderr as global variables, like
below:
FileStream *Stdin = nil;
FileStream *Stdout = nil;
FileStream *Stderr = nil;
@implementation FileStream
+ (void)initialize
{
Stdin = [[FileStream new] initWithFd:0];
Stdout = [[FileStream new] initWithFd:1];
Stderr = [[FileStream new] initWithFd:2];
}
/* Other methods here */
@end
In this example, the initialization of Stdin, Stdout and
Stderr in +initialize occurs too late. The programmer can
send a message to one of these objects before the variables are actually
initialized, thus sending messages to the nil object. The
+initialize method which actually initializes the global
variables is not invoked until the first message is sent to the class
object. The solution would require these variables to be initialized
just before entering main.
The correct solution of the above problem is to use the +load
method instead of +initialize:
@implementation FileStream
+ (void)load
{
Stdin = [[FileStream new] initWithFd:0];
Stdout = [[FileStream new] initWithFd:1];
Stderr = [[FileStream new] initWithFd:2];
}
/* Other methods here */
@end
The +load is a method that is not overridden by categories. If a
class and a category of it both implement +load, both methods are
invoked. This allows some additional initializations to be performed in
a category.
This mechanism is not intended to be a replacement for +initialize.
You should be aware of its limitations when you decide to use it
instead of +initialize.
+loadThe +load implementation in the GNU runtime guarantees you the following
things:
@"this is a
constant string");
+load implementation of all super classes of a class are executed before the +load of that class is executed;
+load implementation of a class is executed before the
+load implementation of any category.
In particular, the following things, even if they can work in a particular case, are not guaranteed:
You should make no assumptions about receiving +load in sibling
classes when you write +load of a class. The order in which
sibling classes receive +load is not guaranteed.
The order in which +load and +initialize are called could
be problematic if this matters. If you don't allocate objects inside
+load, it is guaranteed that +load is called before
+initialize. If you create an object inside +load the
+initialize method of object's class is invoked even if
+load was not invoked. Note if you explicitly call +load
on a class, +initialize will be called first. To avoid possible
problems try to implement only one of these methods.
The +load method is also invoked when a bundle is dynamically
loaded into your running program. This happens automatically without any
intervening operation from you. When you write bundles and you need to
write +load you can safely create and send messages to objects whose
classes already exist in the running program. The same restrictions as
above apply to classes defined in bundle.
The Objective-C compiler generates type encodings for all the types. These type encodings are used at runtime to find out information about selectors and methods and about objects and classes.
The types are encoded in the following way:
char
| c
|
unsigned char
| C
|
short
| s
|
unsigned short
| S
|
int
| i
|
unsigned int
| I
|
long
| l
|
unsigned long
| L
|
long long
| q
|
unsigned long long
| Q
|
float
| f
|
double
| d
|
void
| v
|
id
| @
|
Class
| #
|
SEL
| :
|
char*
| *
|
| unknown type | ?
|
| bit-fields | b followed by the starting position of the bit-field, the type of the bit-field and the size of the bit-field (the bit-fields encoding was changed from the NeXT's compiler encoding, see below)
|
The encoding of bit-fields has changed to allow bit-fields to be properly handled by the runtime functions that compute sizes and alignments of types that contain bit-fields. The previous encoding contained only the size of the bit-field. Using only this information it is not possible to reliably compute the size occupied by the bit-field. This is very important in the presence of the Boehm's garbage collector because the objects are allocated using the typed memory facility available in this collector. The typed memory allocation requires information about where the pointers are located inside the object.
The position in the bit-field is the position, counting in bits, of the bit closest to the beginning of the structure.
The non-atomic types are encoded as follows:
| pointers | ^ followed by the pointed type.
|
| arrays | [ followed by the number of elements in the array followed by the type of the elements followed by ]
|
| structures | { followed by the name of the structure (or ? if the structure is unnamed), the = sign, the type of the members and by }
|
| unions | ( followed by the name of the structure (or ? if the union is unnamed), the = sign, the type of the members followed by )
|
Here are some types and their encodings, as they are generated by the compiler on an i386 machine:
| Objective-C type | Compiler encoding
|
int a[10]; | [10i]
|
struct {
int i;
float f[3];
int a:3;
int b:2;
char c;
}
| {?=i[3f]b128i3b131i2c}
|
In addition to the types the compiler also encodes the type specifiers. The table below describes the encoding of the current Objective-C type specifiers:
| Specifier | Encoding
|
const
| r
|
in
| n
|
inout
| N
|
out
| o
|
bycopy
| O
|
oneway
| V
|
The type specifiers are encoded just before the type. Unlike types however, the type specifiers are only encoded when they appear in method argument types.
Support for a new memory management policy has been added by using a powerful conservative garbage collector, known as the Boehm-Demers-Weiser conservative garbage collector. It is available from http://www.hpl.hp.com/personal/Hans_Boehm/gc/.
To enable the support for it you have to configure the compiler using an additional argument, --enable-objc-gc. You need to have garbage collector installed before building the compiler. This will build an additional runtime library which has several enhancements to support the garbage collector. The new library has a new name, libobjc_gc.a to not conflict with the non-garbage-collected library.
When the garbage collector is used, the objects are allocated using the so-called typed memory allocation mechanism available in the Boehm-Demers-Weiser collector. This mode requires precise information on where pointers are located inside objects. This information is computed once per class, immediately after the class has been initialized.
There is a new runtime function class_ivar_set_gcinvisible()
which can be used to declare a so-called weak pointer
reference. Such a pointer is basically hidden for the garbage collector;
this can be useful in certain situations, especially when you want to
keep track of the allocated objects, yet allow them to be
collected. This kind of pointers can only be members of objects, you
cannot declare a global pointer as a weak reference. Every type which is
a pointer type can be declared a weak pointer, including id,
Class and SEL.
Here is an example of how to use this feature. Suppose you want to implement a class whose instances hold a weak pointer reference; the following class does this:
@interface WeakPointer : Object
{
const void* weakPointer;
}
- initWithPointer:(const void*)p;
- (const void*)weakPointer;
@end
@implementation WeakPointer
+ (void)initialize
{
class_ivar_set_gcinvisible (self, "weakPointer", YES);
}
- initWithPointer:(const void*)p
{
weakPointer = p;
return self;
}
- (const void*)weakPointer
{
return weakPointer;
}
@end
Weak pointers are supported through a new type character specifier
represented by the ! character. The
class_ivar_set_gcinvisible() function adds or removes this
specifier to the string type description of the instance variable named
as argument.
GNU Objective-C provides constant string objects that are generated directly by the compiler. You declare a constant string object by prefixing a C constant string with the character @:
id myString = @"this is a constant string object";
The constant string objects are by default instances of the
NXConstantString class which is provided by the GNU Objective-C
runtime. To get the definition of this class you must include the
objc/NXConstStr.h header file.
User defined libraries may want to implement their own constant string
class. To be able to support them, the GNU Objective-C compiler provides
a new command line options -fconstant-string-class=class-name.
The provided class should adhere to a strict structure, the same
as NXConstantString's structure:
@interface MyConstantStringClass
{
Class isa;
char *c_string;
unsigned int len;
}
@end
NXConstantString inherits from Object; user class
libraries may choose to inherit the customized constant string class
from a different class than Object. There is no requirement in
the methods the constant string class has to implement, but the final
ivar layout of the class must be the compatible with the given
structure.
When the compiler creates the statically allocated constant string
object, the c_string field will be filled by the compiler with
the string; the length field will be filled by the compiler with
the string length; the isa pointer will be filled with
NULL by the compiler, and it will later be fixed up automatically
at runtime by the GNU Objective-C runtime library to point to the class
which was set by the -fconstant-string-class option when the
object file is loaded (if you wonder how it works behind the scenes, the
name of the class to use, and the list of static objects to fixup, are
stored by the compiler in the object file in a place where the GNU
runtime library will find them at runtime).
As a result, when a file is compiled with the -fconstant-string-class option, all the constant string objects will be instances of the class specified as argument to this option. It is possible to have multiple compilation units referring to different constant string classes, neither the compiler nor the linker impose any restrictions in doing this.
This is a feature of the Objective-C compiler rather than of the runtime, anyway since it is documented nowhere and its existence was forgotten, we are documenting it here.
The keyword @compatibility_alias allows you to define a class name
as equivalent to another class name. For example:
@compatibility_alias WOApplication GSWApplication;
tells the compiler that each time it encounters WOApplication as
a class name, it should replace it with GSWApplication (that is,
WOApplication is just an alias for GSWApplication).
There are some constraints on how this can be used—
WOApplication (the alias) must not be an existing class;
GSWApplication (the real class) must be an existing class.
Binary compatibility encompasses several related concepts:
The application binary interface implemented by a C or C++ compiler affects code generation and runtime support for:
In addition, the application binary interface implemented by a C++ compiler affects code generation and runtime support for:
Some GCC compilation options cause the compiler to generate code that does not conform to the platform's default ABI. Other options cause different program behavior for implementation-defined features that are not covered by an ABI. These options are provided for consistency with other compilers that do not follow the platform's default ABI or the usual behavior of implementation-defined features for the platform. Be very careful about using such options.
Most platforms have a well-defined ABI that covers C code, but ABIs that cover C++ functionality are not yet common.
Starting with GCC 3.2, GCC binary conventions for C++ are based on a
written, vendor-neutral C++ ABI that was designed to be specific to
64-bit Itanium but also includes generic specifications that apply to
any platform.
This C++ ABI is also implemented by other compiler vendors on some
platforms, notably GNU/Linux and BSD systems.
We have tried hard to provide a stable ABI that will be compatible with
future GCC releases, but it is possible that we will encounter problems
that make this difficult. Such problems could include different
interpretations of the C++ ABI by different vendors, bugs in the ABI, or
bugs in the implementation of the ABI in different compilers.
GCC's -Wabi switch warns when G++ generates code that is
probably not compatible with the C++ ABI.
The C++ library used with a C++ compiler includes the Standard C++ Library, with functionality defined in the C++ Standard, plus language runtime support. The runtime support is included in a C++ ABI, but there is no formal ABI for the Standard C++ Library. Two implementations of that library are interoperable if one follows the de-facto ABI of the other and if they are both built with the same compiler, or with compilers that conform to the same ABI for C++ compiler and runtime support.
When G++ and another C++ compiler conform to the same C++ ABI, but the implementations of the Standard C++ Library that they normally use do not follow the same ABI for the Standard C++ Library, object files built with those compilers can be used in the same program only if they use the same C++ library. This requires specifying the location of the C++ library header files when invoking the compiler whose usual library is not being used. The location of GCC's C++ header files depends on how the GCC build was configured, but can be seen by using the G++ -v option. With default configuration options for G++ 3.3 the compile line for a different C++ compiler needs to include
-Igcc_install_directory/include/c++/3.3
Similarly, compiling code with G++ that must use a C++ library other than the GNU C++ library requires specifying the location of the header files for that other library.
The most straightforward way to link a program to use a particular C++ library is to use a C++ driver that specifies that C++ library by default. The g++ driver, for example, tells the linker where to find GCC's C++ library (libstdc++) plus the other libraries and startup files it needs, in the proper order.
If a program must use a different C++ library and it's not possible to do the final link using a C++ driver that uses that library by default, it is necessary to tell g++ the location and name of that library. It might also be necessary to specify different startup files and other runtime support libraries, and to suppress the use of GCC's support libraries with one or more of the options -nostdlib, -nostartfiles, and -nodefaultlibs.
gcov is a tool you can use in conjunction with GCC to test code coverage in your programs.
gcov is a test coverage program. Use it in concert with GCC to analyze your programs to help create more efficient, faster running code and to discover untested parts of your program. You can use gcov as a profiling tool to help discover where your optimization efforts will best affect your code. You can also use gcov along with the other profiling tool, gprof, to assess which parts of your code use the greatest amount of computing time.
Profiling tools help you analyze your code's performance. Using a profiler such as gcov or gprof, you can find out some basic performance statistics, such as:
Once you know these things about how your code works when compiled, you can look at each module to see which modules should be optimized. gcov helps you determine where to work on optimization.
Software developers also use coverage testing in concert with testsuites, to make sure software is actually good enough for a release. Testsuites can verify that a program works as expected; a coverage program tests to see how much of the program is exercised by the testsuite. Developers can then determine what kinds of test cases need to be added to the testsuites to create both better testing and a better final product.
You should compile your code without optimization if you plan to use gcov because the optimization, by combining some lines of code into one function, may not give you as much information as you need to look for `hot spots' where the code is using a great deal of computer time. Likewise, because gcov accumulates statistics by line (at the lowest resolution), it works best with a programming style that places only one statement on each line. If you use complicated macros that expand to loops or to other control structures, the statistics are less helpful—they only report on the line where the macro call appears. If your complex macros behave like functions, you can replace them with inline functions to solve this problem.
gcov creates a logfile called sourcefile.gcov which indicates how many times each line of a source file sourcefile.c has executed. You can use these logfiles along with gprof to aid in fine-tuning the performance of your programs. gprof gives timing information you can use along with the information you get from gcov.
gcov works only on code compiled with GCC. It is not compatible with any other profiling or test coverage mechanism.
gcov [options] sourcefile
gcov accepts the following options:
-h--help-v--version-b--branch-probabilities-c--branch-counts-n--no-output-l--long-file-names-p--preserve-paths-f--function-summaries-o directory|file--object-directory directory--object-file filegcov should be run with the current directory the same as that when you invoked the compiler. Otherwise it will not be able to locate the source files. gcov produces files called mangledname.gcov in the current directory. These contain the coverage information of the source file they correspond to. One .gcov file is produced for each source file containing code, which was compiled to produce the data files. The .gcov files contain the ':' separated fields along with program source code. The format is
execution_count:line_number:source line text
Additional block information may succeed each line, when requested by command line option. The execution_count is - for lines containing no code and ##### for lines which were never executed. Some lines of information at the start have line_number of zero.
When printing percentages, 0% and 100% are only printed when the values are exactly 0% and 100% respectively. Other values which would conventionally be rounded to 0% or 100% are instead printed as the nearest non-boundary value.
When using gcov, you must first compile your program with two special GCC options: -fprofile-arcs -ftest-coverage. This tells the compiler to generate additional information needed by gcov (basically a flow graph of the program) and also includes additional code in the object files for generating the extra profiling information needed by gcov. These additional files are placed in the directory where the object file is located.
Running the program will cause profile output to be generated. For each source file compiled with -fprofile-arcs, an accompanying .da file will be placed in the object file directory.
Running gcov with your program's source file names as arguments will now produce a listing of the code along with frequency of execution for each line. For example, if your program is called tmp.c, this is what you see when you use the basic gcov facility:
$ gcc -fprofile-arcs -ftest-coverage tmp.c
$ a.out
$ gcov tmp.c
90.00% of 10 source lines executed in file tmp.c
Creating tmp.c.gcov.
The file tmp.c.gcov contains output from gcov. Here is a sample:
-: 0:Source:tmp.c
-: 0:Object:tmp.bb
-: 1:#include <stdio.h>
-: 2:
-: 3:int main (void)
1: 4:{
1: 5: int i, total;
-: 6:
1: 7: total = 0;
-: 8:
11: 9: for (i = 0; i < 10; i++)
10: 10: total += i;
-: 11:
1: 12: if (total != 45)
#####: 13: printf ("Failure\n");
-: 14: else
1: 15: printf ("Success\n");
1: 16: return 0;
1: 17:}
When you use the -b option, your output looks like this:
$ gcov -b tmp.c
90.00% of 10 source lines executed in file tmp.c
80.00% of 5 branches executed in file tmp.c
80.00% of 5 branches taken at least once in file tmp.c
50.00% of 2 calls executed in file tmp.c
Creating tmp.c.gcov.
Here is a sample of a resulting tmp.c.gcov file:
-: 0:Source:tmp.c
-: 0:Object:tmp.bb
-: 1:#include <stdio.h>
-: 2:
-: 3:int main (void)
1: 4:{
1: 5: int i, total;
-: 6:
1: 7: total = 0;
-: 8:
11: 9: for (i = 0; i < 10; i++)
branch 0: taken 90%
branch 1: taken 100%
branch 2: taken 100%
10: 10: total += i;
-: 11:
1: 12: if (total != 45)
branch 0: taken 100%
#####: 13: printf ("Failure\n");
call 0: never executed
branch 1: never executed
-: 14: else
1: 15: printf ("Success\n");
call 0: returns 100%
1: 16: return 0;
1: 17:}
For each basic block, a line is printed after the last line of the basic block describing the branch or call that ends the basic block. There can be multiple branches and calls listed for a single source line if there are multiple basic blocks that end on that line. In this case, the branches and calls are each given a number. There is no simple way to map these branches and calls back to source constructs. In general, though, the lowest numbered branch or call will correspond to the leftmost construct on the source line.
For a branch, if it was executed at least once, then a percentage indicating the number of times the branch was taken divided by the number of times the branch was executed will be printed. Otherwise, the message “never executed” is printed.
For a call, if it was executed at least once, then a percentage
indicating the number of times the call returned divided by the number
of times the call was executed will be printed. This will usually be
100%, but may be less for functions call exit or longjmp,
and thus may not return every time they are called.
The execution counts are cumulative. If the example program were executed again without removing the .da file, the count for the number of times each line in the source was executed would be added to the results of the previous run(s). This is potentially useful in several ways. For example, it could be used to accumulate data over a number of program runs as part of a test verification suite, or to provide more accurate long-term information over a large number of program runs.
The data in the .da files is saved immediately before the program exits. For each source file compiled with -fprofile-arcs, the profiling code first attempts to read in an existing .da file; if the file doesn't match the executable (differing number of basic block counts) it will ignore the contents of the file. It then adds in the new execution counts and finally writes the data to the file.
If you plan to use gcov to help optimize your code, you must first compile your program with two special GCC options: -fprofile-arcs -ftest-coverage. Aside from that, you can use any other GCC options; but if you want to prove that every single line in your program was executed, you should not compile with optimization at the same time. On some machines the optimizer can eliminate some simple code lines by combining them with other lines. For example, code like this:
if (a != b)
c = 1;
else
c = 0;
can be compiled into one instruction on some machines. In this case, there is no way for gcov to calculate separate execution counts for each line because there isn't separate code for each line. Hence the gcov output looks like this if you compiled the program with optimization:
100: 12:if (a != b)
100: 13: c = 1;
100: 14:else
100: 15: c = 0;
The output shows that this block of code, combined by optimization, executed 100 times. In one sense this result is correct, because there was only one instruction representing all four of these lines. However, the output does not indicate how many times the result was 0 and how many times the result was 1.
gcov uses three files for doing profiling. The names of these files are derived from the original source file by substituting the file suffix with either .bb, .bbg, or .da. All of these files are placed in the same directory as the source file, and contain data stored in a platform-independent method.
The .bb and .bbg files are generated when the source file is compiled with the GCC -ftest-coverage option. The .bb file contains a list of source files (including headers), functions within those files, and line numbers corresponding to each basic block in the source file.
The .bb file format consists of several lists of 4-byte integers which correspond to the line numbers of each basic block in the file. Each list is terminated by a line number of 0. A line number of −1 is used to designate that the source file name (padded to a 4-byte boundary and followed by another −1) follows. In addition, a line number of −2 is used to designate that the name of a function (also padded to a 4-byte boundary and followed by a −2) follows.
The .bbg file is used to reconstruct the program flow graph for the source file. It contains a list of the program flow arcs (possible branches taken from one basic block to another) for each function which, in combination with the .bb file, enables gcov to reconstruct the program flow.
In the .bbg file, the format is:
name of function #0
checksum of function #0
number of basic blocks for function #0 (4-byte number)
total number of arcs for function #0 (4-byte number)
count of arcs in basic block #0 (4-byte number)
destination basic block of arc #0 (4-byte number)
flag bits (4-byte number)
destination basic block of arc #1 (4-byte number)
flag bits (4-byte number)
...
destination basic block of arc #N (4-byte number)
flag bits (4-byte number)
count of arcs in basic block #1 (4-byte number)
destination basic block of arc #0 (4-byte number)
flag bits (4-byte number)
...
A −1 (stored as a 4-byte number) is used to separate each function's list of basic blocks, and to verify that the file has been read correctly.
The function name is stored as a −1 (4 bytes), the length (4 bytes), the name itself (padded to 4-byte boundary) followed by a −1 (4 bytes).
The flags are defined as follows:
The .da file is generated when a program containing object files built with the GCC -fprofile-arcs option is executed. A separate .da file is created for each source file compiled with this option, and the name of the .da file is stored as an absolute pathname in the resulting object file. This path name is derived from the object file name by substituting a .da suffix.
The .da consists of one or more blocks with the following structure:
"magic" number −123 (4-byte number)
number of functions (4-byte number)
length of the "extension block" in bytes
extension block (variable length)
name of function #0 (the same format as in .bbg file)
checksum of function #0
number of instrumented arcs (4-byte number)
count of arc #0 (8-byte number)
count of arc #1 (8-byte number)
...
count of arc #M_0 (8-byte number)
name of function #1 (the same format as in .bbg file)
checksum of function #1
...
Multiple program runs might merge data into a single block, or might append a new block. The current structure of the extension block is as follows:
number of instrumented arcs in whole program (4-byte number)
sum all of instrumented arcs in whole program (8-byte number)
maximal value of counter in whole program (8-byte number)
number of instrumented arcs in the object file (4-byte number)
sum all of instrumented arcs in the object file (8-byte number)
maximal value of counter in the object file (8-byte number)
All three of these files use the functions in gcov-io.h to store integers; the functions in this header provide a machine-independent mechanism for storing and retrieving data from a stream.
This section describes known problems that affect users of GCC. Most of these are not GCC bugs per se—if they were, we would fix them. But the result for a user may be like the result of a bug.
Some of these problems are due to bugs in other software, some are missing features that are too much work to add, and some are places where people's opinions differ as to what is best.
fixincludes script interacts badly with automounters; if the
directory of system header files is automounted, it tends to be
unmounted while fixincludes is running. This would seem to be a
bug in the automounter. We don't know any good way to work around it.
fixproto script will sometimes add prototypes for the
sigsetjmp and siglongjmp functions that reference the
jmp_buf type before that type is defined. To work around this,
edit the offending file and place the typedef in front of the
prototypes.
You may run into problems with cross compilation on certain machines, for several reasons.
The compiler writes these integer constants by examining the floating point value as an integer and printing that integer, because this is simple to write and independent of the details of the floating point representation. But this does not work if the compiler is running on a different machine with an incompatible floating point format, or even a different byte-ordering.
In addition, correct constant folding of floating point values requires representing them in the target machine's format. (The C standard does not quite require this, but in practice it is the only way to win.)
It is now possible to overcome these problems by defining macros such
as REAL_VALUE_TYPE. But doing so is a substantial amount of
work for each target machine.
See Cross Compilation and Floating Point (GNU Compiler Collection (GCC) Internals).
This section lists various difficulties encountered in using GCC together with other compilers or with the assemblers, linkers, libraries and debuggers on certain systems.
An area where the difference is most apparent is name mangling. The use of different name mangling is intentional, to protect you from more subtle problems. Compilers differ as to many internal details of C++ implementation, including: how class instances are laid out, how multiple inheritance is implemented, and how virtual function calls are handled. If the name encoding were made the same, your programs would link against libraries provided from other compilers—but the programs would then crash when run. Incompatible libraries are then detected at link time, rather than at run time.
double on an 8-byte
boundary, and it expects every double to be so aligned. The Sun
compiler usually gives double values 8-byte alignment, with one
exception: function arguments of type double may not be aligned.
As a result, if a function compiled with Sun CC takes the address of an
argument of type double and passes this pointer of type
double * to a function compiled with GCC, dereferencing the
pointer may cause a fatal signal.
One way to solve this problem is to compile your entire program with GCC.
Another solution is to modify the function that is compiled with
Sun CC to copy the argument into a local variable; local variables
are always properly aligned. A third solution is to modify the function
that uses the pointer to dereference it via the following function
access_double instead of directly with *:
inline double
access_double (double *unaligned_ptr)
{
union d2i { double d; int i[2]; };
union d2i *p = (union d2i *) unaligned_ptr;
union d2i u;
u.i[0] = p->i[0];
u.i[1] = p->i[1];
return u.d;
}
Storing into the pointer can be done likewise with the same union.
malloc function in the libmalloc.a library
may allocate memory that is only 4 byte aligned. Since GCC on the
SPARC assumes that doubles are 8 byte aligned, this may result in a
fatal signal if doubles are stored in memory allocated by the
libmalloc.a library.
The solution is to not use the libmalloc.a library. Use instead
malloc and related functions from libc.a; they do not have
this problem.
_dlclose, _dlsym or _dlopen
when linking, compile and link against the file
mit/util/misc/dlsym.c from the MIT version of X windows.
cc does not
compile GCC correctly. We do not yet know why. However, GCC
compiled on earlier HP-UX versions works properly on HP-UX 9.01 and can
compile itself properly on 9.01.
alloca or variable-size arrays. This is because GCC doesn't
generate HP-UX unwind descriptors for such functions. It may even be
impossible to generate them.
(warning) Use of GR3 when
frame >= 8192 may cause conflict.
These warnings are harmless and can be safely ignored.
extern int foo;
... foo ...
static int foo;
will cause the linker to report an undefined symbol foo.
Although this behavior differs from most other systems, it is not a
bug because redefining an extern variable as static
is undefined in ISO C.
GCC uses the same convention as the Ultrix C compiler. You can use these options to produce code compatible with the Fortran compiler:
-fcall-saved-r2 -fcall-saved-r3 -fcall-saved-r4 -fcall-saved-r5
ecvt, fcvt and gcvt. Given valid
floating point numbers, they sometimes print NaN.
Certain programs have problems compiling.
#ifdef __STDC__
#define NeedFunctionPrototypes 0
#endif
You can prevent this problem by linking GCC with the GNU malloc (which thus replaces the malloc that comes with the system). GNU malloc is available as a separate package, and also in the file src/gmalloc.c in the GNU Emacs 19 distribution.
If you have installed GNU malloc as a separate library package, use this option when you relink GCC:
MALLOC=/usr/local/lib/libgmalloc.a
Alternatively, if you have compiled gmalloc.c from Emacs 19, copy the object file to gmalloc.o and use this option when you relink GCC:
MALLOC=gmalloc.o
There are several noteworthy incompatibilities between GNU C and K&R (non-ISO) versions of C.
One consequence is that you cannot call mktemp with a string
constant argument. The function mktemp always alters the
string its argument points to.
Another consequence is that sscanf does not work on some systems
when passed a string constant as its format control string or input.
This is because sscanf incorrectly tries to write into the string
constant. Likewise fscanf and scanf.
The best solution to these problems is to change the program to use
char-array variables with initialization strings for these
purposes instead of string constants. But if this is not possible,
you can use the -fwritable-strings flag, which directs GCC
to handle string constants the same way most C compilers do.
-2147483648 is positive.
This is because 2147483648 cannot fit in the type int, so
(following the ISO C rules) its data type is unsigned long int.
Negating this value yields 2147483648 again.
#define foo(a) "a"
will produce output "a" regardless of what the argument a is.
setjmp and longjmp, the only automatic
variables guaranteed to remain valid are those declared
volatile. This is a consequence of automatic register
allocation. Consider this function:
jmp_buf j;
foo ()
{
int a, b;
a = fun1 ();
if (setjmp (j))
return a;
a = fun2 ();
/* longjmp (j) may occur in fun3. */
return a + fun3 ();
}
Here a may or may not be restored to its first value when the
longjmp occurs. If a is allocated in a register, then
its first value is restored; otherwise, it keeps the last value stored
in it.
If you use the -W option with the -O option, you will get a warning when GCC thinks such a problem might be possible.
foobar (
#define luser
hack)
ISO C does not permit such a construct.
In some other C compilers, a extern declaration affects all the
rest of the file even if it happens within a block.
long, etc., with a typedef name,
as shown here:
typedef int foo;
typedef long foo bar;
In ISO C, this is not allowed: long and other type modifiers
require an explicit int.
typedef int foo;
typedef foo foo;
#if 0
You can't expect this to work.
#endif
The best solution to such a problem is to put the text into an actual C comment delimited by /*...*/.
time, so it did not matter what type your program declared it to
return. But in systems with ISO C headers, time is declared to
return time_t, and if that is not the same as long, then
long time (); is erroneous.
The solution is to change your program to use appropriate system headers
(<time.h> on systems with ISO C headers) and not to declare
time if the system header files declare it, or failing that to
use time_t as the return type of time.
float, PCC converts it to
a double. GCC actually returns a float. If you are concerned
with PCC compatibility, you should declare your functions to return
double; you might as well say what you mean.
The method used by GCC is as follows: a structure or union which is
1, 2, 4 or 8 bytes long is returned like a scalar. A structure or union
with any other size is stored into an address supplied by the caller
(usually in a special, fixed register, but on some machines it is passed
on the stack). The machine-description macros STRUCT_VALUE and
STRUCT_INCOMING_VALUE tell GCC where to pass this address.
By contrast, PCC on most target machines returns structures and unions of any size by copying the data into an area of static storage, and then returning the address of that storage as if it were a pointer value. The caller must copy the data from that memory area to the place where the value is wanted. GCC does not use this method because it is slower and nonreentrant.
On some newer machines, PCC uses a reentrant convention for all structure and union returning. GCC on most of these machines uses a compatible convention when returning structures and unions in memory, but still returns small structures and unions in registers.
You can tell GCC to use a compatible convention for all structure and union returning with the option -fpcc-struct-return.
A preprocessing token is a preprocessing number if it begins with a digit and is followed by letters, underscores, digits, periods and e+, e-, E+, E-, p+, p-, P+, or P- character sequences. (In strict C89 mode, the sequences p+, p-, P+ and P- cannot appear in preprocessing numbers.)
To make the above program fragment valid, place whitespace in front of the minus sign. This whitespace will end the preprocessing number.
GCC needs to install corrected versions of some system header files. This is because most target systems have some header files that won't work with GCC unless they are changed. Some have bugs, some are incompatible with ISO C, and some depend on special features of other compilers.
Installing GCC automatically creates and installs the fixed header
files, by running a program called fixincludes (or for certain
targets an alternative such as fixinc.svr4). Normally, you
don't need to pay attention to this. But there are cases where it
doesn't do the right thing automatically.
The programs that fix the header files do not understand this special way of using symbolic links; therefore, the directory of fixed header files is good only for the machine model used to build it.
In SunOS 4, only programs that look inside the kernel will notice the difference between machine models. Therefore, for most purposes, you need not be concerned about this.
It is possible to make separate sets of fixed header files for the different machine models, and arrange a structure of symbolic links so as to use the proper set, but you'll have to do this by hand.
fixincludes script to fail.
This means you will encounter problems due to bugs in the system header files. It may be no comfort that they aren't GCC's fault, but it does mean that there's nothing for us to do about them.
GCC by itself attempts to be a conforming freestanding implementation. See Language Standards Supported by GCC, for details of what this means. Beyond the library facilities required of such an implementation, the rest of the C library is supplied by the vendor of the operating system. If that C library doesn't conform to the C standards, then your programs might get warnings (especially when using -Wall) that you don't expect.
For example, the sprintf function on SunOS 4.1.3 returns
char * while the C standard says that sprintf returns an
int. The fixincludes program could make the prototype for
this function match the Standard, but that would be wrong, since the
function will still return char *.
If you need a Standard compliant library, then you need to find one, as
GCC does not provide one. The GNU C library (called glibc)
provides ISO C, POSIX, BSD, SystemV and X/Open compatibility for
GNU/Linux and HURD-based GNU systems; no recent version of it supports
other systems, though some very old versions did. Version 2.2 of the
GNU C library includes nearly complete C99 support. You could also ask
your operating system vendor if newer libraries are available.
These problems are perhaps regrettable, but we don't know any practical way around them.
This occurs because sometimes GCC optimizes the variable out of existence. There is no way to tell the debugger how to compute the value such a variable “would have had”, and it is not clear that would be desirable anyway. So GCC simply does not mention the eliminated variable when it writes debugging information.
You have to expect a certain amount of disagreement between the executable and your source code, when you use optimization.
int foo (struct mumble *);
struct mumble { ... };
int foo (struct mumble *x)
{ ... }
This code really is erroneous, because the scope of struct
mumble in the prototype is limited to the argument list containing it.
It does not refer to the struct mumble defined with file scope
immediately below—they are two unrelated types with similar names in
different scopes.
But in the definition of foo, the file-scope type is used
because that is available to be inherited. Thus, the definition and
the prototype do not match, and you get an error.
This behavior may seem silly, but it's what the ISO standard specifies.
It is easy enough for you to make your code work by moving the
definition of struct mumble above the prototype. It's not worth
being incompatible with ISO C just to avoid an error for the example
shown above.
If you care about controlling the amount of memory that is accessed, use volatile but do not use bit-fields.
If new system header files are installed, nothing automatically arranges
to update the corrected header files. You will have to reinstall GCC
to fix the new header files. More specifically, go to the build
directory and delete the files stmp-fixinc and
stmp-headers, and the subdirectory include; then do
make install again.
double in memory.
Compiled code moves values between memory and floating point registers
at its convenience, and moving them into memory truncates them.
You can partially avoid this problem by using the -ffloat-store option (see Optimize Options).
C++ is a complex language and an evolving one, and its standard definition (the ISO C++ standard) was only recently completed. As a result, your C++ compiler may occasionally surprise you, even when its behavior is correct. This section discusses some areas that frequently give rise to questions of this sort.
When a class has static data members, it is not enough to declare the static member; you must also define it. For example:
class Foo
{
...
void method();
static int bar;
};
This declaration only establishes that the class Foo has an
int named Foo::bar, and a member function named
Foo::method. But you still need to define both
method and bar elsewhere. According to the ISO
standard, you must supply an initializer in one (and only one) source
file, such as:
int Foo::bar = 0;
Other C++ compilers may not correctly implement the standard behavior. As a result, when you switch to g++ from one of these compilers, you may discover that a program that appeared to work correctly in fact does not conform to the standard: g++ reports as undefined symbols any static data members that lack definitions.
It is dangerous to use pointers or references to portions of a
temporary object. The compiler may very well delete the object before
you expect it to, leaving a pointer to garbage. The most common place
where this problem crops up is in classes like string classes,
especially ones that define a conversion function to type char *
or const char *—which is one reason why the standard
string class requires you to call the c_str member
function. However, any class that returns a pointer to some internal
structure is potentially subject to this problem.
For example, a program may use a function strfunc that returns
string objects, and another function charfunc that
operates on pointers to char:
string strfunc ();
void charfunc (const char *);
void
f ()
{
const char *p = strfunc().c_str();
...
charfunc (p);
...
charfunc (p);
}
In this situation, it may seem reasonable to save a pointer to the C
string returned by the c_str member function and use that rather
than call c_str repeatedly. However, the temporary string
created by the call to strfunc is destroyed after p is
initialized, at which point p is left pointing to freed memory.
Code like this may run successfully under some other compilers, particularly obsolete cfront-based compilers that delete temporaries along with normal local variables. However, the GNU C++ behavior is standard-conforming, so if your program depends on late destruction of temporaries it is not portable.
The safe way to write such code is to give the temporary a name, which forces it to remain until the end of the scope of the name. For example:
const string& tmp = strfunc ();
charfunc (tmp.c_str ());
When a base class is virtual, only one subobject of the base class belongs to each full object. Also, the constructors and destructors are invoked only once, and called from the most-derived class. However, such objects behave unspecified when being assigned. For example:
struct Base{
char *name;
Base(char *n) : name(strdup(n)){}
Base& operator= (const Base& other){
free (name);
name = strdup (other.name);
}
};
struct A:virtual Base{
int val;
A():Base("A"){}
};
struct B:virtual Base{
int bval;
B():Base("B"){}
};
struct Derived:public A, public B{
Derived():Base("Derived"){}
};
void func(Derived &d1, Derived &d2)
{
d1 = d2;
}
The C++ standard specifies that Base::Base is only called once when constructing or copy-constructing a Derived object. It is unspecified whether Base::operator= is called more than once when the implicit copy-assignment for Derived objects is invoked (as it is inside func in the example).
g++ implements the “intuitive” algorithm for copy-assignment: assign all
direct bases, then assign all members. In that algorithm, the virtual
base subobject can be encountered more than once. In the example, copying
proceeds in the following order: val, name (via
strdup), bval, and name again.
If application code relies on copy-assignment, a user-defined copy-assignment operator removes any uncertainties. With such an operator, the application can define whether and how the virtual base subobject is assigned.
The conversion programs protoize and unprotoize can sometimes change a source file in a way that won't work unless you rearrange it.
If this happens, compiler error messages should show you where the new references are, so fixing the file by hand is straightforward.
You can find all the places where this problem might occur by compiling the program with the -Wconversion option. It prints a warning whenever an argument is converted.
You can generally work around this problem by using protoize step by step, each time specifying a different set of -D options for compilation, until all of the functions have been converted. There is no automatic way to verify that you have got them all, however.
If you plan on converting source files which contain such code, it is recommended that you first make sure that each conditionally compiled region of source code which contains an alternative function header also contains at least one additional follower token (past the final right parenthesis of the function header). This should circumvent the problem.
This section lists changes that people frequently request, but which we do not make because we think GCC is better without them.
Such a feature would work only occasionally—only for calls that appear in the same file as the called function, following the definition. The only way to check all calls reliably is to add a prototype for the function. But adding a prototype eliminates the motivation for this feature. So the feature is not worthwhile.
Shift count operands are probably signed more often than unsigned. Warning about this would cause far more annoyance than good.
Such assignments must be very common; warning about them would cause more annoyance than good.
Coming as I do from a Lisp background, I balk at the idea that there is
something dangerous about discarding a value. There are functions that
return values which some callers may find useful; it makes no sense to
clutter the program with a cast to void whenever the value isn't
useful.
This would cause storage layout to be incompatible with most other C compilers. And it doesn't seem very important, given that you can get the same result in other ways. The case where it matters most is when the enumeration-valued object is inside a structure, and in that case you can specify a field width explicitly.
The ISO C standard leaves it up to the implementation whether a bit-field
declared plain int is signed or not. This in effect creates two
alternative dialects of C.
The GNU C compiler supports both dialects; you can specify the signed dialect with -fsigned-bitfields and the unsigned dialect with -funsigned-bitfields. However, this leaves open the question of which dialect to use by default.
Currently, the preferred dialect makes plain bit-fields signed, because
this is simplest. Since int is the same as signed int in
every other context, it is cleanest for them to be the same in bit-fields
as well.
Some computer manufacturers have published Application Binary Interface standards which specify that plain bit-fields should be unsigned. It is a mistake, however, to say anything about this issue in an ABI. This is because the handling of plain bit-fields distinguishes two dialects of C. Both dialects are meaningful on every type of machine. Whether a particular object file was compiled using signed bit-fields or unsigned is of no concern to other object files, even if they access the same bit-fields in the same data structures.
A given program is written in one or the other of these two dialects. The program stands a chance to work on most any machine if it is compiled with the proper dialect. It is unlikely to work at all if compiled with the wrong dialect.
Many users appreciate the GNU C compiler because it provides an environment that is uniform across machines. These users would be inconvenienced if the compiler treated plain bit-fields differently on certain machines.
Occasionally users write programs intended only for a particular machine type. On these occasions, the users would benefit if the GNU C compiler were to support by default the same dialect as the other compilers on that machine. But such applications are rare. And users writing a program to run on more than one type of machine cannot possibly benefit from this kind of compatibility.
This is why GCC does and will treat plain bit-fields in the same fashion on all types of machines (by default).
There are some arguments for making bit-fields unsigned by default on all machines. If, for example, this becomes a universal de facto standard, it would make sense for GCC to go along with it. This is something to be considered in the future.
(Of course, users strongly concerned about portability should indicate explicitly in each bit-field whether it is signed or not. In this way, they write programs which have the same meaning in both C dialects.)
__STDC__ when -ansi is not used.
Currently, GCC defines __STDC__ unconditionally. This provides
good results in practice.
Programmers normally use conditionals on __STDC__ to ask whether
it is safe to use certain features of ISO C, such as function
prototypes or ISO token concatenation. Since plain gcc supports
all the features of ISO C, the correct answer to these questions is
“yes”.
Some users try to use __STDC__ to check for the availability of
certain library facilities. This is actually incorrect usage in an ISO
C program, because the ISO C standard says that a conforming
freestanding implementation should define __STDC__ even though it
does not have the library facilities. gcc -ansi -pedantic is a
conforming freestanding implementation, and it is therefore required to
define __STDC__, even though it does not come with an ISO C
library.
Sometimes people say that defining __STDC__ in a compiler that
does not completely conform to the ISO C standard somehow violates the
standard. This is illogical. The standard is a standard for compilers
that claim to support ISO C, such as gcc -ansi—not for other
compilers such as plain gcc. Whatever the ISO C standard says
is relevant to the design of plain gcc without -ansi only
for pragmatic reasons, not as a requirement.
GCC normally defines __STDC__ to be 1, and in addition
defines __STRICT_ANSI__ if you specify the -ansi option,
or a -std option for strict conformance to some version of ISO C.
On some hosts, system include files use a different convention, where
__STDC__ is normally 0, but is 1 if the user specifies strict
conformance to the C Standard. GCC follows the host convention when
processing system include files, but when processing user files it follows
the usual GNU C convention.
__STDC__ in C++.
Programs written to compile with C++-to-C translators get the
value of __STDC__ that goes with the C compiler that is
subsequently used. These programs must test __STDC__
to determine what kind of C preprocessor that compiler uses:
whether they should concatenate tokens in the ISO C fashion
or in the traditional fashion.
These programs work properly with GNU C++ if __STDC__ is defined.
They would not work otherwise.
In addition, many header files are written to provide prototypes in ISO
C but not in traditional C. Many of these header files can work without
change in C++ provided __STDC__ is defined. If __STDC__
is not defined, they will all fail, and will all need to be changed to
test explicitly for C++ as well.
Historically, GCC has not deleted “empty” loops under the assumption that the most likely reason you would put one in a program is to have a delay, so deleting them will not make real programs run any faster.
However, the rationale here is that optimization of a nonempty loop cannot produce an empty one, which holds for C but is not always the case for C++.
Moreover, with -funroll-loops small “empty” loops are already removed, so the current behavior is both sub-optimal and inconsistent and will change in the future.
It is never safe to depend on the order of evaluation of side effects. For example, a function call like this may very well behave differently from one compiler to another:
void func (int, int);
int i = 2;
func (i++, i++);
There is no guarantee (in either the C or the C++ standard language
definitions) that the increments will be evaluated in any particular
order. Either increment might happen first. func might get the
arguments 2, 3, or it might get 3, 2, or even 2, 2.
Strictly speaking, there is no prohibition in the ISO C standard against allowing structures with volatile fields in registers, but it does not seem to make any sense and is probably not what you wanted to do. So the compiler will give an error message in this case.
Some ISO C testsuites report failure when the compiler does not produce an error message for a certain program.
ISO C requires a “diagnostic” message for certain kinds of invalid programs, but a warning is defined by GCC to count as a diagnostic. If GCC produces a warning but not an error, that is correct ISO C support. If test suites call this “failure”, they should be run with the GCC option -pedantic-errors, which will turn these warnings into errors.
The GNU compiler can produce two kinds of diagnostics: errors and warnings. Each kind has a different purpose:
Warnings may indicate danger points where you should check to make sure that your program really does what you intend; or the use of obsolete features; or the use of nonstandard features of GNU C or C++. Many warnings are issued only if you ask for them, with one of the -W options (for instance, -Wall requests a variety of useful warnings).
GCC always tries to compile your program if possible; it never gratuitously rejects a program whose meaning is clear merely because (for instance) it fails to conform to a standard. In some cases, however, the C and C++ standards specify that certain extensions are forbidden, and a diagnostic must be issued by a conforming compiler. The -pedantic option tells GCC to issue warnings in such cases; -pedantic-errors says to make them errors instead. This does not mean that all non-ISO constructs get warnings or errors.
See Options to Request or Suppress Warnings, for more detail on these and related command-line options.
Your bug reports play an essential role in making GCC reliable.
When you encounter a problem, the first thing to do is to see if it is already known. See Trouble. If it isn't known, then you should report the problem.
If you are not sure whether you have found a bug, here are some guidelines:
asm statement), that is a compiler bug, unless the
compiler reports errors (not just warnings) which would ordinarily
prevent the assembler from being run.
However, you must double-check to make sure, because you may have a program whose behavior is undefined, which happened by chance to give the desired results with another C or C++ compiler.
For example, in many nonoptimizing compilers, you can write x;
at the end of a function instead of return x;, with the same
results. But the value of the function is undefined if return
is omitted; it is not a bug when GCC produces different results.
Problems often result from expressions with two increment operators,
as in f (*p++, *p++). Your previous compiler might have
interpreted that expression the way you intended; GCC might
interpret it another way. Neither compiler is wrong. The bug is
in your code.
After you have localized the error to a single source line, it should be easy to check for these things. If your program is correct and well defined, you have found a compiler bug.
Bugs should be reported to our bug database. Please refer to http://gcc.gnu.org/bugs.html for up-to-date instructions how to submit bug reports. Copies of this file in HTML (bugs.html) and plain text (BUGS) are also part of GCC releases.
If you need help installing, using or changing GCC, there are two ways to find it:
For further information, see http://gcc.gnu.org/faq.html#support.
If you would like to help pretest GCC releases to assure they work well, our current development sources are available by CVS (see http://gcc.gnu.org/cvs.html). Source and binary snapshots are also available for FTP; see http://gcc.gnu.org/snapshots.html.
If you would like to work on improvements to GCC, please read the advice at these URLs:
http://gcc.gnu.org/contribute.html
http://gcc.gnu.org/contributewhy.html
for information on how to make useful contributions and avoid duplication of effort. Suggested projects are listed at http://gcc.gnu.org/projects/.
If you want to have more free software a few years from now, it makes sense for you to help encourage people to contribute funds for its development. The most effective approach known is to encourage commercial redistributors to donate.
Users of free software systems can boost the pace of development by encouraging for-a-fee distributors to donate part of their selling price to free software developers—the Free Software Foundation, and others.
The way to convince distributors to do this is to demand it and expect it from them. So when you compare distributors, judge them partly by how much they give to free software development. Show distributors they must compete to be the one who gives the most.
To make this approach work, you must insist on numbers that you can compare, such as, “We will donate ten dollars to the Frobnitz project for each disk sold.” Don't be satisfied with a vague promise, such as “A portion of the profits are donated,” since it doesn't give a basis for comparison.
Even a precise fraction “of the profits from this disk” is not very meaningful, since creative accounting and unrelated business decisions can greatly alter what fraction of the sales price counts as profit. If the price you pay is $50, ten percent of the profit is probably less than a dollar; it might be a few cents, or nothing at all.
Some redistributors do development work themselves. This is useful too; but to keep everyone honest, you need to inquire how much they do, and what kind. Some kinds of development make much more long-term difference than others. For example, maintaining a separate version of a program contributes very little; maintaining the standard version of a program for the whole community contributes much. Easy new ports contribute little, since someone else would surely do them; difficult ports such as adding a new CPU to the GNU Compiler Collection contribute more; major new features or packages contribute the most.
By establishing the idea that supporting further development is “the proper thing to do” when distributing free software for a fee, we can assure a steady flow of resources into making more free software.
Copyright © 1994 Free Software Foundation, Inc.
Verbatim copying and redistribution of this section is permitted
without royalty; alteration is not permitted.
The GNU Project was launched in 1984 to develop a complete Unix-like operating system which is free software: the GNU system. (GNU is a recursive acronym for “GNU's Not Unix”; it is pronounced “guh-NEW”.) Variants of the GNU operating system, which use the kernel Linux, are now widely used; though these systems are often referred to as “Linux”, they are more accurately called GNU/Linux systems.
For more information, see:
http://www.gnu.org/
http://www.gnu.org/gnu/linux-and-gnu.html
Copyright © 1989, 1991 Free Software Foundation, Inc.
59 Temple Place - Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software—to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.
Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.
The precise terms and conditions for copying, distribution and modification follow.
Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.
These requirements apply to the modified work as a whole. If identifiable sections of that work are not derived from the Program, and can be reasonably considered independent and separate works in themselves, then this License, and its terms, do not apply to those sections when you distribute them as separate works. But when you distribute the same sections as part of a whole which is a work based on the Program, the distribution of the whole must be on the terms of this License, whose permissions for other licensees extend to the entire whole, and thus to each and every part regardless of who wrote it.
Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Program.
In addition, mere aggregation of another work not based on the Program with the Program (or with a work based on the Program) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.
The source code for a work means the preferred form of the work for making modifications to it. For an executable work, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the executable. However, as a special exception, the source code distributed need not include anything that is normally distributed (in either source or binary form) with the major components (compiler, kernel, and so on) of the operating system on which the executable runs, unless that component itself accompanies the executable.
If distribution of executable or object code is made by offering access to copy from a designated place, then offering equivalent access to copy the source code from the same place counts as distribution of the source code, even though third parties are not compelled to copy the source along with the object code.
If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply and the section as a whole is intended to apply in other circumstances.
It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system, which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice.
This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.
Each version is given a distinguishing version number. If the Program specifies a version number of this License which applies to it and “any later version”, you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Program does not specify a version number of this License, you may choose any version ever published by the Free Software Foundation.
If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found.
one line to give the program's name and a brief idea of what it does.
Copyright (C) year name of author
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software Foundation,
Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) year name of author
Gnomovision comes with ABSOLUTELY NO WARRANTY; for details
type `show w'.
This is free software, and you are welcome to redistribute it
under certain conditions; type `show c' for details.
The hypothetical commands show w and show c should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than show w and show c; they could even be mouse-clicks or menu items—whatever suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a “copyright disclaimer” for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program
`Gnomovision' (which makes passes at compilers) written by James Hacker.
signature of Ty Coon, 1 April 1989
Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.
Copyright © 2000,2001,2002 Free Software Foundation, Inc.
59 Temple Place, Suite 330, Boston, MA 02111-1307, USA
Everyone is permitted to copy and distribute verbatim copies
of this license document, but changing it is not allowed.
The purpose of this License is to make a manual, textbook, or other functional and useful document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.
This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.
We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.
This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”. You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.
A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.
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The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License. If a section does not fit the above definition of Secondary then it is not allowed to be designated as Invariant. The Document may contain zero Invariant Sections. If the Document does not identify any Invariant Sections then there are none.
The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License. A Front-Cover Text may be at most 5 words, and a Back-Cover Text may be at most 25 words.
A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, that is suitable for revising the document straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup, or absence of markup, has been arranged to thwart or discourage subsequent modification by readers is not Transparent. An image format is not Transparent if used for any substantial amount of text. A copy that is not “Transparent” is called “Opaque”.
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The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.
A section “Entitled XYZ” means a named subunit of the Document whose title either is precisely XYZ or contains XYZ in parentheses following text that translates XYZ in another language. (Here XYZ stands for a specific section name mentioned below, such as “Acknowledgements”, “Dedications”, “Endorsements”, or “History”.) To “Preserve the Title” of such a section when you modify the Document means that it remains a section “Entitled XYZ” according to this definition.
The Document may include Warranty Disclaimers next to the notice which states that this License applies to the Document. These Warranty Disclaimers are considered to be included by reference in this License, but only as regards disclaiming warranties: any other implication that these Warranty Disclaimers may have is void and has no effect on the meaning of this License.
You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.
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If you publish printed copies (or copies in media that commonly have printed covers) of the Document, numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.
If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.
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It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.
You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:
If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.
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You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice, and that you preserve all their Warranty Disclaimers.
The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.
In the combination, you must combine any sections Entitled “History” in the various original documents, forming one section Entitled “History”; likewise combine any sections Entitled “Acknowledgements”, and any sections Entitled “Dedications”. You must delete all sections Entitled “Endorsements.”
You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.
You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.
A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, is called an “aggregate” if the copyright resulting from the compilation is not used to limit the legal rights of the compilation's users beyond what the individual works permit. When the Document is included an aggregate, this License does not apply to the other works in the aggregate which are not themselves derivative works of the Document.
If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one half of the entire aggregate, the Document's Cover Texts may be placed on covers that bracket the Document within the aggregate, or the electronic equivalent of covers if the Document is in electronic form. Otherwise they must appear on printed covers that bracket the whole aggregate.
Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License, and all the license notices in the Document, and any Warrany Disclaimers, provided that you also include the original English version of this License and the original versions of those notices and disclaimers. In case of a disagreement between the translation and the original version of this License or a notice or disclaimer, the original version will prevail.
If a section in the Document is Entitled “Acknowledgements”, “Dedications”, or “History”, the requirement (section 4) to Preserve its Title (section 1) will typically require changing the actual title.
You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.
Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.
To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page:
Copyright (C) year your name.
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2
or any later version published by the Free Software Foundation;
with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.
A copy of the license is included in the section entitled ``GNU
Free Documentation License''.
If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the “with...Texts.” line with this:
with the Invariant Sections being list their titles, with
the Front-Cover Texts being list, and with the Back-Cover Texts
being list.
If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation.
If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.
The GCC project would like to thank its many contributors. Without them the project would not have been nearly as successful as it has been. Any omissions in this list are accidental. Feel free to contact law@redhat.com or gerald@pfeifer.com if you have been left out or some of your contributions are not listed. Please keep this list in alphabetical order.
restrict support, and serving as release manager for GCC 3.x.
In addition to the above, all of which also contributed time and energy in testing GCC, we would like to thank the following for their contributions to testing:
And finally we'd like to thank everyone who uses the compiler, submits bug reports and generally reminds us why we're doing this work in the first place.
GCC's command line options are indexed here without any initial - or --. Where an option has both positive and negative forms (such as -foption and -fno-option), relevant entries in the manual are indexed under the most appropriate form; it may sometimes be useful to look up both forms.
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Optionsdh: Debugging OptionsdI: Preprocessor Optionsdi: Debugging Optionsdj: Debugging Optionsdk: Debugging OptionsdL: Debugging Optionsdl: Debugging OptionsdM: Preprocessor Optionsdm: Debugging OptionsdM: Debugging OptionsdN: Preprocessor OptionsdN: Debugging Optionsdn: Debugging Optionsdo: Debugging OptionsdP: Debugging Optionsdp: Debugging OptionsdR: Debugging Optionsdr: Debugging OptionsdS: Debugging Optionsds: Debugging OptionsdT: Debugging Optionsdt: Debugging Optionsdu: Debugging Optionsdumpmachine: Debugging Optionsdumpspecs: Debugging Optionsdumpversion: Debugging Optionsdv: Debugging OptionsdW: Debugging Optionsdw: Debugging Optionsdx: Debugging OptionsdX: Debugging Optionsdy: Debugging Optionsdylib_file: Darwin Optionsdylinker_install_name: Darwin Optionsdynamic: Darwin Optionsdynamiclib: Darwin Optionsdz: Debugging OptionsE: Link OptionsE: Overall OptionsEB: ARC OptionsEB: MIPS OptionsEL: ARC OptionsEL: MIPS Optionsexported_symbols_list: Darwin Optionsfabi-version: C++ Dialect Optionsfalign-functions: Optimize Optionsfalign-jumps: Optimize Optionsfalign-labels: Optimize Optionsfalign-loops: Optimize Optionsfalt-external-templates: C++ Dialect Optionsfargument-alias: Code Gen Optionsfargument-noalias: Code Gen Optionsfargument-noalias-global: Code Gen Optionsfbounds-check: Code Gen Optionsfbranch-probabilities: Optimize Optionsfcall-saved: Interoperationfcall-saved: Code Gen Optionsfcall-used: Code Gen Optionsfcaller-saves: Optimize Optionsfcheck-new: C++ Dialect Optionsfcommon: Variable Attributesfcond-mismatch: C Dialect Optionsfconserve-space: C++ Dialect Optionsfconstant-string-class: Objective-C Dialect Optionsfcse-follow-jumps: Optimize Optionsfcse-skip-blocks: Optimize Optionsfdata-sections: Optimize Optionsfdelayed-branch: Optimize Optionsfdelete-null-pointer-checks: Optimize Optionsfdiagnostics-show-location: Language Independent Optionsfdollars-in-identifiers: Interoperationfdollars-in-identifiers: C++ Dialect Optionsfdump-class-hierarchy: Debugging Optionsfdump-translation-unit: Debugging Optionsfdump-tree: Debugging Optionsfdump-unnumbered: Debugging Optionsfeliminate-dwarf2-dups: Debugging Optionsfexceptions: Code Gen Optionsfexpensive-optimizations: Optimize Optionsfexternal-templates: C++ Dialect Optionsffast-math: Optimize Optionsffinite-math-only: Optimize Optionsffixed: Code Gen Optionsffloat-store: Disappointmentsffloat-store: Optimize Optionsffor-scope: C++ Dialect Optionsfforce-addr: Optimize Optionsfforce-mem: Optimize Optionsffreestanding: Function Attributesffreestanding: C Dialect Optionsffreestanding: Standardsffunction-sections: Optimize Optionsfgcse: Optimize Optionsfgcse-lm: Optimize Optionsfgcse-sm: Optimize Optionsfgnu-runtime: Objective-C Dialect Optionsfhosted: C Dialect Optionsfilelist: Darwin Optionsfinhibit-size-directive: Code Gen Optionsfinline-functions: Optimize Optionsfinline-limit: Optimize Optionsfinstrument-functions: Function Attributesfinstrument-functions: Code Gen Optionsfkeep-inline-functions: Inlinefkeep-inline-functions: Optimize Optionsfkeep-static-consts: Optimize Optionsflat_namespace: Darwin Optionsfleading-underscore: Code Gen Optionsfloop-optimize: Optimize Optionsfmem-report: Debugging Optionsfmessage-length: Language Independent Optionsfmove-all-movables: Optimize Optionsfms-extensions: C++ Dialect Optionsfms-extensions: C Dialect Optionsfnew-ra: Optimize Optionsfnext-runtime: Objective-C Dialect Optionsfno-access-control: C++ Dialect Optionsfno-asm: C Dialect Optionsfno-branch-count-reg: Optimize Optionsfno-builtin: Other Builtinsfno-builtin: C Dialect Optionsfno-common: Variable Attributesfno-common: Code Gen Optionsfno-const-strings: C++ Dialect Optionsfno-cprop-registers: Optimize Optionsfno-default-inline: Inlinefno-default-inline: Optimize Optionsfno-default-inline: C++ Dialect Optionsfno-defer-pop: Optimize Optionsfno-elide-constructors: C++ Dialect Optionsfno-enforce-eh-specs: C++ Dialect Optionsfno-for-scope: C++ Dialect Optionsfno-function-cse: Optimize Optionsfno-gnu-keywords: C++ Dialect Optionsfno-gnu-linker: Code Gen Optionsfno-guess-branch-probability: Optimize Optionsfno-ident: Code Gen Optionsfno-implement-inlines: C++ Interfacefno-implement-inlines: C++ Dialect Optionsfno-implicit-inline-templates: C++ Dialect Optionsfno-implicit-templates: Template Instantiationfno-implicit-templates: C++ Dialect Optionsfno-inline: Optimize Optionsfno-math-errno: Optimize Optionsfno-nonansi-builtins: C++ Dialect Optionsfno-operator-names: C++ Dialect Optionsfno-optional-diags: C++ Dialect Optionsfno-peephole: Optimize Optionsfno-peephole2: Optimize Optionsfno-rtti: C++ Dialect Optionsfno-sched-interblock: Optimize Optionsfno-sched-spec: Optimize Optionsfno-show-column: Preprocessor Optionsfno-signed-bitfields: C Dialect Optionsfno-stack-limit: Code Gen Optionsfno-trapping-math: Optimize Optionsfno-unsigned-bitfields: C Dialect Optionsfno-weak: C++ Dialect Optionsfno-zero-initialized-in-bss: Optimize Optionsfnon-call-exceptions: Code Gen Optionsfomit-frame-pointer: Optimize Optionsfoptimize-register-move: Optimize Optionsfoptimize-sibling-calls: Optimize Optionsforce_cpusubtype_ALL: Darwin Optionsforce_flat_namespace: Darwin Optionsfpack-struct: Code Gen Optionsfpcc-struct-return: Incompatibilitiesfpcc-struct-return: Code Gen Optionsfpermissive: C++ Dialect OptionsfPIC: Code Gen Optionsfpic: Code Gen Optionsfprefetch-loop-arrays: Optimize Optionsfpreprocessed: Preprocessor Optionsfprofile-arcs: Other Builtinsfprofile-arcs: Debugging Optionsfrandom-string: Debugging Optionsfreduce-all-givs: Optimize Optionsfreg-struct-return: Code Gen Optionsfregmove: Optimize Optionsfrename-registers: Optimize Optionsfreorder-blocks: Optimize Optionsfreorder-functions: Optimize Optionsfrepo: Template Instantiationfrepo: C++ Dialect Optionsfrerun-cse-after-loop: Optimize Optionsfrerun-loop-opt: Optimize Optionsfsched-spec-load: Optimize Optionsfsched-spec-load-dangerous: Optimize Optionsfsched-verbose: Debugging Optionsfschedule-insns: Optimize Optionsfschedule-insns2: Optimize Optionsfshared-data: Code Gen Optionsfshort-double: Code Gen Optionsfshort-enums: Non-bugsfshort-enums: Type Attributesfshort-enums: Code Gen Optionsfshort-wchar: Code Gen Optionsfsignaling-nans: Optimize Optionsfsigned-bitfields: Non-bugsfsigned-bitfields: C Dialect Optionsfsigned-char: C Dialect Optionsfsingle-precision-constant: Optimize Optionsfssa: Optimize Optionsfssa-ccp: Optimize Optionsfssa-dce: Optimize Optionsfstack-check: Code Gen Optionsfstack-limit-register: Code Gen Optionsfstack-limit-symbol: Code Gen Optionsfstats: C++ Dialect Optionsfstrength-reduce: Optimize Optionsfstrict-aliasing: Optimize Optionsfsyntax-only: Warning Optionsftabstop: Preprocessor Optionsftemplate-depth: C++ Dialect Optionsftest-coverage: Debugging Optionsfthread-jumps: Optimize Optionsftime-report: Debugging Optionsftracer: Optimize Optionsftrapv: Code Gen Optionsfunroll-all-loops: Optimize Optionsfunroll-loops: Non-bugsfunroll-loops: Optimize Optionsfunsafe-math-optimizations: Optimize Optionsfunsigned-bitfields: Non-bugsfunsigned-bitfields: C Dialect Optionsfunsigned-char: C Dialect Optionsfunwind-tables: Code Gen Optionsfuse-cxa-atexit: C++ Dialect Optionsfverbose-asm: Code Gen Optionsfvolatile: Code Gen Optionsfvolatile-global: Code Gen Optionsfvolatile-static: Code Gen Optionsfvtable-gc: C++ Dialect Optionsfwritable-strings: Incompatibilitiesfwritable-strings: C Dialect OptionsG: System V OptionsG: MIPS OptionsG: RS/6000 and PowerPC OptionsG: M32R/D Optionsg: Debugging Optionsgcoff: Debugging Optionsgdwarf: Debugging Optionsgdwarf+: Debugging Optionsgdwarf-2: Debugging Optionsgen-decls: Objective-C Dialect Optionsggdb: Debugging Optionsgnu-ld: HPPA Optionsgstabs: Debugging Optionsgstabs+: Debugging Optionsgvms: Debugging Optionsgxcoff: Debugging Optionsgxcoff+: Debugging OptionsH: Preprocessor Optionsheaderpad_max_install_names: Darwin Optionshelp: Preprocessor Optionshelp: Overall Optionshp-ld: HPPA OptionsI: Directory OptionsI: Preprocessor OptionsI-: Directory OptionsI-: Preprocessor Optionsidirafter: Preprocessor Optionsif-conversion: Optimize Optionsif-conversion2: Optimize Optionsimacros: Preprocessor Optionsimage_base: Darwin Optionsinclude: Preprocessor Optionsinit: Darwin Optionsinstall_name: Darwin Optionsiprefix: Preprocessor Optionsisystem: Preprocessor Optionsiwithprefix: Preprocessor Optionsiwithprefixbefore: Preprocessor Optionskeep_private_externs: Darwin OptionsL: Directory Optionsl: Link Optionslobjc: Link OptionsM: Preprocessor Optionsm1: SH Optionsm10: PDP-11 Optionsm128bit-long-double: i386 and x86-64 Optionsm16-bit: CRIS Optionsm2: SH Optionsm210: MCore Optionsm3: SH Optionsm31: S/390 and zSeries Optionsm32: i386 and x86-64 Optionsm32: SPARC Optionsm32-bit: CRIS Optionsm32032: NS32K Optionsm32081: NS32K Optionsm32332: NS32K Optionsm32381: NS32K Optionsm32532: NS32K Optionsm32r: M32R/D Optionsm32rx: M32R/D Optionsm340: MCore Optionsm386: i386 and x86-64 Optionsm3dnow: i386 and x86-64 Optionsm3e: SH Optionsm4: SH Optionsm4-nofpu: SH Optionsm4-single: SH Optionsm4-single-only: SH Optionsm40: PDP-11 Optionsm45: PDP-11 Optionsm4650: MIPS Optionsm486: i386 and x86-64 Optionsm4byte-functions: MCore Optionsm5200: M680x0 Optionsm64: S/390 and zSeries Optionsm64: i386 and x86-64 Optionsm64: SPARC Optionsm68000: M680x0 Optionsm68020: M680x0 Optionsm68020-40: M680x0 Optionsm68020-60: M680x0 Optionsm68030: M680x0 Optionsm68040: M680x0 Optionsm68060: M680x0 Optionsm6811: M68hc1x Optionsm6812: M68hc1x Optionsm68881: M680x0 Optionsm68hc11: M68hc1x Optionsm68hc12: M68hc1x Optionsm68hcs12: M68hc1x Optionsm68S12: M68hc1x Optionsm8-bit: CRIS Optionsm88000: M88K Optionsm88100: M88K Optionsm88110: M88K Optionsm96bit-long-double: i386 and x86-64 Optionsmabi-mmixware: MMIX Optionsmabi=32: MIPS Optionsmabi=64: MIPS Optionsmabi=altivec: RS/6000 and PowerPC Optionsmabi=eabi: MIPS Optionsmabi=gnu: MMIX Optionsmabi=meabi: MIPS 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Optionsmasm-compat: Intel 960 Optionsmasm-optimize: D30V Optionsmasm=dialect: i386 and x86-64 Optionsmauto-incdec: M68hc1x Optionsmauto-pic: IA-64 Optionsmb: SH Optionsmb-step: IA-64 Optionsmbackchain: S/390 and zSeries Optionsmbase-addresses: MMIX Optionsmbcopy: PDP-11 Optionsmbig: TMS320C3x/C4x Optionsmbig: RS/6000 and PowerPC Optionsmbig-endian: Xtensa Optionsmbig-endian: IA-64 Optionsmbig-endian: MCore Optionsmbig-endian: RS/6000 and PowerPC Optionsmbig-endian: ARM Optionsmbig-memory: TMS320C3x/C4x Optionsmbig-pic: M88K Optionsmbig-switch: V850 Optionsmbig-switch: HPPA Optionsmbigtable: SH Optionsmbit-align: RS/6000 and PowerPC Optionsmbitfield: NS32K Optionsmbitfield: M680x0 Optionsmbk: TMS320C3x/C4x Optionsmbooleans: Xtensa Optionsmbranch-cheap: PDP-11 Optionsmbranch-cost: D30V Optionsmbranch-expensive: PDP-11 Optionsmbranch-likely: MIPS Optionsmbranch-predict: MMIX Optionsmbroken-saverestore: SPARC Optionsmbuild-constants: DEC Alpha Optionsmbwx: DEC Alpha Optionsmc68000: M680x0 Optionsmc68020: M680x0 Optionsmca: Intel 960 Optionsmcall-aix: RS/6000 and PowerPC Optionsmcall-gnu: RS/6000 and PowerPC Optionsmcall-lib-mul: RT Optionsmcall-linux: RS/6000 and PowerPC Optionsmcall-netbsd: RS/6000 and PowerPC Optionsmcall-prologues: AVR Optionsmcall-solaris: RS/6000 and PowerPC Optionsmcall-sysv: RS/6000 and PowerPC Optionsmcall-sysv-eabi: RS/6000 and PowerPC Optionsmcall-sysv-noeabi: RS/6000 and PowerPC Optionsmcallee-super-interworking: ARM Optionsmcaller-super-interworking: ARM Optionsmcallgraph-data: MCore Optionsmcc-init: CRIS Optionsmcf: Intel 960 Optionsmcheck-zero-division: M88K Optionsmcix: DEC Alpha Optionsmcmodel=embmedany: SPARC Optionsmcmodel=kernel: i386 and x86-64 Optionsmcmodel=large: i386 and x86-64 Optionsmcmodel=medany: SPARC Optionsmcmodel=medium: i386 and x86-64 Optionsmcmodel=medlow: SPARC Optionsmcmodel=medmid: SPARC Optionsmcmodel=small: i386 and x86-64 Optionsmcode-align: Intel 960 Optionsmcode-model=large: M32R/D Optionsmcode-model=medium: M32R/D Optionsmcode-model=small: M32R/D Optionsmcomplex-addr: Intel 960 Optionsmcond-exec: FRV Optionsmcond-exec: D30V Optionsmcond-move: FRV Optionsmconst-align: CRIS Optionsmconstant-gp: IA-64 Optionsmcpu: FRV Optionsmcpu: CRIS Optionsmcpu: ARC Optionsmcpu: TMS320C3x/C4x Optionsmcpu: DEC Alpha Optionsmcpu: i386 and x86-64 Optionsmcpu: RS/6000 and PowerPC Optionsmcpu: ARM Optionsmcpu: SPARC Optionsmcpu32: M680x0 Optionsmcypress: SPARC OptionsMD: Preprocessor Optionsmdalign: SH Optionsmdata: ARC Optionsmdata-align: CRIS Optionsmdb: TMS320C3x/C4x Optionsmdebug: S/390 and zSeries Optionsmdec-asm: PDP-11 Optionsmdensity: Xtensa Optionsmdisable-callt: V850 Optionsmdisable-fpregs: HPPA Optionsmdisable-indexing: HPPA Optionsmdiv: MCore Optionsmdouble: FRV Optionsmdouble-float: MIPS Optionsmdp-isr-reload: TMS320C3x/C4x Optionsmdwarf2-asm: IA-64 Optionsmdword: FRV Optionsmeabi: RS/6000 and PowerPC Optionsmelf: MMIX Optionsmelf: CRIS Optionsmelinux: CRIS Optionsmelinux-stacksize: CRIS Optionsmemb: RS/6000 and PowerPC Optionsmembedded-data: MIPS Optionsmembedded-pic: MIPS Optionsmentry: MIPS Optionsmep: V850 Optionsmepsilon: MMIX Optionsmetrax100: CRIS Optionsmetrax4: CRIS Optionsmexplicit-relocs: DEC Alpha Optionsmextmem: D30V Optionsmextmemory: D30V OptionsMF: Preprocessor Optionsmfast-fix: TMS320C3x/C4x Optionsmfast-indirect-calls: HPPA Optionsmfaster-structs: SPARC Optionsmfix: DEC Alpha Optionsmfix7000: MIPS Optionsmfixed-cc: FRV Optionsmfixed-range: IA-64 Optionsmflat: SPARC Optionsmfloat-ieee: DEC Alpha Optionsmfloat-vax: DEC Alpha Optionsmfloat32: PDP-11 Optionsmfloat64: PDP-11 Optionsmflush-func: MIPS Optionsmfmovd: SH Optionsmfp: ARM Optionsmfp-arg-in-fpregs: RT Optionsmfp-arg-in-gregs: RT Optionsmfp-reg: DEC Alpha Optionsmfp-rounding-mode: DEC Alpha Optionsmfp-trap-mode: DEC Alpha Optionsmfp32: MIPS Optionsmfp64: MIPS Optionsmfpa: M680x0 Optionsmfpe: ARM Optionsmfpr-32: FRV Optionsmfpr-64: FRV Optionsmfpu: PDP-11 Optionsmfpu: SPARC Optionsmfull-fp-blocks: RT Optionsmfull-toc: RS/6000 and PowerPC Optionsmfused-madd: Xtensa Optionsmfused-madd: MIPS Optionsmfused-madd: RS/6000 and PowerPC Optionsmg: VAX OptionsMG: Preprocessor Optionsmgas: DEC Alpha Optionsmgas: HPPA Optionsmgas: MIPS Optionsmgnu: VAX Optionsmgnu-as: IA-64 Optionsmgnu-ld: IA-64 Optionsmgotplt: CRIS Optionsmgp32: MIPS Optionsmgp64: MIPS Optionsmgpopt: MIPS Optionsmgpr-32: FRV Optionsmgpr-64: FRV Optionsmh: H8/300 Optionsmhalf-pic: MIPS Optionsmhandle-large-shift: M88K Optionsmhard-float: Xtensa Optionsmhard-float: FRV Optionsmhard-float: S/390 and zSeries Optionsmhard-float: MIPS Optionsmhard-float: RS/6000 and PowerPC Optionsmhard-float: ARM Optionsmhard-float: SPARC Optionsmhard-quad-float: SPARC Optionsmhardlit: MCore Optionsmhc-struct-return: RT Optionsmhimem: NS32K Optionsmhitachi: SH Optionsmic-compat: Intel 960 Optionsmic2.0-compat: Intel 960 Optionsmic3.0-compat: Intel 960 Optionsmidentify-revision: M88K Optionsmieee: SH Optionsmieee: DEC Alpha Optionsmieee-compare: NS32K Optionsmieee-conformant: DEC Alpha Optionsmieee-fp: i386 and x86-64 Optionsmieee-with-inexact: DEC Alpha Optionsmimpure-text: SPARC Optionsmin-line-mul: RT Optionsminit-stack: AVR Optionsminline-all-stringops: i386 and x86-64 Optionsminline-float-divide-max-throughput: IA-64 Optionsminline-float-divide-min-latency: IA-64 Optionsminline-int-divide-max-throughput: IA-64 Optionsminline-int-divide-min-latency: IA-64 Optionsminmax: M68hc1x Optionsmint16: PDP-11 Optionsmint32: PDP-11 Optionsmint32: H8/300 Optionsmint64: MIPS Optionsmintel-asm: Intel 960 Optionsmips1: MIPS Optionsmips16: MIPS Optionsmips2: MIPS Optionsmips3: MIPS Optionsmips32: MIPS Optionsmips4: MIPS Optionsmips64: MIPS Optionsmisel: RS/6000 and PowerPC Optionsmisize: SH Optionsmjump-in-delay: HPPA Optionsmka: Intel 960 Optionsmkb: Intel 960 Optionsmknuthdiv: MMIX Optionsml: SH Optionsmlarge-data: DEC Alpha Optionsmleaf-procedures: Intel 960 Optionsmlibfuncs: MMIX Optionsmlibrary-pic: FRV Optionsmlinker-opt: HPPA Optionsmlinux: CRIS Optionsmlittle: RS/6000 and PowerPC Optionsmlittle-endian: Xtensa Optionsmlittle-endian: IA-64 Optionsmlittle-endian: MCore Optionsmlittle-endian: RS/6000 and PowerPC Optionsmlittle-endian: ARM Optionsmlittle-endian: SPARC Optionsmlive-g0: SPARC Optionsmlong-calls: V850 Optionsmlong-calls: MIPS Optionsmlong-calls: ARM Optionsmlong-calls: M68hc1x Optionsmlong-double-64: Intel 960 Optionsmlong-load-store: HPPA Optionsmlong32: MIPS Optionsmlong64: MIPS Optionsmlongcall: RS/6000 and PowerPC Optionsmlongcalls: Xtensa Optionsmloop-unsigned: TMS320C3x/C4x OptionsMM: Preprocessor Optionsmmac16: Xtensa Optionsmmad: MIPS Optionsmmangle-cpu: ARC Optionsmmax: DEC Alpha Optionsmmax-stack-frame: CRIS Optionsmmc: Intel 960 Optionsmmcu: AVR OptionsMMD: Preprocessor Optionsmmedia: FRV Optionsmmemcpy: MIPS Optionsmmemory-latency: DEC Alpha Optionsmmemparm: TMS320C3x/C4x Optionsmminimal-toc: RS/6000 and PowerPC Optionsmminimum-fp-blocks: RT Optionsmminmax: Xtensa Optionsmmips-as: MIPS Optionsmmips-tfile: MIPS Optionsmmmx: i386 and x86-64 Optionsmmpyi: TMS320C3x/C4x Optionsmmul-bug-workaround: CRIS Optionsmmul16: Xtensa Optionsmmul32: Xtensa Optionsmmuladd: FRV Optionsmmult-bug: MN10300 Optionsmmulti-add: NS32K Optionsmmulti-cond-exec: FRV Optionsmmultiple: RS/6000 and PowerPC Optionsmmvcle: S/390 and zSeries Optionsmmvme: RS/6000 and PowerPC Optionsmn: H8/300 Optionsmnested-cond-exec: FRV Optionsmnew-mnemonics: RS/6000 and PowerPC Optionsmno-3dnow: i386 and x86-64 Optionsmno-4byte-functions: MCore Optionsmno-abicalls: MIPS Optionsmno-abshi: PDP-11 Optionsmno-ac0: PDP-11 Optionsmno-align-double: i386 and x86-64 Optionsmno-align-int: M680x0 Optionsmno-align-stringops: i386 and x86-64 Optionsmno-alignment-traps: ARM Optionsmno-altivec: RS/6000 and PowerPC Optionsmno-am33: MN10300 Optionsmno-app-regs: V850 Optionsmno-app-regs: SPARC Optionsmno-asm-optimize: D30V Optionsmno-backchain: S/390 and zSeries Optionsmno-base-addresses: MMIX Optionsmno-bit-align: RS/6000 and PowerPC Optionsmno-bk: TMS320C3x/C4x Optionsmno-booleans: Xtensa Optionsmno-branch-likely: MIPS Optionsmno-branch-predict: MMIX Optionsmno-bwx: DEC Alpha Optionsmno-callgraph-data: MCore Optionsmno-check-zero-division: M88K Optionsmno-cix: DEC Alpha Optionsmno-code-align: Intel 960 Optionsmno-complex-addr: Intel 960 Optionsmno-cond-exec: FRV Optionsmno-cond-move: FRV Optionsmno-const-align: CRIS Optionsmno-crt0: MN10300 Optionsmno-data-align: CRIS Optionsmno-db: TMS320C3x/C4x Optionsmno-debug: S/390 and zSeries Optionsmno-density: Xtensa Optionsmno-div: MCore Optionsmno-double: FRV Optionsmno-dwarf2-asm: IA-64 Optionsmno-dword: FRV Optionsmno-eabi: RS/6000 and PowerPC Optionsmno-eflags: FRV Optionsmno-embedded-data: MIPS Optionsmno-embedded-pic: MIPS Optionsmno-ep: V850 Optionsmno-epsilon: MMIX Optionsmno-explicit-relocs: DEC Alpha Optionsmno-fancy-math-387: i386 and x86-64 Optionsmno-fast-fix: TMS320C3x/C4x Optionsmno-faster-structs: SPARC Optionsmno-fix: DEC Alpha Optionsmno-flat: SPARC Optionsmno-float32: PDP-11 Optionsmno-float64: PDP-11 Optionsmno-fp-in-toc: RS/6000 and PowerPC Optionsmno-fp-regs: DEC Alpha Optionsmno-fp-ret-in-387: i386 and x86-64 Optionsmno-fpu: SPARC Optionsmno-fused-madd: Xtensa Optionsmno-fused-madd: MIPS Optionsmno-fused-madd: RS/6000 and PowerPC Optionsmno-gnu-as: IA-64 Optionsmno-gnu-ld: IA-64 Optionsmno-gotplt: CRIS Optionsmno-gpopt: MIPS Optionsmno-half-pic: MIPS Optionsmno-hardlit: MCore Optionsmno-ieee-compare: NS32K Optionsmno-ieee-fp: i386 and x86-64 Optionsmno-int16: PDP-11 Optionsmno-int32: PDP-11 Optionsmno-interrupts: AVR Optionsmno-knuthdiv: MMIX Optionsmno-leaf-procedures: Intel 960 Optionsmno-libfuncs: MMIX Optionsmno-long-calls: V850 Optionsmno-long-calls: HPPA Optionsmno-long-calls: MIPS Optionsmno-long-calls: ARM Optionsmno-long-calls: M68hc1x Optionsmno-longcall: RS/6000 and PowerPC Optionsmno-longcalls: Xtensa Optionsmno-loop-unsigned: TMS320C3x/C4x Optionsmno-mac16: Xtensa Optionsmno-mad: MIPS Optionsmno-max: DEC Alpha Optionsmno-media: FRV Optionsmno-memcpy: MIPS Optionsmno-minmax: Xtensa Optionsmno-mips-tfile: MIPS Optionsmno-mips16: MIPS Optionsmno-mmx: i386 and x86-64 Optionsmno-mpyi: TMS320C3x/C4x Optionsmno-mul-bug-workaround: CRIS Optionsmno-mul16: Xtensa Optionsmno-mul32: Xtensa Optionsmno-muladd: FRV Optionsmno-mult-bug: MN10300 Optionsmno-multi-cond-exec: FRV Optionsmno-multiple: RS/6000 and PowerPC Optionsmno-mvcle: S/390 and zSeries Optionsmno-nested-cond-exec: FRV Optionsmno-nsa: Xtensa Optionsmno-ocs-debug-info: M88K Optionsmno-ocs-frame-position: M88K Optionsmno-optimize-arg-area: M88K Optionsmno-pack: FRV Optionsmno-parallel-insns: TMS320C3x/C4x Optionsmno-parallel-mpy: TMS320C3x/C4x Optionsmno-pic: IA-64 Optionsmno-power: RS/6000 and PowerPC Optionsmno-power2: RS/6000 and PowerPC Optionsmno-powerpc: RS/6000 and PowerPC Optionsmno-powerpc-gfxopt: RS/6000 and PowerPC Optionsmno-powerpc-gpopt: RS/6000 and PowerPC Optionsmno-powerpc64: RS/6000 and PowerPC Optionsmno-prolog-function: V850 Optionsmno-prologue-epilogue: CRIS Optionsmno-prototype: RS/6000 and PowerPC Optionsmno-push-args: i386 and x86-64 Optionsmno-register-names: IA-64 Optionsmno-regnames: RS/6000 and PowerPC Optionsmno-relax-immediate: MCore Optionsmno-relocatable: RS/6000 and PowerPC Optionsmno-relocatable-lib: RS/6000 and PowerPC Optionsmno-rnames: MIPS Optionsmno-rptb: TMS320C3x/C4x Optionsmno-rpts: TMS320C3x/C4x Optionsmno-scc: FRV Optionsmno-sched-prolog: ARM Optionsmno-sdata: IA-64 Optionsmno-sdata: RS/6000 and PowerPC Optionsmno-serialize-volatile: Xtensa Optionsmno-serialize-volatile: M88K Optionsmno-sext: Xtensa Optionsmno-short-load-bytes: ARM Optionsmno-short-load-words: ARM Optionsmno-side-effects: CRIS Optionsmno-single-exit: MMIX Optionsmno-slow-bytes: MCore Optionsmno-small-exec: S/390 and zSeries Optionsmno-soft-float: DEC Alpha Optionsmno-space-regs: HPPA Optionsmno-split: PDP-11 Optionsmno-split-addresses: MIPS Optionsmno-sse: i386 and x86-64 Optionsmno-stack-align: CRIS Optionsmno-stack-bias: SPARC Optionsmno-stats: MIPS Optionsmno-strict-align: Intel 960 Optionsmno-strict-align: RS/6000 and PowerPC Optionsmno-strict-align: M680x0 Optionsmno-string: RS/6000 and PowerPC Optionsmno-sum-in-toc: RS/6000 and PowerPC Optionsmno-svr3-shlib: i386 and x86-64 Optionsmno-tablejump: AVR Optionsmno-tail-call: Intel 960 Optionsmno-target-align: Xtensa Optionsmno-text-section-literals: Xtensa Optionsmno-toc: RS/6000 and PowerPC Optionsmno-toplevel-symbols: MMIX Optionsmno-unaligned-doubles: SPARC Optionsmno-underscores: M88K Optionsmno-uninit-const-in-rodata: MIPS Optionsmno-update: RS/6000 and PowerPC Optionsmno-vliw-branch: FRV Optionsmno-volatile-asm-stop: IA-64 Optionsmno-wide-bitfields: MCore Optionsmno-xl-call: RS/6000 and PowerPC Optionsmno-zero-extend: MMIX Optionsmnobitfield: NS32K Optionsmnobitfield: M680x0 Optionsmnohc-struct-return: RT Optionsmnohimem: NS32K Optionsmnomacsave: SH Optionsmnominmax: M68hc1x Optionsmnomulti-add: 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Dialect Optionstrigraphs: Preprocessor Optionstrigraphs: C Dialect Optionstwolevel_namespace: Darwin Optionsu: Link OptionsU: Preprocessor Optionsumbrella: Darwin Optionsundef: Preprocessor Optionsundefined: Darwin Optionsunexported_symbols_list: Darwin OptionsV: Target Optionsv: Preprocessor Optionsv: Overall Optionsversion: Preprocessor Optionsversion: Overall OptionsW: Incompatibilitiesw: Preprocessor OptionsW: Warning Optionsw: Warning OptionsWa: Assembler OptionsWabi: C++ Dialect OptionsWaggregate-return: Warning OptionsWall: Standard LibrariesWall: Preprocessor OptionsWall: Warning OptionsWbad-function-cast: Warning OptionsWcast-align: Warning OptionsWcast-qual: Warning OptionsWchar-subscripts: Warning OptionsWcomment: Preprocessor OptionsWcomment: Warning OptionsWcomments: Preprocessor OptionsWconversion: Protoize CaveatsWconversion: Warning OptionsWctor-dtor-privacy: C++ Dialect OptionsWdisabled-optimization: Warning OptionsWdiv-by-zero: Warning 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Warning OptionsWno-deprecated: C++ Dialect OptionsWno-deprecated-declarations: Warning OptionsWno-div-by-zero: Warning OptionsWno-format-extra-args: Warning OptionsWno-format-y2k: Warning OptionsWno-format-zero-length: Warning OptionsWno-import: Warning OptionsWno-long-long: Warning OptionsWno-multichar: Warning OptionsWno-non-template-friend: C++ Dialect OptionsWno-pmf-conversions: Bound member functionsWno-pmf-conversions: C++ Dialect OptionsWno-protocol: Objective-C Dialect OptionsWnon-virtual-dtor: C++ Dialect OptionsWnonnull: Warning OptionsWold-style-cast: C++ Dialect OptionsWoverloaded-virtual: C++ Dialect OptionsWp: Preprocessor OptionsWpacked: Warning OptionsWpadded: Warning OptionsWparentheses: Warning OptionsWpointer-arith: Pointer ArithWpointer-arith: Warning OptionsWredundant-decls: Warning OptionsWreorder: C++ Dialect OptionsWreturn-type: Warning OptionsWselector: Objective-C Dialect OptionsWsequence-point: Warning OptionsWshadow: Warning OptionsWsign-compare: Warning OptionsWsign-promo: C++ Dialect OptionsWstrict-aliasing: Warning OptionsWstrict-prototypes: Warning OptionsWswitch: Warning OptionsWswitch-enum: Warning OptionsWswitch-switch: Warning OptionsWsynth: C++ Dialect OptionsWsystem-headers: Preprocessor OptionsWsystem-headers: Warning OptionsWtraditional: Preprocessor OptionsWtraditional: Warning OptionsWtrigraphs: Preprocessor OptionsWtrigraphs: Warning OptionsWundeclared-selector: Objective-C Dialect OptionsWundef: Preprocessor OptionsWundef: Warning OptionsWuninitialized: Warning OptionsWunknown-pragmas: Warning OptionsWunreachable-code: Warning OptionsWunused: Warning OptionsWunused-function: Warning OptionsWunused-label: Warning OptionsWunused-macros: Preprocessor OptionsWunused-parameter: Warning OptionsWunused-value: Warning OptionsWunused-variable: Warning OptionsWwrite-strings: Warning Optionsx: Preprocessor Optionsx: Overall OptionsXlinker: Link OptionsYm: System V OptionsYP: System V Options#pragma implementation: C++ Interface#pragma implementation, implied: C++ Interface#pragma interface: C++ Interface#pragma, reason for not using: Function Attributes<?: Min and Max>?: Min and Max?: extensions: Conditionals?: extensions: Lvalues__builtin_apply: Constructing Calls__builtin_apply_args: Constructing Calls__builtin_choose_expr: Other Builtins__builtin_constant_p: Other Builtins__builtin_expect: Other Builtins__builtin_frame_address: Return Address__builtin_huge_val: Other Builtins__builtin_huge_valf: Other Builtins__builtin_huge_vall: Other Builtins__builtin_inf: Other Builtins__builtin_inff: Other Builtins__builtin_infl: Other Builtins__builtin_isgreater: Other Builtins__builtin_isgreaterequal: Other Builtins__builtin_isless: Other Builtins__builtin_islessequal: Other Builtins__builtin_islessgreater: Other Builtins__builtin_isunordered: Other Builtins__builtin_nan: Other Builtins__builtin_nanf: Other Builtins__builtin_nanl: Other Builtins__builtin_nans: Other Builtins__builtin_nansf: Other Builtins__builtin_nansl: Other Builtins__builtin_prefetch: Other Builtins__builtin_return: Constructing Calls__builtin_return_address: Return Address__builtin_types_compatible_p: Other Builtins__complex__ keyword: Complex__declspec(dllexport): Function Attributes__declspec(dllimport): Function Attributes__extension__: Alternate Keywords__func__ identifier: Function Names__FUNCTION__ identifier: Function Names__imag__ keyword: Complex__PRETTY_FUNCTION__ identifier: Function Names__real__ keyword: Complex__STDC_HOSTED__: Standards_Complex keyword: Complex_Exit: Other Builtins_exit: Other Builtinsabort: Other Builtinsabs: Other Builtinsaddress_operand: Simple Constraintsalias attribute: Function Attributesaligned attribute: Type Attributesaligned attribute: Variable Attributesalloca: Other Builtinsalloca vs variable-length arrays: Variable Lengthalways_inline function attribute: Function Attributesasm constraints: Constraintsasm expressions: Extended Asminline for C++ member fns: Inlinebcmp: Other Builtinsbzero: Other Builtinsc++: Invoking G++inline: InlineC_INCLUDE_PATH: Environment Variablescimag: Other Builtinscimagf: Other Builtinscimagl: Other Builtinscleanup attribute: Variable Attributescommon attribute: Variable AttributesCOMPILER_PATH: Environment Variablesconj: Other Builtinsconjf: Other Builtinsconjl: Other Builtinsconst applied to function: Function Attributesconst function attribute: Function Attributesasm: Constraintsconstructor function attribute: Function Attributescos: Other Builtinscosf: Other Builtinscosl: Other BuiltinsCPATH: Environment VariablesCPLUS_INCLUDE_PATH: Environment Variablescreal: Other Builtinscrealf: Other Builtinscreall: Other BuiltinsDEPENDENCIES_OUTPUT: Environment Variablesdeprecated attribute: Variable Attributesdeprecated attribute.: Function Attributesdestructor function attribute: Function Attributesexit: Other Builtinsexp: Other Builtinsexpf: Other Builtinsexpl: Other Builtinsasm: Extended Asm?:: Conditionals?:: Lvaluesfabs: Other Builtinsfabsf: Other Builtinsfabsl: Other Builtinsffs: Other Builtinsfloat as function value type: Incompatibilitiesformat function attribute: Function Attributesformat_arg function attribute: Function Attributesfprintf: Other Builtinsfprintf_unlocked: Other Builtinsfputs: Other Builtinsfputs_unlocked: Other Builtinsfscanf, and constant strings: Incompatibilitiesprintf, scanf, strftime or strfmon style arguments: Function Attributesg++: Invoking G++GCC_EXEC_PREFIX: Environment Variableslongjmp: Global Reg Varsimaxabs: Other Builtins#pragma implementation: C++ Interfaceindex: Other Builtinsinline automatic for C++ member fns: Inlinelabs: Other BuiltinsLANG: Environment VariablesLC_ALL: Environment VariablesLC_CTYPE: Environment VariablesLC_MESSAGES: Environment VariablesLIBRARY_PATH: Environment VariablesLL integer suffix: Long Longllabs: Other Builtinslog: Other Builtinslogf: Other Builtinslogl: Other Builtinslong long data types: Long Longlongjmp: Global Reg Varslongjmp incompatibilities: Incompatibilitieslongjmp warnings: Warning Optionsasm: Extended Asmmalloc attribute: Function Attributesinline: Inlinememcmp: Other Builtinsmemcpy: Other Builtinsmemset: Other Builtinsmktemp, and constant strings: Incompatibilitiesmode attribute: Variable Attributesno_instrument_function function attribute: Function Attributesnocommon attribute: Variable Attributesnoinline function attribute: Function Attributesnonnull function attribute: Function Attributesnoreturn function attribute: Function Attributesnothrow function attribute: Function AttributesOBJC_INCLUDE_PATH: Environment Variablesasm: Constraintspacked attribute: Variable Attributesprintf: Other Builtinsprintf_unlocked: Other Builtinspure function attribute: Function Attributesputchar: Other Builtinsputs: Other Builtinsqsort, and global register variables: Global Reg Varslongjmp: Global Reg Varsregparm attribute: Function Attributesrindex: Other Builtinsscanf: Other Builtinsscanf, and constant strings: Incompatibilitiessection function attribute: Function Attributessection variable attribute: Variable Attributessetjmp: Global Reg Varssetjmp incompatibilities: Incompatibilitiesshared variable attribute: Variable Attributessin: Other Builtinssinf: Other Builtinssinl: Other Builtinssizeof: Typeofsnprintf: Other Builtinssprintf: Other Builtinssqrt: Other Builtinssqrtf: Other Builtinssqrtl: Other Builtinssscanf: Other Builtinssscanf, and constant strings: Incompatibilitiesstrcat: Other Builtinsstrchr: Other Builtinsstrcmp: Other Builtinsstrcpy: Other Builtinsstrcspn: Other Builtinsstrlen: Other Builtinsstrncat: Other Builtinsstrncmp: Other Builtinsstrncpy: Other Builtinsstrpbrk: Other Builtinsstrrchr: Other Builtinsstrspn: Other Builtinsstrstr: Other BuiltinsSUNPRO_DEPENDENCIES: Environment Variablestls_model attribute: Variable AttributesTMPDIR: Environment Variablestypeof: TypeofULL integer suffix: Long Longunused attribute.: Function Attributesused attribute.: Function Attributeslongjmp: Global Reg Varsvisibility attribute: Function Attributesvolatile applied to function: Function Attributesvprintf: Other Builtinsvscanf: Other Builtinsvsnprintf: Other Builtinsvsprintf: Other Builtinsvsscanf: Other Builtinsweak attribute: Function Attributes[1] On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous.
[2] Future versions of GCC may zero-extend, or use
a target-defined ptr_extend pattern. Do not rely on sign extension.
[3] The analogous feature in Fortran is called an assigned goto, but that name seems inappropriate in C, where one can do more than simply store label addresses in label variables.
[4] A file's basename was the name stripped of all leading path information and of trailing suffixes, such as .h or .C or .cc.