This manual documents the use of gfortran, the GNU Fortran 95 compiler. You can find in this manual how to invoke gfortran, as well as its features and incompatibilities.
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Gfortran is the GNU Fortran 95 compiler front end, designed initially as a free replacement for, or alternative to, the unix f95 command; gfortran is command you'll use to invoke the compiler.
Gfortran is still in an early state of development. gfortran can generate code for most constructs and expressions, but much work remains to be done.
When gfortran is finished, it will do everything you expect from any decent compiler:
The compiler will also attempt to diagnose cases where the user's program contains a correct usage of the language, but instructs the computer to do something questionable. This kind of diagnostics message is called a warning message.
Gfortran consists of several components:
GCC used to be the GNU “C” Compiler, but is now known as the GNU Compiler Collection. GCC provides the GNU system with a very versatile compiler middle end (shared optimization passes), and with back ends (code generators) for many different computer architectures and operating systems. The code of the middle end and back end are shared by all compiler front ends that are in the GNU Compiler Collection.
A GCC front end is essentially a source code parser and a pass to generate a representation of the semantics of the program in the source code in the GCC language independent intermediate language, called GENERIC.
The parser takes a source file written in a particular computer language, reads and parses it, and tries to make sure that the source code conforms to the language rules. Once the correctness of a program has been established, the compiler will build a data structure known as the Abstract Syntax tree, or just AST or “tree” for short. This data structure represents the whole program or a subroutine or a function. The “tree” is passed to the GCC middle end, which will perform optimization passes on it, pass the optimized AST and generate assembly for the program unit.
Different phases in this translation process can be, and in fact are merged in many compiler front ends. GNU Fortran 95 has a strict separation between the parser and code generator.
The goal of the gfortran project is to build a new front end for GCC: A Fortran 95 front end. In a non-gfortran installation, gcc will not be able to compile Fortran 95 source code (only the “C” front end has to be compiled if you want to build GCC, all other languages are optional). If you build GCC with gfortran, gcc will recognize .f/.f90/.f95 source files and accepts Fortran 95 specific command line options.
Why do we write a compiler front end from scratch? There's a fine Fortran 77 compiler in the GNU Compiler Collection that accepts some features of the Fortran 90 standard as extensions. Why not start from there and revamp it?
One of the reasons is that Craig Burley, the author of G77, has decided to stop working on the G77 front end. On Craig explains the reasons for his decision to stop working on G77 in one of the pages in his homepage. Among the reasons is a lack of interest in improvements to g77. Users appear to be quite satisfied with g77 as it is. While g77 is still being maintained (by Toon Moene), it is unlikely that sufficient people will be willing to completely rewrite the existing code.
But there are other reasons to start from scratch. Many people, including Craig Burley, no longer agreed with certain design decisions in the G77 front end. Also, the interface of g77 to the back end is written in a style which is confusing and not up to date on recommended practice. In fact, a full rewrite had already been planned for GCC 3.0.
When Craig decided to stop, it just seemed to be a better idea to start a new project from scratch, because it was expected to be easier to maintain code we develop ourselves than to do a major overhaul of g77 first, and then build a Fortran 95 compiler out of it.
The gfortran command supports all the options supported by the gcc command. Only options specific to gfortran are documented here.
See GCC Command Options (Using the GNU Compiler Collection (GCC)), for information on the non-Fortran-specific aspects of the gcc command (and, therefore, the gfortran command).
All gcc and gfortran options are accepted both by gfortran and by gcc (as well as any other drivers built at the same time, such as g++), since adding gfortran to the gcc distribution enables acceptance of gfortran options by all of the relevant drivers.
In some cases, options have 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.
Here is a summary of all the options specific to GNU Fortran, grouped by type. Explanations are in the following sections.
-ffree-form -fno-fixed-form
-fdollar-ok -fimplicit-none -fmax-identifier-length
-std=std
-ffixed-line-length-n -ffixed-line-length-none
-i8 -r8 -d8
-fsyntax-only -pedantic -pedantic-errors
-w -Wall -Waliasing -Wconversion
-Wimplicit-interface -Wnonstd-intrinsics -Wsurprising -Wunderflow
-Wunused-labels -Wline-truncation
-Werror -W
-fdump-parse-tree
-Idir -Mdir
-fno-underscoring -fno-second-underscore
-fbounds-check -fmax-stack-var-size=n
-fpackderived -frepack-arrays
The following options control the dialect of Fortran that the compiler accepts:
-ffree-form-ffixed-form-fdollar-ok-ffixed-line-length-nPopular values for n include 72 (the standard and the default), 80 (card image), and 132 (corresponds to “extended-source” options in some popular compilers). n may be none, meaning that the entire line is meaningful and that continued character constants never have implicit spaces appended to them to fill out the line. -ffixed-line-length-0 means the same thing as -ffixed-line-length-none.
-fmax-identifier-length=n-fimplicit-none-std=std-i8-r8-d8INTEGER
and REAL kinds to KIND=8. The -d8 option is
equivalent to specifying both -i8 and -r8.
When -r8 is specified, the DOUBLE PRECISION kind is set
to KIND=16 if the target supports a 16 byte floating point format.
If no such format exists, the DOUBLE PRECISION kind is unchanged.
Warnings are diagnostic messages that report constructions which are not inherently erroneous but which are risky or suggest there might 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.
These options control the amount and kinds of warnings produced by GNU Fortran:
-fsyntax-only-pedanticValid FORTRAN 95 programs should compile properly with or without this option. However, without this option, certain GNU extensions and traditional Fortran features are supported as well. With this option, many of them are rejected.
Some users try to use -pedantic to check programs for conformance. They soon find that it does not do quite what they want—it finds some nonstandard practices, but not all. However, improvements to gfortran in this area are welcome.
This should be used in conjunction with -std=std.
-pedantic-errors-w-Wall-Waliasingbar to
modify either parameter.
INTEGER A
CALL BAR(A,A)
-Wconversion-Wimplicit-interface-Wnonstd-intrinsic-WsurprisingThis currently produces a warning under the following circumstances:
-Wunderflow-Wunused-labels-Werror-WSee Options to Request or Suppress Warnings (Using the GNU Compiler Collection (GCC)), for information on more options offered by the GBE shared by gfortran, gcc and other GNU compilers.
Some of these have no effect when compiling programs written in Fortran.
GNU Fortran has various special options that are used for debugging either your program or gfortran
-fdump-parse-treeSee Options for Debugging Your Program or GCC (Using the GNU Compiler Collection (GCC)), for more information on debugging options.
There options affect how affect how gfortran searches
for files specified via the INCLUDE directive, and where it searches
for previously compiled modules.
It also affects the search paths used by cpp when used to preprocess Fortran source.
-IdirINCLUDE directive
(as well as of the #include directive of the cpp
preprocessor).
Also note that the general behavior of -I and
INCLUDE is pretty much the same as of -I with
#include in the cpp preprocessor, with regard to
looking for header.gcc files and other such things.
This path is also used to search for .mod files when previously
compiled modules are required by a USE statement.
See Options for Directory Search (Using the GNU Compiler Collection (GCC)), for information on the -I option.
-Mdir-JdirUSE
statement.
The default is the current directory.
-J is an alias for -M to avoid conflicts with existing GCC options.
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.
-fno-underscoringWith -funderscoring in effect, gfortran appends two underscores to names with underscores and one underscore to external names with no underscores. (gfortran also appends two underscores to internal names with underscores to avoid naming collisions with external names. The -fno-second-underscore option disables appending of the second underscore in all cases.)
This is done to ensure compatibility with code produced by many UNIX Fortran compilers, including f2c which perform the same transformations.
Use of -fno-underscoring is not recommended unless you are experimenting with issues such as integration of (GNU) Fortran into existing system environments (vis-a-vis existing libraries, tools, and so on).
For example, with -funderscoring, and assuming other defaults like -fcase-lower and that j() and max_count() are external functions while my_var and lvar are local variables, a statement like
I = J() + MAX_COUNT (MY_VAR, LVAR)
is implemented as something akin to:
i = j_() + max_count__(&my_var__, &lvar);
With -fno-underscoring, the same statement is implemented as:
i = j() + max_count(&my_var, &lvar);
Use of -fno-underscoring allows direct specification of user-defined names while debugging and when interfacing gfortran code with other languages.
Note that just because the names match does not mean that the interface implemented by gfortran for an external name matches the interface implemented by some other language for that same name. That is, getting code produced by gfortran to link to code produced by some other compiler using this or any other method can be only a small part of the overall solution—getting the code generated by both compilers to agree on issues other than naming can require significant effort, and, unlike naming disagreements, linkers normally cannot detect disagreements in these other areas.
Also, note that with -fno-underscoring, the lack of appended underscores introduces the very real possibility that a user-defined external name will conflict with a name in a system library, which could make finding unresolved-reference bugs quite difficult in some cases—they might occur at program run time, and show up only as buggy behavior at run time.
In future versions of gfortran we hope to improve naming and linking issues so that debugging always involves using the names as they appear in the source, even if the names as seen by the linker are mangled to prevent accidental linking between procedures with incompatible interfaces.
-fno-second-underscoreThis option has no effect if -fno-underscoring is in effect.
Otherwise, with this option, an external name such as MAX_COUNT is implemented as a reference to the link-time external symbol max_count_, instead of max_count__.
-fbounds-checkIn the future this may also include other forms of checking, eg. checking substring references.
-fmax-stack-var-size=nThis option currently only affects local arrays declared with constant bounds, and may not apply to all character variables. Future versions of gfortran may improve this behavior.
The default value for n is 32768.
-fpackderived-frepack-arraysThis should result in faster accesses to the array. However it can introduce significant overhead to the function call, especially when the passed data is discontiguous.
See Options for Code Generation Conventions (Using the GNU Compiler Collection (GCC)), for information on more options offered by the GBE shared by gfortran gcc and other GNU compilers.
GNU Fortran 95 currently does not make use of any environment variables to control its operation above and beyond those that affect the operation of gcc.
See Environment Variables Affecting GCC (Using the GNU Compiler Collection (GCC)), for information on environment variables.
As soon as gfortran can parse all of the statements correctly, it will be in the “larva” state. When we generate code, the “puppa” state. When gfortran is done, we'll see if it will be a beautiful butterfly, or just a big bug....–Andy Vaught, April 2000
The start of the GNU Fortran 95 project was announced on the GCC homepage in March 18, 2000 (even though Andy had already been working on it for a while, or course).
Gfortran is currently reaching the stage where is is able to compile real world programs. However it is still under development and has many rough edges.
Some intrinsic functions map directly to library functions, and in most cases the name of the library function used depends on the type of the arguments. For some intrinsics we generate inline code, and for others, such as sin, cos and sqrt, we rely on the backend to use special instructions in the floating point unit of the CPU if available, or to fall back to a call to libm if these are not available.
Implementation of some non-elemental intrinsic functions (eg. DOT_PRODUCT, AVERAGE) is not yet optimal. This is hard because we have to make decisions whether to use inline code (good for small arrays as no function call overhead occurs) or generate function calls (good for large arrays as it allows use of hand-optimized assembly routines, SIMD instructions, etc.)
The IO library is still under development. The following features should be usable for real programs:
Usable with bugs:
Not recommended:
Many Fortran programs only use a small subset of the available IO capabilities, so your mileage may vary.
Here's a list of proposed extensions for gfortran, in no particular order. Most of these are necessary to be fully compatible with existing Fortran compilers, but they are not part of the official J3 Fortran 95 standard.
Makefile info.
gfortran implements a number of extensions over standard Fortran. This chapter contains information on their syntax and meaning.
gfortran allows old-style kind specifications in declarations. These look like:
TYPESPEC*k x,y,z
where TYPESPEC is a basic type, and where k is a valid kind
number for that type. The statement then declares x, y
and z to be of type TYPESPEC with kind k. In
other words, it is equivalent to the standard conforming declaration
TYPESPEC(k) x,y,z
gfortran allows old-style initialization of variables of the form:
INTEGER*4 i/1/,j/2/
REAL*8 x(2,2) /3*0.,1./
These are only allowed in declarations without double colons
(::), as these were introduced in Fortran 90 which also
introduced a new syntax for variable initializations. The syntax for
the individual initializers is as for the DATA statement, but
unlike in a DATA statement, an initializer only applies to the
variable immediately preceding. In other words, something like
INTEGER I,J/2,3/ is not valid.
Examples of standard conforming code equivalent to the above example, are:
! Fortran 90
INTEGER(4) :: i = 1, j = 2
REAL(8) :: x(2,2) = RESHAPE((/0.,0.,0.,1./),SHAPE(x))
! Fortran 77
INTEGER i, j
DOUBLE PRECISION x(2,2)
DATA i,j,x /1,2,3*0.,1./
Free software is only possible if people contribute to efforts to create it. We're always in need of more people helping out with ideas and comments, writing documentation and contributing code.
If you want to contribute to GNU Fortran 95, have a look at the long lists of projects you can take on. Some of these projects are small, some of them are large; some are completely orthogonal to the rest of what is happening on gfortran, but others are “mainstream” projects in need of enthusiastic hackers. All of these projects are important! We'll eventually get around to the things here, but they are also things doable by someone who is willing and able.
Most of the parser was hand-crafted by Andy Vaught, who is also the initiator of the whole project. Thanks Andy! Most of the interface with GCC was written by Paul Brook.
The following individuals have contributed code and/or ideas and significant help to the gfortran project (in no particular order):
The following people have contributed bug reports, smaller or larger patches, and much needed feedback and encouragement for the gfortran project:
Many other individuals have helped debug, test and improve gfortran over the past two years, and we welcome you to do the same! If you already have done so, and you would like to see your name listed in the list above, please contact us.
If you wish to work on the runtime libraries, please contact a project maintainer.
The GNU Fortran 95 Compiler aims to be a conforming implementation of ISO/IEC 1539:1997 (Fortran 95).
In the future it may also support other variants and extensions to the Fortran language. This includes ANSI Fortran 77, Fortran 90, Fortran 2000 (not yet finalized), and OpenMP.