GNAT User's Guide for Unix Platforms

GNAT User's Guide

GNAT User's Guide for Unix Platforms

GNAT, The GNU Ada 95 Compiler

GNAT Version for GCC 3.3.4

Ada Core Technologies, Inc. Copyright © 1995-2002, Free Software Foundation

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 Free Documentation License", with the Front-Cover Texts being "GNAT User's Guide for Unix Platforms", and with no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License".

• About This Guide:
• Getting Started with GNAT:
• The GNAT Compilation Model:
• Compiling Using gcc:
• Binding Using gnatbind:
• Linking Using gnatlink:
• The GNAT Make Program gnatmake:
• Renaming Files Using gnatchop:
• Configuration Pragmas:
• Handling Arbitrary File Naming Conventions Using gnatname:
• GNAT Project Manager:
• Elaboration Order Handling in GNAT:
• The Cross-Referencing Tools gnatxref and gnatfind:
• File Name Krunching Using gnatkr:
• Preprocessing Using gnatprep:
• The GNAT Library Browser gnatls:
• GNAT and Libraries:
• Using the GNU make Utility:
• Finding Memory Problems with gnatmem:
• Finding Memory Problems with GNAT Debug Pool:
• Creating Sample Bodies Using gnatstub:
• Reducing the Size of Ada Executables with gnatelim:
• Other Utility Programs:
• Running and Debugging Ada Programs:
• Inline Assembler:
• Performance Considerations:
• GNU Free Documentation License:
• Index:

  --- The Detailed Node Listing ---

  About This Guide

• What This Guide Contains:
• What You Should Know before Reading This Guide:
• Related Information:
• Conventions:

  Getting Started with GNAT

• Running GNAT:
• Running a Simple Ada Program:
• Running a Program with Multiple Units:
• Using the gnatmake Utility:

  The GNAT Compilation Model

• Source Representation:
• Foreign Language Representation:
• File Naming Rules:
• Using Other File Names:
• Alternative File Naming Schemes:
• Generating Object Files:
• Source Dependencies:
• The Ada Library Information Files:
• Binding an Ada Program:
• Mixed Language Programming:
• Building Mixed Ada & C++ Programs:
• Comparison between GNAT and C/C++ Compilation Models:
• Comparison between GNAT and Conventional Ada Library Models:

  Foreign Language Representation

• Latin-1:
• Other 8-Bit Codes:
• Wide Character Encodings:

  Compiling Ada Programs With gcc

• Compiling Programs:
• Switches for gcc:
• Search Paths and the Run-Time Library (RTL):
• Order of Compilation Issues:
• Examples:

  Switches for gcc

• Output and Error Message Control:
• Debugging and Assertion Control:
• Run-Time Checks:
• Stack Overflow Checking:
• Run-Time Control:
• Validity Checking:
• Style Checking:
• Using gcc for Syntax Checking:
• Using gcc for Semantic Checking:
• Compiling Ada 83 Programs:
• Character Set Control:
• File Naming Control:
• Subprogram Inlining Control:
• Auxiliary Output Control:
• Debugging Control:
• Units to Sources Mapping Files:

  Binding Ada Programs With gnatbind

• Running gnatbind:
• Generating the Binder Program in C:
• Consistency-Checking Modes:
• Binder Error Message Control:
• Elaboration Control:
• Output Control:
• Binding with Non-Ada Main Programs:
• Binding Programs with No Main Subprogram:
• Summary of Binder Switches:
• Command-Line Access:
• Search Paths for gnatbind:
• Examples of gnatbind Usage:

  Linking Using gnatlink

• Running gnatlink:
• Switches for gnatlink:
• Setting Stack Size from gnatlink:
• Setting Heap Size from gnatlink:

  The GNAT Make Program gnatmake

• Running gnatmake:
• Switches for gnatmake:
• Mode Switches for gnatmake:
• Notes on the Command Line:
• How gnatmake Works:
• Examples of gnatmake Usage:

  Renaming Files Using gnatchop

• Handling Files with Multiple Units:
• Operating gnatchop in Compilation Mode:
• Command Line for gnatchop:
• Switches for gnatchop:
• Examples of gnatchop Usage:

  Configuration Pragmas

• Handling of Configuration Pragmas:
• The Configuration Pragmas Files:

  Handling Arbitrary File Naming Conventions Using gnatname

• Arbitrary File Naming Conventions:
• Running gnatname:
• Switches for gnatname:
• Examples of gnatname Usage:

  GNAT Project Manager

• Introduction:
• Examples of Project Files:
• Project File Syntax:
• Objects and Sources in Project Files:
• Importing Projects:
• Project Extension:
• External References in Project Files:
• Packages in Project Files:
• Variables from Imported Projects:
• Naming Schemes:
• Library Projects:
• Switches Related to Project Files:
• Tools Supporting Project Files:
• An Extended Example:
• Project File Complete Syntax:

  Elaboration Order Handling in GNAT

• Elaboration Code in Ada 95:
• Checking the Elaboration Order in Ada 95:
• Controlling the Elaboration Order in Ada 95:
• Controlling Elaboration in GNAT - Internal Calls:
• Controlling Elaboration in GNAT - External Calls:
• Default Behavior in GNAT - Ensuring Safety:
• Elaboration Issues for Library Tasks:
• Mixing Elaboration Models:
• What to Do If the Default Elaboration Behavior Fails:
• Elaboration for Access-to-Subprogram Values:
• Summary of Procedures for Elaboration Control:
• Other Elaboration Order Considerations:

  The Cross-Referencing Tools gnatxref and gnatfind

• gnatxref Switches:
• gnatfind Switches:
• Project Files for gnatxref and gnatfind:
• Regular Expressions in gnatfind and gnatxref:
• Examples of gnatxref Usage:
• Examples of gnatfind Usage:

  File Name Krunching Using gnatkr

• About gnatkr:
• Using gnatkr:
• Krunching Method:
• Examples of gnatkr Usage:

  Preprocessing Using gnatprep

• Using gnatprep:
• Switches for gnatprep:
• Form of Definitions File:
• Form of Input Text for gnatprep:

  The GNAT Library Browser gnatls

• Running gnatls:
• Switches for gnatls:
• Examples of gnatls Usage:

  GNAT and Libraries

• Creating an Ada Library:
• Installing an Ada Library:
• Using an Ada Library:
• Creating an Ada Library to be Used in a Non-Ada Context:
• Rebuilding the GNAT Run-Time Library:

  Using the GNU make Utility

• Using gnatmake in a Makefile:
• Automatically Creating a List of Directories:
• Generating the Command Line Switches:
• Overcoming Command Line Length Limits:

  Finding Memory Problems with gnatmem

• Running gnatmem (GDB Mode):
• Running gnatmem (GMEM Mode):
• Switches for gnatmem:
• Examples of gnatmem Usage:
• GDB and GMEM Modes:
• Implementation Note:

  Finding Memory Problems with GNAT Debug Pool

  Creating Sample Bodies Using gnatstub

• Running gnatstub:
• Switches for gnatstub:

  Reducing the Size of Ada Executables with gnatelim

• About gnatelim:
• Eliminate Pragma:
• Tree Files:
• Preparing Tree and Bind Files for gnatelim:
• Running gnatelim:
• Correcting the List of Eliminate Pragmas:
• Making Your Executables Smaller:
• Summary of the gnatelim Usage Cycle:

  Other Utility Programs

• Using Other Utility Programs with GNAT:
• The gnatpsta Utility Program:
• The External Symbol Naming Scheme of GNAT:
• Ada Mode for Glide:
• Converting Ada Files to html with gnathtml:

  Running and Debugging Ada Programs

• The GNAT Debugger GDB:
• Running GDB:
• Introduction to GDB Commands:
• Using Ada Expressions:
• Calling User-Defined Subprograms:
• Using the Next Command in a Function:
• Ada Exceptions:
• Ada Tasks:
• Debugging Generic Units:
• GNAT Abnormal Termination or Failure to Terminate:
• Naming Conventions for GNAT Source Files:
• Getting Internal Debugging Information:
• Stack Traceback:

  Inline Assembler

• Basic Assembler Syntax:
• A Simple Example of Inline Assembler:
• Output Variables in Inline Assembler:
• Input Variables in Inline Assembler:
• Inlining Inline Assembler Code:
• Other Asm Functionality:
• A Complete Example:

  Performance Considerations

• Controlling Run-Time Checks:
• Optimization Levels:
• Debugging Optimized Code:
• Inlining of Subprograms:

• Index:

About This Guide

This guide describes the use of GNAT, a compiler and software development toolset for the full Ada 95 programming language. It describes the features of the compiler and tools, and details how to use them to build Ada 95 applications.

• What This Guide Contains:
• What You Should Know before Reading This Guide:
• Related Information:
• Conventions:

What This Guide Contains

This guide contains the following chapters:

• Getting Started with GNAT, describes how to get started compiling and running Ada programs with the GNAT Ada programming environment.
• The GNAT Compilation Model, describes the compilation model used by GNAT.
• Compiling Using gcc, describes how to compile Ada programs with gcc, the Ada compiler.
• Binding Using gnatbind, describes how to perform binding of Ada programs with gnatbind, the GNAT binding utility.
• Linking Using gnatlink, describes gnatlink, a program that provides for linking using the GNAT run-time library to construct a program. gnatlink can also incorporate foreign language object units into the executable.
• The GNAT Make Program gnatmake, describes gnatmake, a utility that automatically determines the set of sources needed by an Ada compilation unit, and executes the necessary compilations binding and link.
• Renaming Files Using gnatchop, describes gnatchop, a utility that allows you to preprocess a file that contains Ada source code, and split it into one or more new files, one for each compilation unit.
• Configuration Pragmas, describes the configuration pragmas handled by GNAT.
• Handling Arbitrary File Naming Conventions Using gnatname, shows how to override the default GNAT file naming conventions, either for an individual unit or globally.
• GNAT Project Manager, describes how to use project files to organize large projects.
• Elaboration Order Handling in GNAT, describes how GNAT helps you deal with elaboration order issues.
• The Cross-Referencing Tools gnatxref and gnatfind, discusses gnatxref and gnatfind, two tools that provide an easy way to navigate through sources.
• File Name Krunching Using gnatkr, describes the gnatkr file name krunching utility, used to handle shortened file names on operating systems with a limit on the length of names.
• Preprocessing Using gnatprep, describes gnatprep, a preprocessor utility that allows a single source file to be used to generate multiple or parameterized source files, by means of macro substitution.
• The GNAT Library Browser gnatls, describes gnatls, a utility that displays information about compiled units, including dependences on the corresponding sources files, and consistency of compilations.
• GNAT and Libraries, describes the process of creating and using Libraries with GNAT. It also describes how to recompile the GNAT run-time library.
• Using the GNU make Utility, describes some techniques for using the GNAT toolset in Makefiles.
• Finding Memory Problems with gnatmem, describes gnatmem, a utility that monitors dynamic allocation and deallocation activity in a program, and displays information about incorrect deallocations and sources of possible memory leaks.
• Finding Memory Problems with GNAT Debug Pool, describes how to use the GNAT-specific Debug Pool in order to detect as early as possible the use of incorrect memory references.
• Creating Sample Bodies Using gnatstub, discusses gnatstub, a utility that generates empty but compilable bodies for library units.
• Reducing the Size of Ada Executables with gnatelim, describes gnatelim, a tool which detects unused subprograms and helps the compiler to create a smaller executable for the program.
• Other Utility Programs, discusses several other GNAT utilities, including gnatpsta.
• Running and Debugging Ada Programs, describes how to run and debug Ada programs.
• Inline Assembler, shows how to use the inline assembly facility in an Ada program.
• Performance Considerations, reviews the trade offs between using defaults or options in program development.

What You Should Know before Reading This Guide

This user's guide assumes that you are familiar with Ada 95 language, as described in the International Standard ANSI/ISO/IEC-8652:1995, Jan 1995.

Related Information

For further information about related tools, refer to the following documents:

• GNAT Reference Manual, which contains all reference material for the GNAT implementation of Ada 95.
• Ada 95 Language Reference Manual, which contains all reference material for the Ada 95 programming language.
• Debugging with GDB contains all details on the use of the GNU source-level debugger.
• GNU Emacs Manual contains full information on the extensible editor and programming environment Emacs.

Conventions

Following are examples of the typographical and graphic conventions used in this guide:

• Functions, utility program names, standard names, and classes.
• Option flags
• File Names, button names, and field names.
• Variables.
• Emphasis.
• [optional information or parameters]
• Examples are described by text

            and then shown this way.
            

Commands that are entered by the user are preceded in this manual by the characters "$ " (dollar sign followed by space). If your system uses this sequence as a prompt, then the commands will appear exactly as you see them in the manual. If your system uses some other prompt, then the command will appear with the $ replaced by whatever prompt character you are using.

1 Getting Started with GNAT

This chapter describes some simple ways of using GNAT to build executable Ada programs.

• Running GNAT:
• Running a Simple Ada Program:

• Running a Program with Multiple Units:

• Using the gnatmake Utility:
• Introduction to Glide and GVD:

1.1 Running GNAT

Three steps are needed to create an executable file from an Ada source file:

1. The source file(s) must be compiled.
2. The file(s) must be bound using the GNAT binder.
3. All appropriate object files must be linked to produce an executable.

All three steps are most commonly handled by using the gnatmake utility program that, given the name of the main program, automatically performs the necessary compilation, binding and linking steps.

1.2 Running a Simple Ada Program

Any text editor may be used to prepare an Ada program. If Glide is used, the optional Ada mode may be helpful in laying out the program. The program text is a normal text file. We will suppose in our initial example that you have used your editor to prepare the following standard format text file:

     

     with Ada.Text_IO; use Ada.Text_IO;
     procedure Hello is
     begin
        Put_Line ("Hello WORLD!");
     end Hello;
     
     

This file should be named hello.adb. With the normal default file naming conventions, GNAT requires that each file contain a single compilation unit whose file name is the unit name, with periods replaced by hyphens; the extension is ads for a spec and adb for a body. You can override this default file naming convention by use of the special pragma Source_File_Name (see Using Other File Names). Alternatively, if you want to rename your files according to this default convention, which is probably more convenient if you will be using GNAT for all your compilations, then the gnatchop utility can be used to generate correctly-named source files (see Renaming Files Using gnatchop).

You can compile the program using the following command ($ is used as the command prompt in the examples in this document):

     $ gcc -c hello.adb
     

gcc is the command used to run the compiler. This compiler is capable of compiling programs in several languages, including Ada 95 and C. It assumes that you have given it an Ada program if the file extension is either .ads or .adb, and it will then call the GNAT compiler to compile the specified file.

The -c switch is required. It tells gcc to only do a compilation. (For C programs, gcc can also do linking, but this capability is not used directly for Ada programs, so the -c switch must always be present.)

This compile command generates a file hello.o, which is the object file corresponding to your Ada program. It also generates an "Ada Library Information" file hello.ali, which contains additional information used to check that an Ada program is consistent. To build an executable file, use gnatbind to bind the program and gnatlink to link it. The argument to both gnatbind and gnatlink is the name of the ali file, but the default extension of .ali can be omitted. This means that in the most common case, the argument is simply the name of the main program:

     $ gnatbind hello
     $ gnatlink hello
     

A simpler method of carrying out these steps is to use gnatmake, a master program that invokes all the required compilation, binding and linking tools in the correct order. In particular, gnatmake automatically recompiles any sources that have been modified since they were last compiled, or sources that depend on such modified sources, so that "version skew" is avoided.

     $ gnatmake hello.adb
     

The result is an executable program called hello, which can be run by entering:

     $ hello
     

assuming that the current directory is on the search path for executable programs.

and, if all has gone well, you will see

     Hello WORLD!
     

appear in response to this command.

1.3 Running a Program with Multiple Units

Consider a slightly more complicated example that has three files: a main program, and the spec and body of a package:

     

     
package Greetings is
        procedure Hello;
        procedure Goodbye;
     end Greetings;
     
     with Ada.Text_IO; use Ada.Text_IO;
     package body Greetings is
        procedure Hello is
        begin
           Put_Line ("Hello WORLD!");
        end Hello;
     
        procedure Goodbye is
        begin
           Put_Line ("Goodbye WORLD!");
        end Goodbye;
     end Greetings;

     
with Greetings;
     procedure Gmain is
     begin
        Greetings.Hello;
        Greetings.Goodbye;
     end Gmain;
     
     

Following the one-unit-per-file rule, place this program in the following three separate files:

greetings.ads

spec of package Greetings

greetings.adb

body of package Greetings

gmain.adb

body of main program

To build an executable version of this program, we could use four separate steps to compile, bind, and link the program, as follows:

     $ gcc -c gmain.adb
     $ gcc -c greetings.adb
     $ gnatbind gmain
     $ gnatlink gmain
     

Note that there is no required order of compilation when using GNAT. In particular it is perfectly fine to compile the main program first. Also, it is not necessary to compile package specs in the case where there is an accompanying body; you only need to compile the body. If you want to submit these files to the compiler for semantic checking and not code generation, then use the -gnatc switch:

        $ gcc -c greetings.ads -gnatc
     

Although the compilation can be done in separate steps as in the above example, in practice it is almost always more convenient to use the gnatmake tool. All you need to know in this case is the name of the main program's source file. The effect of the above four commands can be achieved with a single one:

     $ gnatmake gmain.adb
     

In the next section we discuss the advantages of using gnatmake in more detail.

1.4 Using the gnatmake Utility

If you work on a program by compiling single components at a time using gcc, you typically keep track of the units you modify. In order to build a consistent system, you compile not only these units, but also any units that depend on the units you have modified. For example, in the preceding case, if you edit gmain.adb, you only need to recompile that file. But if you edit greetings.ads, you must recompile both greetings.adb and gmain.adb, because both files contain units that depend on greetings.ads.

gnatbind will warn you if you forget one of these compilation steps, so that it is impossible to generate an inconsistent program as a result of forgetting to do a compilation. Nevertheless it is tedious and error-prone to keep track of dependencies among units. One approach to handle the dependency-bookkeeping is to use a makefile. However, makefiles present maintenance problems of their own: if the dependencies change as you change the program, you must make sure that the makefile is kept up-to-date manually, which is also an error-prone process.

The gnatmake utility takes care of these details automatically. Invoke it using either one of the following forms:

     $ gnatmake gmain.adb
     $ gnatmake gmain
     

The argument is the name of the file containing the main program; you may omit the extension. gnatmake examines the environment, automatically recompiles any files that need recompiling, and binds and links the resulting set of object files, generating the executable file, gmain. In a large program, it can be extremely helpful to use gnatmake, because working out by hand what needs to be recompiled can be difficult.

Note that gnatmake takes into account all the Ada 95 rules that establish dependencies among units. These include dependencies that result from inlining subprogram bodies, and from generic instantiation. Unlike some other Ada make tools, gnatmake does not rely on the dependencies that were found by the compiler on a previous compilation, which may possibly be wrong when sources change. gnatmake determines the exact set of dependencies from scratch each time it is run.

1.5 Introduction to Glide and GVD

Although it is possible to develop programs using only the command line interface (gnatmake, etc.) a graphical Interactive Development Environment can make it easier for you to compose, navigate, and debug programs. This section describes the main features of Glide, the GNAT graphical IDE, and also shows how to use the basic commands in GVD, the GNU Visual Debugger. Additional information may be found in the on-line help for these tools.

• Building a New Program with Glide:
• Simple Debugging with GVD:
• Other Glide Features:

1.5.1 Building a New Program with Glide

The simplest way to invoke Glide is to enter glide at the command prompt. It will generally be useful to issue this as a background command, thus allowing you to continue using your command window for other purposes while Glide is running:

     $ glide&
     

Glide will start up with an initial screen displaying the top-level menu items as well as some other information. The menu selections are as follows

• Buffers
• Files
• Tools
• Edit
• Search
• Mule
• Glide
• Help

For this introductory example, you will need to create a new Ada source file. First, select the Files menu. This will pop open a menu with around a dozen or so items. To create a file, select the Open file... choice. Depending on the platform, you may see a pop-up window where you can browse to an appropriate directory and then enter the file name, or else simply see a line at the bottom of the Glide window where you can likewise enter the file name. Note that in Glide, when you attempt to open a non-existent file, the effect is to create a file with that name. For this example enter hello.adb as the name of the file.

A new buffer will now appear, occupying the entire Glide window, with the file name at the top. The menu selections are slightly different from the ones you saw on the opening screen; there is an Entities item, and in place of Glide there is now an Ada item. Glide uses the file extension to identify the source language, so adb indicates an Ada source file.

You will enter some of the source program lines explicitly, and use the syntax-oriented template mechanism to enter other lines. First, type the following text:

     with Ada.Text_IO; use Ada.Text_IO;
     procedure Hello is
     begin
     

Observe that Glide uses different colors to distinguish reserved words from identifiers. Also, after the procedure Hello is line, the cursor is automatically indented in anticipation of declarations. When you enter begin, Glide recognizes that there are no declarations and thus places begin flush left. But after the begin line the cursor is again indented, where the statement(s) will be placed.

The main part of the program will be a for loop. Instead of entering the text explicitly, however, use a statement template. Select the Ada item on the top menu bar, move the mouse to the Statements item, and you will see a large selection of alternatives. Choose for loop. You will be prompted (at the bottom of the buffer) for a loop name; simply press the <Enter> key since a loop name is not needed. You should see the beginning of a for loop appear in the source program window. You will now be prompted for the name of the loop variable; enter a line with the identifier ind (lower case). Note that, by default, Glide capitalizes the name (you can override such behavior if you wish, although this is outside the scope of this introduction). Next, Glide prompts you for the loop range; enter a line containing 1..5 and you will see this also appear in the source program, together with the remaining elements of the for loop syntax.

Next enter the statement (with an intentional error, a missing semicolon) that will form the body of the loop:

     Put_Line("Hello, World" & Integer'Image(I))
     

Finally, type end Hello; as the last line in the program. Now save the file: choose the File menu item, and then the Save buffer selection. You will see a message at the bottom of the buffer confirming that the file has been saved.

You are now ready to attempt to build the program. Select the Ada item from the top menu bar. Although we could choose simply to compile the file, we will instead attempt to do a build (which invokes gnatmake) since, if the compile is successful, we want to build an executable. Thus select Ada build. This will fail because of the compilation error, and you will notice that the Glide window has been split: the top window contains the source file, and the bottom window contains the output from the GNAT tools. Glide allows you to navigate from a compilation error to the source file position corresponding to the error: click the middle mouse button (or simultaneously press the left and right buttons, on a two-button mouse) on the diagnostic line in the tool window. The focus will shift to the source window, and the cursor will be positioned on the character at which the error was detected.

Correct the error: type in a semicolon to terminate the statement. Although you can again save the file explicitly, you can also simply invoke Ada => Build and you will be prompted to save the file. This time the build will succeed; the tool output window shows you the options that are supplied by default. The GNAT tools' output (e.g., object and ALI files, executable) will go in the directory from which Glide was launched.

To execute the program, choose Ada and then Run. You should see the program's output displayed in the bottom window:

     Hello, world 1
     Hello, world 2
     Hello, world 3
     Hello, world 4
     Hello, world 5
     

1.5.2 Simple Debugging with GVD

This section describes how to set breakpoints, examine/modify variables, and step through execution.

In order to enable debugging, you need to pass the -g switch to both the compiler and to gnatlink. If you are using the command line, passing -g to gnatmake will have this effect. You can then launch GVD, e.g. on the hello program, by issuing the command:

     $ gvd hello
     

If you are using Glide, then -g is passed to the relevant tools by default when you do a build. Start the debugger by selecting the Ada menu item, and then Debug.

GVD comes up in a multi-part window. One pane shows the names of files comprising your executable; another pane shows the source code of the current unit (initially your main subprogram), another pane shows the debugger output and user interactions, and the fourth pane (the data canvas at the top of the window) displays data objects that you have selected.

To the left of the source file pane, you will notice green dots adjacent to some lines. These are lines for which object code exists and where breakpoints can thus be set. You set/reset a breakpoint by clicking the green dot. When a breakpoint is set, the dot is replaced by an X in a red circle. Clicking the circle toggles the breakpoint off, and the red circle is replaced by the green dot.

For this example, set a breakpoint at the statement where Put_Line is invoked.

Start program execution by selecting the Run button on the top menu bar. (The Start button will also start your program, but it will cause program execution to break at the entry to your main subprogram.) Evidence of reaching the breakpoint will appear: the source file line will be highlighted, and the debugger interactions pane will display a relevant message.

You can examine the values of variables in several ways. Move the mouse over an occurrence of Ind in the for loop, and you will see the value (now 1) displayed. Alternatively, right-click on Ind and select Display Ind; a box showing the variable's name and value will appear in the data canvas.

Although a loop index is a constant with respect to Ada semantics, you can change its value in the debugger. Right-click in the box for Ind, and select the Set Value of Ind item. Enter 2 as the new value, and press OK. The box for Ind shows the update.

Press the Step button on the top menu bar; this will step through one line of program text (the invocation of Put_Line), and you can observe the effect of having modified Ind since the value displayed is 2.

Remove the breakpoint, and resume execution by selecting the Cont button. You will see the remaining output lines displayed in the debugger interaction window, along with a message confirming normal program termination.

1.5.3 Other Glide Features

You may have observed that some of the menu selections contain abbreviations; e.g., (C-x C-f) for Open file... in the Files menu. These are shortcut keys that you can use instead of selecting menu items. The <C> stands for <Ctrl>; thus (C-x C-f) means <Ctrl-x> followed by <Ctrl-f>, and this sequence can be used instead of selecting Files and then Open file....

To abort a Glide command, type <Ctrl-g>.

If you want Glide to start with an existing source file, you can either launch Glide as above and then open the file via Files => Open file..., or else simply pass the name of the source file on the command line:

     $ glide hello.adb&
     

While you are using Glide, a number of buffers exist. You create some explicitly; e.g., when you open/create a file. Others arise as an effect of the commands that you issue; e.g., the buffer containing the output of the tools invoked during a build. If a buffer is hidden, you can bring it into a visible window by first opening the Buffers menu and then selecting the desired entry.

If a buffer occupies only part of the Glide screen and you want to expand it to fill the entire screen, then click in the buffer and then select Files => One Window.

If a window is occupied by one buffer and you want to split the window to bring up a second buffer, perform the following steps:

• Select Files => Split Window; this will produce two windows each of which holds the original buffer (these are not copies, but rather different views of the same buffer contents)
• With the focus in one of the windows, select the desired buffer from the Buffers menu

To exit from Glide, choose Files => Exit.

2 The GNAT Compilation Model

• Source Representation:
• Foreign Language Representation:
• File Naming Rules:
• Using Other File Names:
• Alternative File Naming Schemes:
• Generating Object Files:
• Source Dependencies:
• The Ada Library Information Files:
• Binding an Ada Program:
• Mixed Language Programming:
• Building Mixed Ada & C++ Programs:
• Comparison between GNAT and C/C++ Compilation Models:
• Comparison between GNAT and Conventional Ada Library Models:

This chapter describes the compilation model used by GNAT. Although similar to that used by other languages, such as C and C++, this model is substantially different from the traditional Ada compilation models, which are based on a library. The model is initially described without reference to the library-based model. If you have not previously used an Ada compiler, you need only read the first part of this chapter. The last section describes and discusses the differences between the GNAT model and the traditional Ada compiler models. If you have used other Ada compilers, this section will help you to understand those differences, and the advantages of the GNAT model.

2.1 Source Representation

Ada source programs are represented in standard text files, using Latin-1 coding. Latin-1 is an 8-bit code that includes the familiar 7-bit ASCII set, plus additional characters used for representing foreign languages (see Foreign Language Representation for support of non-USA character sets). The format effector characters are represented using their standard ASCII encodings, as follows:

VT

Vertical tab, 16#0B#

HT

Horizontal tab, 16#09#

CR

Carriage return, 16#0D#

LF

Line feed, 16#0A#

FF

Form feed, 16#0C#

Source files are in standard text file format. In addition, GNAT will recognize a wide variety of stream formats, in which the end of physical physical lines is marked by any of the following sequences: LF, CR, CR-LF, or LF-CR. This is useful in accommodating files that are imported from other operating systems.

The end of a source file is normally represented by the physical end of file. However, the control character 16#1A# (SUB) is also recognized as signalling the end of the source file. Again, this is provided for compatibility with other operating systems where this code is used to represent the end of file.

Each file contains a single Ada compilation unit, including any pragmas associated with the unit. For example, this means you must place a package declaration (a package spec) and the corresponding body in separate files. An Ada compilation (which is a sequence of compilation units) is represented using a sequence of files. Similarly, you will place each subunit or child unit in a separate file.

2.2 Foreign Language Representation

GNAT supports the standard character sets defined in Ada 95 as well as several other non-standard character sets for use in localized versions of the compiler (see Character Set Control).

• Latin-1:
• Other 8-Bit Codes:
• Wide Character Encodings:

2.2.1 Latin-1

The basic character set is Latin-1. This character set is defined by ISO standard 8859, part 1. The lower half (character codes 16#00# ... 16#7F#) is identical to standard ASCII coding, but the upper half is used to represent additional characters. These include extended letters used by European languages, such as French accents, the vowels with umlauts used in German, and the extra letter A-ring used in Swedish.

For a complete list of Latin-1 codes and their encodings, see the source file of library unit Ada.Characters.Latin_1 in file a-chlat1.ads. You may use any of these extended characters freely in character or string literals. In addition, the extended characters that represent letters can be used in identifiers.

2.2.2 Other 8-Bit Codes

GNAT also supports several other 8-bit coding schemes:

Latin-2

Latin-2 letters allowed in identifiers, with uppercase and lowercase equivalence.

Latin-3

Latin-3 letters allowed in identifiers, with uppercase and lowercase equivalence.

Latin-4

Latin-4 letters allowed in identifiers, with uppercase and lowercase equivalence.

Latin-5

Latin-4 letters (Cyrillic) allowed in identifiers, with uppercase and lowercase equivalence.

IBM PC (code page 437)

This code page is the normal default for PCs in the U.S. It corresponds to the original IBM PC character set. This set has some, but not all, of the extended Latin-1 letters, but these letters do not have the same encoding as Latin-1. In this mode, these letters are allowed in identifiers with uppercase and lowercase equivalence.

IBM PC (code page 850)

This code page is a modification of 437 extended to include all the Latin-1 letters, but still not with the usual Latin-1 encoding. In this mode, all these letters are allowed in identifiers with uppercase and lowercase equivalence.

Full Upper 8-bit

Any character in the range 80-FF allowed in identifiers, and all are considered distinct. In other words, there are no uppercase and lowercase equivalences in this range. This is useful in conjunction with certain encoding schemes used for some foreign character sets (e.g. the typical method of representing Chinese characters on the PC).

No Upper-Half

No upper-half characters in the range 80-FF are allowed in identifiers. This gives Ada 83 compatibility for identifier names.

For precise data on the encodings permitted, and the uppercase and lowercase equivalences that are recognized, see the file csets.adb in the GNAT compiler sources. You will need to obtain a full source release of GNAT to obtain this file.

2.2.3 Wide Character Encodings

GNAT allows wide character codes to appear in character and string literals, and also optionally in identifiers, by means of the following possible encoding schemes:

Hex Coding

In this encoding, a wide character is represented by the following five character sequence:

          ESC a b c d
          

Where a, b, c, d are the four hexadecimal characters (using uppercase letters) of the wide character code. For example, ESC A345 is used to represent the wide character with code 16#A345#. This scheme is compatible with use of the full Wide_Character set.

Upper-Half Coding

The wide character with encoding 16#abcd# where the upper bit is on (in other words, "a" is in the range 8-F) is represented as two bytes, 16#ab# and 16#cd#. The second byte cannot be a format control character, but is not required to be in the upper half. This method can be also used for shift-JIS or EUC, where the internal coding matches the external coding.

Shift JIS Coding

A wide character is represented by a two-character sequence, 16#ab# and 16#cd#, with the restrictions described for upper-half encoding as described above. The internal character code is the corresponding JIS character according to the standard algorithm for Shift-JIS conversion. Only characters defined in the JIS code set table can be used with this encoding method.

EUC Coding

A wide character is represented by a two-character sequence 16#ab# and 16#cd#, with both characters being in the upper half. The internal character code is the corresponding JIS character according to the EUC encoding algorithm. Only characters defined in the JIS code set table can be used with this encoding method.

UTF-8 Coding

A wide character is represented using UCS Transformation Format 8 (UTF-8) as defined in Annex R of ISO 10646-1/Am.2. Depending on the character value, the representation is a one, two, or three byte sequence:

          16#0000#-16#007f#: 2#0xxxxxxx#
          16#0080#-16#07ff#: 2#110xxxxx# 2#10xxxxxx#
          16#0800#-16#ffff#: 2#1110xxxx# 2#10xxxxxx# 2#10xxxxxx#
          
          

where the xxx bits correspond to the left-padded bits of the 16-bit character value. Note that all lower half ASCII characters are represented as ASCII bytes and all upper half characters and other wide characters are represented as sequences of upper-half (The full UTF-8 scheme allows for encoding 31-bit characters as 6-byte sequences, but in this implementation, all UTF-8 sequences of four or more bytes length will be treated as illegal).

Brackets Coding

In this encoding, a wide character is represented by the following eight character sequence:

          [ " a b c d " ]
          

Where a, b, c, d are the four hexadecimal characters (using uppercase letters) of the wide character code. For example, ["A345"] is used to represent the wide character with code 16#A345#. It is also possible (though not required) to use the Brackets coding for upper half characters. For example, the code 16#A3# can be represented as ["A3"].

This scheme is compatible with use of the full Wide_Character set, and is also the method used for wide character encoding in the standard ACVC (Ada Compiler Validation Capability) test suite distributions.

Note: Some of these coding schemes do not permit the full use of the Ada 95 character set. For example, neither Shift JIS, nor EUC allow the use of the upper half of the Latin-1 set.

2.3 File Naming Rules

The default file name is determined by the name of the unit that the file contains. The name is formed by taking the full expanded name of the unit and replacing the separating dots with hyphens and using lowercase for all letters.

An exception arises if the file name generated by the above rules starts with one of the characters a,g,i, or s, and the second character is a minus. In this case, the character tilde is used in place of the minus. The reason for this special rule is to avoid clashes with the standard names for child units of the packages System, Ada, Interfaces, and GNAT, which use the prefixes s- a- i- and g- respectively.

The file extension is .ads for a spec and .adb for a body. The following list shows some examples of these rules.

main.ads

Main (spec)

main.adb

Main (body)

arith_functions.ads

Arith_Functions (package spec)

arith_functions.adb

Arith_Functions (package body)

func-spec.ads

Func.Spec (child package spec)

func-spec.adb

Func.Spec (child package body)

main-sub.adb

Sub (subunit of Main)

a~bad.adb

A.Bad (child package body)

Following these rules can result in excessively long file names if corresponding unit names are long (for example, if child units or subunits are heavily nested). An option is available to shorten such long file names (called file name "krunching"). This may be particularly useful when programs being developed with GNAT are to be used on operating systems with limited file name lengths. See Using gnatkr.

Of course, no file shortening algorithm can guarantee uniqueness over all possible unit names; if file name krunching is used, it is your responsibility to ensure no name clashes occur. Alternatively you can specify the exact file names that you want used, as described in the next section. Finally, if your Ada programs are migrating from a compiler with a different naming convention, you can use the gnatchop utility to produce source files that follow the GNAT naming conventions. (For details see Renaming Files Using gnatchop.)

2.4 Using Other File Names

In the previous section, we have described the default rules used by GNAT to determine the file name in which a given unit resides. It is often convenient to follow these default rules, and if you follow them, the compiler knows without being explicitly told where to find all the files it needs.

However, in some cases, particularly when a program is imported from another Ada compiler environment, it may be more convenient for the programmer to specify which file names contain which units. GNAT allows arbitrary file names to be used by means of the Source_File_Name pragma. The form of this pragma is as shown in the following examples:

     

     pragma Source_File_Name (My_Utilities.Stacks,
       Spec_File_Name => "myutilst_a.ada");
     pragma Source_File_name (My_Utilities.Stacks,
       Body_File_Name => "myutilst.ada");
     
     

As shown in this example, the first argument for the pragma is the unit name (in this example a child unit). The second argument has the form of a named association. The identifier indicates whether the file name is for a spec or a body; the file name itself is given by a string literal.

The source file name pragma is a configuration pragma, which means that normally it will be placed in the gnat.adc file used to hold configuration pragmas that apply to a complete compilation environment. For more details on how the gnat.adc file is created and used see Handling of Configuration Pragmas

GNAT allows completely arbitrary file names to be specified using the source file name pragma. However, if the file name specified has an extension other than .ads or .adb it is necessary to use a special syntax when compiling the file. The name in this case must be preceded by the special sequence -x followed by a space and the name of the language, here ada, as in:

     $ gcc -c -x ada peculiar_file_name.sim
     

gnatmake handles non-standard file names in the usual manner (the non-standard file name for the main program is simply used as the argument to gnatmake). Note that if the extension is also non-standard, then it must be included in the gnatmake command, it may not be omitted.

2.5 Alternative File Naming Schemes

In the previous section, we described the use of the Source_File_Name pragma to allow arbitrary names to be assigned to individual source files. However, this approach requires one pragma for each file, and especially in large systems can result in very long gnat.adc files, and also create a maintenance problem.

GNAT also provides a facility for specifying systematic file naming schemes other than the standard default naming scheme previously described. An alternative scheme for naming is specified by the use of Source_File_Name pragmas having the following format:

     pragma Source_File_Name (
        Spec_File_Name  => FILE_NAME_PATTERN
      [,Casing          => CASING_SPEC]
      [,Dot_Replacement => STRING_LITERAL]);
     
     pragma Source_File_Name (
        Body_File_Name  => FILE_NAME_PATTERN
      [,Casing          => CASING_SPEC]
      [,Dot_Replacement => STRING_LITERAL]);
     
     pragma Source_File_Name (
        Subunit_File_Name  => FILE_NAME_PATTERN
      [,Casing             => CASING_SPEC]
      [,Dot_Replacement    => STRING_LITERAL]);
     
     FILE_NAME_PATTERN ::= STRING_LITERAL
     CASING_SPEC ::= Lowercase | Uppercase | Mixedcase
     
     

The FILE_NAME_PATTERN string shows how the file name is constructed. It contains a single asterisk character, and the unit name is substituted systematically for this asterisk. The optional parameter Casing indicates whether the unit name is to be all upper-case letters, all lower-case letters, or mixed-case. If no Casing parameter is used, then the default is all lower-case.

The optional Dot_Replacement string is used to replace any periods that occur in subunit or child unit names. If no Dot_Replacement argument is used then separating dots appear unchanged in the resulting file name. Although the above syntax indicates that the Casing argument must appear before the Dot_Replacement argument, but it is also permissible to write these arguments in the opposite order.

As indicated, it is possible to specify different naming schemes for bodies, specs, and subunits. Quite often the rule for subunits is the same as the rule for bodies, in which case, there is no need to give a separate Subunit_File_Name rule, and in this case the Body_File_name rule is used for subunits as well.

The separate rule for subunits can also be used to implement the rather unusual case of a compilation environment (e.g. a single directory) which contains a subunit and a child unit with the same unit name. Although both units cannot appear in the same partition, the Ada Reference Manual allows (but does not require) the possibility of the two units coexisting in the same environment.

The file name translation works in the following steps:

• If there is a specific Source_File_Name pragma for the given unit, then this is always used, and any general pattern rules are ignored.
• If there is a pattern type Source_File_Name pragma that applies to the unit, then the resulting file name will be used if the file exists. If more than one pattern matches, the latest one will be tried first, and the first attempt resulting in a reference to a file that exists will be used.
• If no pattern type Source_File_Name pragma that applies to the unit for which the corresponding file exists, then the standard GNAT default naming rules are used.

As an example of the use of this mechanism, consider a commonly used scheme in which file names are all lower case, with separating periods copied unchanged to the resulting file name, and specs end with ".1.ada", and bodies end with ".2.ada". GNAT will follow this scheme if the following two pragmas appear:

     pragma Source_File_Name
       (Spec_File_Name => "*.1.ada");
     pragma Source_File_Name
       (Body_File_Name => "*.2.ada");
     

The default GNAT scheme is actually implemented by providing the following default pragmas internally:

     pragma Source_File_Name
       (Spec_File_Name => "*.ads", Dot_Replacement => "-");
     pragma Source_File_Name
       (Body_File_Name => "*.adb", Dot_Replacement => "-");
     

Our final example implements a scheme typically used with one of the Ada 83 compilers, where the separator character for subunits was "__" (two underscores), specs were identified by adding _.ADA, bodies by adding .ADA, and subunits by adding .SEP. All file names were upper case. Child units were not present of course since this was an Ada 83 compiler, but it seems reasonable to extend this scheme to use the same double underscore separator for child units.

     pragma Source_File_Name
       (Spec_File_Name => "*_.ADA",
        Dot_Replacement => "__",
        Casing = Uppercase);
     pragma Source_File_Name
       (Body_File_Name => "*.ADA",
        Dot_Replacement => "__",
        Casing = Uppercase);
     pragma Source_File_Name
       (Subunit_File_Name => "*.SEP",
        Dot_Replacement => "__",
        Casing = Uppercase);
     

2.6 Generating Object Files

An Ada program consists of a set of source files, and the first step in compiling the program is to generate the corresponding object files. These are generated by compiling a subset of these source files. The files you need to compile are the following:

• If a package spec has no body, compile the package spec to produce the object file for the package.
• If a package has both a spec and a body, compile the body to produce the object file for the package. The source file for the package spec need not be compiled in this case because there is only one object file, which contains the code for both the spec and body of the package.
• For a subprogram, compile the subprogram body to produce the object file for the subprogram. The spec, if one is present, is as usual in a separate file, and need not be compiled.
• In the case of subunits, only compile the parent unit. A single object file is generated for the entire subunit tree, which includes all the subunits.
• Compile child units independently of their parent units (though, of course, the spec of all the ancestor unit must be present in order to compile a child unit).
• Compile generic units in the same manner as any other units. The object files in this case are small dummy files that contain at most the flag used for elaboration checking. This is because GNAT always handles generic instantiation by means of macro expansion. However, it is still necessary to compile generic units, for dependency checking and elaboration purposes.

The preceding rules describe the set of files that must be compiled to generate the object files for a program. Each object file has the same name as the corresponding source file, except that the extension is .o as usual.

You may wish to compile other files for the purpose of checking their syntactic and semantic correctness. For example, in the case where a package has a separate spec and body, you would not normally compile the spec. However, it is convenient in practice to compile the spec to make sure it is error-free before compiling clients of this spec, because such compilations will fail if there is an error in the spec.

GNAT provides an option for compiling such files purely for the purposes of checking correctness; such compilations are not required as part of the process of building a program. To compile a file in this checking mode, use the -gnatc switch.

2.7 Source Dependencies

A given object file clearly depends on the source file which is compiled to produce it. Here we are using depends in the sense of a typical make utility; in other words, an object file depends on a source file if changes to the source file require the object file to be recompiled. In addition to this basic dependency, a given object may depend on additional source files as follows:

• If a file being compiled with's a unit X, the object file depends on the file containing the spec of unit X. This includes files that are with'ed implicitly either because they are parents of with'ed child units or they are run-time units required by the language constructs used in a particular unit.
• If a file being compiled instantiates a library level generic unit, the object file depends on both the spec and body files for this generic unit.
• If a file being compiled instantiates a generic unit defined within a package, the object file depends on the body file for the package as well as the spec file.
• If a file being compiled contains a call to a subprogram for which pragma Inline applies and inlining is activated with the -gnatn switch, the object file depends on the file containing the body of this subprogram as well as on the file containing the spec. Note that for inlining to actually occur as a result of the use of this switch, it is necessary to compile in optimizing mode.

  The use of -gnatN activates a more extensive inlining optimization that is performed by the front end of the compiler. This inlining does not require that the code generation be optimized. Like -gnatn, the use of this switch generates additional dependencies.

• If an object file O depends on the proper body of a subunit through inlining or instantiation, it depends on the parent unit of the subunit. This means that any modification of the parent unit or one of its subunits affects the compilation of O.
• The object file for a parent unit depends on all its subunit body files.
• The previous two rules meant that for purposes of computing dependencies and recompilation, a body and all its subunits are treated as an indivisible whole.

  These rules are applied transitively: if unit A with's unit B, whose elaboration calls an inlined procedure in package C, the object file for unit A will depend on the body of C, in file c.adb.

  The set of dependent files described by these rules includes all the files on which the unit is semantically dependent, as described in the Ada 95 Language Reference Manual. However, it is a superset of what the ARM describes, because it includes generic, inline, and subunit dependencies.

  An object file must be recreated by recompiling the corresponding source file if any of the source files on which it depends are modified. For example, if the make utility is used to control compilation, the rule for an Ada object file must mention all the source files on which the object file depends, according to the above definition. The determination of the necessary recompilations is done automatically when one uses gnatmake.

2.8 The Ada Library Information Files

Each compilation actually generates two output files. The first of these is the normal object file that has a .o extension. The second is a text file containing full dependency information. It has the same name as the source file, but an .ali extension. This file is known as the Ada Library Information (ali) file. The following information is contained in the ali file.

• Version information (indicates which version of GNAT was used to compile the unit(s) in question)
• Main program information (including priority and time slice settings, as well as the wide character encoding used during compilation).
• List of arguments used in the gcc command for the compilation
• Attributes of the unit, including configuration pragmas used, an indication of whether the compilation was successful, exception model used etc.
• A list of relevant restrictions applying to the unit (used for consistency) checking.
• Categorization information (e.g. use of pragma Pure).
• Information on all with'ed units, including presence of Elaborate or Elaborate_All pragmas.
• Information from any Linker_Options pragmas used in the unit
• Information on the use of Body_Version or Version attributes in the unit.
• Dependency information. This is a list of files, together with time stamp and checksum information. These are files on which the unit depends in the sense that recompilation is required if any of these units are modified.
• Cross-reference data. Contains information on all entities referenced in the unit. Used by tools like gnatxref and gnatfind to provide cross-reference information.

For a full detailed description of the format of the ali file, see the source of the body of unit Lib.Writ, contained in file lib-writ.adb in the GNAT compiler sources.

2.9 Binding an Ada Program

When using languages such as C and C++, once the source files have been compiled the only remaining step in building an executable program is linking the object modules together. This means that it is possible to link an inconsistent version of a program, in which two units have included different versions of the same header.

The rules of Ada do not permit such an inconsistent program to be built. For example, if two clients have different versions of the same package, it is illegal to build a program containing these two clients. These rules are enforced by the GNAT binder, which also determines an elaboration order consistent with the Ada rules.

The GNAT binder is run after all the object files for a program have been created. It is given the name of the main program unit, and from this it determines the set of units required by the program, by reading the corresponding ALI files. It generates error messages if the program is inconsistent or if no valid order of elaboration exists.

If no errors are detected, the binder produces a main program, in Ada by default, that contains calls to the elaboration procedures of those compilation unit that require them, followed by a call to the main program. This Ada program is compiled to generate the object file for the main program. The name of the Ada file is b~xxx.adb (with the corresponding spec b~xxx.ads) where xxx is the name of the main program unit.

Finally, the linker is used to build the resulting executable program, using the object from the main program from the bind step as well as the object files for the Ada units of the program.

2.10 Mixed Language Programming

• Interfacing to C:
• Calling Conventions:

2.10.1 Interfacing to C

There are two ways to build a program that contains some Ada files and some other language files depending on whether the main program is in Ada or not. If the main program is in Ada, you should proceed as follows:

1. Compile the other language files to generate object files. For instance:

             gcc -c file1.c
             gcc -c file2.c
             

2. Compile the Ada units to produce a set of object files and ALI files. For instance:

             gnatmake -c my_main.adb
             

3. Run the Ada binder on the Ada main program. For instance:

             gnatbind my_main.ali
             

4. Link the Ada main program, the Ada objects and the other language objects. For instance:

             gnatlink my_main.ali file1.o file2.o
             

The three last steps can be grouped in a single command:

     gnatmake my_main.adb -largs file1.o file2.o
     

If the main program is in some language other than Ada, Then you may have more than one entry point in the Ada subsystem. You must use a special option of the binder to generate callable routines to initialize and finalize the Ada units (see Binding with Non-Ada Main Programs). Calls to the initialization and finalization routines must be inserted in the main program, or some other appropriate point in the code. The call to initialize the Ada units must occur before the first Ada subprogram is called, and the call to finalize the Ada units must occur after the last Ada subprogram returns. You use the same procedure for building the program as described previously. In this case, however, the binder only places the initialization and finalization subprograms into file b~xxx.adb instead of the main program. So, if the main program is not in Ada, you should proceed as follows:

1. Compile the other language files to generate object files. For instance:

             gcc -c file1.c
             gcc -c file2.c
             

2. Compile the Ada units to produce a set of object files and ALI files. For instance:

             gnatmake -c entry_point1.adb
             gnatmake -c entry_point2.adb
             

3. Run the Ada binder on the Ada main program. For instance:

             gnatbind -n entry_point1.ali entry_point2.ali
             

4. Link the Ada main program, the Ada objects and the other language objects. You only need to give the last entry point here. For instance:

             gnatlink entry_point2.ali file1.o file2.o
             

2.10.2 Calling Conventions

GNAT follows standard calling sequence conventions and will thus interface to any other language that also follows these conventions. The following Convention identifiers are recognized by GNAT:

• Ada. This indicates that the standard Ada calling sequence will be used and all Ada data items may be passed without any limitations in the case where GNAT is used to generate both the caller and callee. It is also possible to mix GNAT generated code and code generated by another Ada compiler. In this case, the data types should be restricted to simple cases, including primitive types. Whether complex data types can be passed depends on the situation. Probably it is safe to pass simple arrays, such as arrays of integers or floats. Records may or may not work, depending on whether both compilers lay them out identically. Complex structures involving variant records, access parameters, tasks, or protected types, are unlikely to be able to be passed.

  Note that in the case of GNAT running on a platform that supports DEC Ada 83, a higher degree of compatibility can be guaranteed, and in particular records are layed out in an identical manner in the two compilers. Note also that if output from two different compilers is mixed, the program is responsible for dealing with elaboration issues. Probably the safest approach is to write the main program in the version of Ada other than GNAT, so that it takes care of its own elaboration requirements, and then call the GNAT-generated adainit procedure to ensure elaboration of the GNAT components. Consult the documentation of the other Ada compiler for further details on elaboration.

  However, it is not possible to mix the tasking run time of GNAT and DEC Ada 83, All the tasking operations must either be entirely within GNAT compiled sections of the program, or entirely within DEC Ada 83 compiled sections of the program.

• Assembler. Specifies assembler as the convention. In practice this has the same effect as convention Ada (but is not equivalent in the sense of being considered the same convention).
• Asm. Equivalent to Assembler.
• Asm. Equivalent to Assembly.
• COBOL. Data will be passed according to the conventions described in section B.4 of the Ada 95 Reference Manual.
• C. Data will be passed according to the conventions described in section B.3 of the Ada 95 Reference Manual.
• Default. Equivalent to C.
• External. Equivalent to C.
• CPP. This stands for C++. For most purposes this is identical to C. See the separate description of the specialized GNAT pragmas relating to C++ interfacing for further details.
• Fortran. Data will be passed according to the conventions described in section B.5 of the Ada 95 Reference Manual.
• Intrinsic. This applies to an intrinsic operation, as defined in the Ada 95 Reference Manual. If a a pragma Import (Intrinsic) applies to a subprogram, this means that the body of the subprogram is provided by the compiler itself, usually by means of an efficient code sequence, and that the user does not supply an explicit body for it. In an application program, the pragma can only be applied to the following two sets of names, which the GNAT compiler recognizes.
  • Rotate_Left, Rotate_Right, Shift_Left, Shift_Right, Shift_Right_- Arithmetic. The corresponding subprogram declaration must have two formal parameters. The first one must be a signed integer type or a modular type with a binary modulus, and the second parameter must be of type Natural. The return type must be the same as the type of the first argument. The size of this type can only be 8, 16, 32, or 64.
  • binary arithmetic operators: "+", "-", "*", "/" The corresponding operator declaration must have parameters and result type that have the same root numeric type (for example, all three are long_float types). This simplifies the definition of operations that use type checking to perform dimensional checks:

                   type Distance is new Long_Float;
                   type Time     is new Long_Float;
                   type Velocity is new Long_Float;
                   function "/" (D : Distance; T : Time)
                     return Velocity;
                   pragma Import (Intrinsic, "/");
                   

    This common idiom is often programmed with a generic definition and an explicit body. The pragma makes it simpler to introduce such declarations. It incurs no overhead in compilation time or code size, because it is implemented as a single machine instruction.

• Stdcall. This is relevant only to NT/Win95 implementations of GNAT, and specifies that the Stdcall calling sequence will be used, as defined by the NT API.
• DLL. This is equivalent to Stdcall.
• Win32. This is equivalent to Stdcall.
• Stubbed. This is a special convention that indicates that the compiler should provide a stub body that raises Program_Error.

GNAT additionally provides a useful pragma Convention_Identifier that can be used to parametrize conventions and allow additional synonyms to be specified. For example if you have legacy code in which the convention identifier Fortran77 was used for Fortran, you can use the configuration pragma:

        pragma Convention_Identifier (Fortran77, Fortran);
     

And from now on the identifier Fortran77 may be used as a convention identifier (for example in an Import pragma) with the same meaning as Fortran.

2.11 Building Mixed Ada & C++ Programs

Building a mixed application containing both Ada and C++ code may be a challenge for the unaware programmer. As a matter of fact, this interfacing has not been standardized in the Ada 95 reference manual due to the immaturity and lack of standard of C++ at the time. This section gives a few hints that should make this task easier. In particular the first section addresses the differences with interfacing with C. The second section looks into the delicate problem of linking the complete application from its Ada and C++ parts. The last section give some hints on how the GNAT run time can be adapted in order to allow inter-language dispatching with a new C++ compiler.

• Interfacing to C++:
• Linking a Mixed C++ & Ada Program:
• A Simple Example:
• Adapting the Run Time to a New C++ Compiler:

2.11.1 Interfacing to C++

GNAT supports interfacing with C++ compilers generating code that is compatible with the standard Application Binary Interface of the given platform.

Interfacing can be done at 3 levels: simple data, subprograms and classes. In the first 2 cases, GNAT offer a specific Convention CPP that behaves exactly like Convention C. Usually C++ mangle names of subprograms and currently GNAT does not provide any help to solve the demangling problem. This problem can be addressed in 2 ways:

• by modifying the C++ code in order to force a C convention using the extern "C" syntax.
• by figuring out the mangled name and use it as the Link_Name argument of the pragma import.

Interfacing at the class level can be achieved by using the GNAT specific pragmas such as CPP_Class and CPP_Virtual. See the GNAT Reference Manual for additional information.

2.11.2 Linking a Mixed C++ & Ada Program

Usually the linker of the C++ development system must be used to link mixed applications because most C++ systems will resolve elaboration issues (such as calling constructors on global class instances) transparently during the link phase. GNAT has been adapted to ease the use of a foreign linker for the last phase. Three cases can be considered:

1. Using GNAT and G++ (GNU C++ compiler) from the same GCC installation. The c++ linker can simply be called by using the c++ specific driver called c++. Note that this setup is not very common because it may request recompiling the whole GCC tree from sources and it does not allow to upgrade easily to a new version of one compiler for one of the two languages without taking the risk of destabilizing the other.

             $ c++ -c file1.C
             $ c++ -c file2.C
             $ gnatmake ada_unit -largs file1.o file2.o --LINK=c++
             

2. Using GNAT and G++ from 2 different GCC installations. If both compilers are on the PATH, the same method can be used. It is important to be aware that environment variables such as C_INCLUDE_PATH, GCC_EXEC_PREFIX, BINUTILS_ROOT or GCC_ROOT will affect both compilers at the same time and thus may make one of the 2 compilers operate improperly if they are set for the other. In particular it is important that the link command has access to the proper gcc library libgcc.a, that is to say the one that is part of the C++ compiler installation. The implicit link command as suggested in the gnatmake command from the former example can be replaced by an explicit link command with full verbosity in order to verify which library is used:

             $ gnatbind ada_unit
             $ gnatlink -v -v ada_unit file1.o file2.o --LINK=c++
             

   If there is a problem due to interfering environment variables, it can be workaround by using an intermediate script. The following example shows the proper script to use when GNAT has not been installed at its default location and g++ has been installed at its default location:

             $ gnatlink -v -v ada_unit file1.o file2.o --LINK=./my_script
             $ cat ./my_script
             #!/bin/sh
             unset BINUTILS_ROOT
             unset GCC_ROOT
             c++ $*
             

3. Using a non GNU C++ compiler. The same set of command as previously described can be used to insure that the c++ linker is used. Nonetheless, you need to add the path to libgcc explicitely, since some libraries needed by GNAT are located in this directory:

             
             $ gnatlink ada_unit file1.o file2.o --LINK=./my_script
             $ cat ./my_script
             #!/bin/sh
             CC $* `gcc -print-libgcc-file-name`
             
             

   Where CC is the name of the non GNU C++ compiler.

2.11.3 A Simple Example

The following example, provided as part of the GNAT examples, show how to achieve procedural interfacing between Ada and C++ in both directions. The C++ class A has 2 methods. The first method is exported to Ada by the means of an extern C wrapper function. The second method calls an Ada subprogram. On the Ada side, The C++ calls is modelized by a limited record with a layout comparable to the C++ class. The Ada subprogram, in turn, calls the c++ method. So from the C++ main program the code goes back and forth between the 2 languages.

Here are the compilation commands for native configurations:

     $ gnatmake -c simple_cpp_interface
     $ c++ -c cpp_main.C
     $ c++ -c ex7.C
     $ gnatbind -n simple_cpp_interface
     $ gnatlink simple_cpp_interface -o cpp_main --LINK=$(CPLUSPLUS)
           -lstdc++ ex7.o cpp_main.o
     

Here are the corresponding sources:

     
     //cpp_main.C
     
     #include "ex7.h"
     
     extern "C" {
       void adainit (void);
       void adafinal (void);
       void method1 (A *t);
     }
     
     void method1 (A *t)
     {
       t->method1 ();
     }
     
     int main ()
     {
       A obj;
       adainit ();
       obj.method2 (3030);
       adafinal ();
     }
     
     //ex7.h
     
     class Origin {
      public:
       int o_value;
     };
     class A : public Origin {
      public:
       void method1 (void);
       virtual void method2 (int v);
       A();
       int   a_value;
     };
     
     //ex7.C
     
     #include "ex7.h"
     #include <stdio.h>
     
     extern "C" { void ada_method2 (A *t, int v);}
     
     void A::method1 (void)
     {
       a_value = 2020;
       printf ("in A::method1, a_value = %d \n",a_value);
     
     }
     
     void A::method2 (int v)
     {
        ada_method2 (this, v);
        printf ("in A::method2, a_value = %d \n",a_value);
     
     }
     
     A::A(void)
     {
        a_value = 1010;
       printf ("in A::A, a_value = %d \n",a_value);
     }
     
     -- Ada sources
     package body Simple_Cpp_Interface is
     
        procedure Ada_Method2 (This : in out A; V : Integer) is
        begin
           Method1 (This);
           This.A_Value := V;
        end Ada_Method2;
     
     end Simple_Cpp_Interface;
     
     package Simple_Cpp_Interface is
        type A is limited
           record
              O_Value : Integer;
              A_Value : Integer;
           end record;
        pragma Convention (C, A);
     
        procedure Method1 (This : in out A);
        pragma Import (C, Method1);
     
        procedure Ada_Method2 (This : in out A; V : Integer);
        pragma Export (C, Ada_Method2);
     
     end Simple_Cpp_Interface;
     

2.11.4 Adapting the Run Time to a New C++ Compiler

GNAT offers the capability to derive Ada 95 tagged types directly from preexisting C++ classes and . See "Interfacing with C++" in the GNAT reference manual. The mechanism used by GNAT for achieving such a goal has been made user configurable through a GNAT library unit Interfaces.CPP. The default version of this file is adapted to the GNU c++ compiler. Internal knowledge of the virtual table layout used by the new C++ compiler is needed to configure properly this unit. The Interface of this unit is known by the compiler and cannot be changed except for the value of the constants defining the characteristics of the virtual table: CPP_DT_Prologue_Size, CPP_DT_Entry_Size, CPP_TSD_Prologue_Size, CPP_TSD_Entry_Size. Read comments in the source of this unit for more details.

2.12 Comparison between GNAT and C/C++ Compilation Models

The GNAT model of compilation is close to the C and C++ models. You can think of Ada specs as corresponding to header files in C. As in C, you don't need to compile specs; they are compiled when they are used. The Ada with is similar in effect to the #include of a C header.

One notable difference is that, in Ada, you may compile specs separately to check them for semantic and syntactic accuracy. This is not always possible with C headers because they are fragments of programs that have less specific syntactic or semantic rules.

The other major difference is the requirement for running the binder, which performs two important functions. First, it checks for consistency. In C or C++, the only defense against assembling inconsistent programs lies outside the compiler, in a makefile, for example. The binder satisfies the Ada requirement that it be impossible to construct an inconsistent program when the compiler is used in normal mode.

The other important function of the binder is to deal with elaboration issues. There are also elaboration issues in C++ that are handled automatically. This automatic handling has the advantage of being simpler to use, but the C++ programmer has no control over elaboration. Where gnatbind might complain there was no valid order of elaboration, a C++ compiler would simply construct a program that malfunctioned at run time.

2.13 Comparison between GNAT and Conventional Ada Library Models

This section is intended to be useful to Ada programmers who have previously used an Ada compiler implementing the traditional Ada library model, as described in the Ada 95 Language Reference Manual. If you have not used such a system, please go on to the next section.

In GNAT, there is no library in the normal sense. Instead, the set of source files themselves acts as the library. Compiling Ada programs does not generate any centralized information, but rather an object file and a ALI file, which are of interest only to the binder and linker. In a traditional system, the compiler reads information not only from the source file being compiled, but also from the centralized library. This means that the effect of a compilation depends on what has been previously compiled. In particular:

• When a unit is with'ed, the unit seen by the compiler corresponds to the version of the unit most recently compiled into the library.
• Inlining is effective only if the necessary body has already been compiled into the library.
• Compiling a unit may obsolete other units in the library.

In GNAT, compiling one unit never affects the compilation of any other units because the compiler reads only source files. Only changes to source files can affect the results of a compilation. In particular:

• When a unit is with'ed, the unit seen by the compiler corresponds to the source version of the unit that is currently accessible to the compiler.
• Inlining requires the appropriate source files for the package or subprogram bodies to be available to the compiler. Inlining is always effective, independent of the order in which units are complied.
• Compiling a unit never affects any other compilations. The editing of sources may cause previous compilations to be out of date if they depended on the source file being modified.

The most important result of these differences is that order of compilation is never significant in GNAT. There is no situation in which one is required to do one compilation before another. What shows up as order of compilation requirements in the traditional Ada library becomes, in GNAT, simple source dependencies; in other words, there is only a set of rules saying what source files must be present when a file is compiled.

3 Compiling Using gcc

This chapter discusses how to compile Ada programs using the gcc command. It also describes the set of switches that can be used to control the behavior of the compiler.

• Compiling Programs:
• Switches for gcc:
• Search Paths and the Run-Time Library (RTL):
• Order of Compilation Issues:
• Examples:

3.1 Compiling Programs

The first step in creating an executable program is to compile the units of the program using the gcc command. You must compile the following files:

• the body file (.adb) for a library level subprogram or generic subprogram
• the spec file (.ads) for a library level package or generic package that has no body
• the body file (.adb) for a library level package or generic package that has a body

You need not compile the following files

• the spec of a library unit which has a body
• subunits

because they are compiled as part of compiling related units. GNAT package specs when the corresponding body is compiled, and subunits when the parent is compiled. If you attempt to compile any of these files, you will get one of the following error messages (where fff is the name of the file you compiled):

     No code generated for file fff (package spec)
     No code generated for file fff (subunit)
     

The basic command for compiling a file containing an Ada unit is

     $ gcc -c [switches] file name
     

where file name is the name of the Ada file (usually having an extension .ads for a spec or .adb for a body). You specify the -c switch to tell gcc to compile, but not link, the file. The result of a successful compilation is an object file, which has the same name as the source file but an extension of .o and an Ada Library Information (ALI) file, which also has the same name as the source file, but with .ali as the extension. GNAT creates these two output files in the current directory, but you may specify a source file in any directory using an absolute or relative path specification containing the directory information.

gcc is actually a driver program that looks at the extensions of the file arguments and loads the appropriate compiler. For example, the GNU C compiler is cc1, and the Ada compiler is gnat1. These programs are in directories known to the driver program (in some configurations via environment variables you set), but need not be in your path. The gcc driver also calls the assembler and any other utilities needed to complete the generation of the required object files.

It is possible to supply several file names on the same gcc command. This causes gcc to call the appropriate compiler for each file. For example, the following command lists three separate files to be compiled:

     $ gcc -c x.adb y.adb z.c
     

calls gnat1 (the Ada compiler) twice to compile x.adb and y.adb, and cc1 (the C compiler) once to compile z.c. The compiler generates three object files x.o, y.o and z.o and the two ALI files x.ali and y.ali from the Ada compilations. Any switches apply to all the files listed, except for -gnatx switches, which apply only to Ada compilations.

3.2 Switches for gcc

The gcc command accepts switches that control the compilation process. These switches are fully described in this section. First we briefly list all the switches, in alphabetical order, then we describe the switches in more detail in functionally grouped sections.

• Output and Error Message Control:
• Debugging and Assertion Control:
• Run-Time Checks:
• Stack Overflow Checking:
• Run-Time Control:
• Validity Checking:
• Style Checking:
• Using gcc for Syntax Checking:
• Using gcc for Semantic Checking:
• Compiling Ada 83 Programs:
• Character Set Control:
• File Naming Control:
• Subprogram Inlining Control:
• Auxiliary Output Control:
• Debugging Control:
• Units to Sources Mapping Files:

-b target

Compile your program to run on target, which is the name of a system configuration. You must have a GNAT cross-compiler built if target is not the same as your host system.

-Bdir

Load compiler executables (for example, gnat1, the Ada compiler) from dir instead of the default location. Only use this switch when multiple versions of the GNAT compiler are available. See the gcc manual page for further details. You would normally use the -b or -V switch instead.

-c

Compile. Always use this switch when compiling Ada programs.

Note: for some other languages when using gcc, notably in the case of C and C++, it is possible to use use gcc without a -c switch to compile and link in one step. In the case of GNAT, you cannot use this approach, because the binder must be run and gcc cannot be used to run the GNAT binder.

-g

Generate debugging information. This information is stored in the object file and copied from there to the final executable file by the linker, where it can be read by the debugger. You must use the -g switch if you plan on using the debugger.

-Idir

Direct GNAT to search the dir directory for source files needed by the current compilation (see Search Paths and the Run-Time Library (RTL)).

-I-

Except for the source file named in the command line, do not look for source files in the directory containing the source file named in the command line (see Search Paths and the Run-Time Library (RTL)).

-o file

This switch is used in gcc to redirect the generated object file and its associated ALI file. Beware of this switch with GNAT, because it may cause the object file and ALI file to have different names which in turn may confuse the binder and the linker.

-O[n]

n controls the optimization level.

n = 0

No optimization, the default setting if no -O appears

n = 1

Normal optimization, the default if you specify -O without an operand.

n = 2

Extensive optimization

n = 3

Extensive optimization with automatic inlining. This applies only to inlining within a unit. For details on control of inter-unit inlining see See Subprogram Inlining Control.

--RTS=rts-path

Specifies the default location of the runtime library. Same meaning as the equivalent gnatmake flag (see Switches for gnatmake).

-S

Used in place of -c to cause the assembler source file to be generated, using .s as the extension, instead of the object file. This may be useful if you need to examine the generated assembly code.

-v

Show commands generated by the gcc driver. Normally used only for debugging purposes or if you need to be sure what version of the compiler you are executing.

-V ver

Execute ver version of the compiler. This is the gcc version, not the GNAT version.

-gnata

Assertions enabled. Pragma Assert and pragma Debug to be activated.

-gnatA

Avoid processing gnat.adc. If a gnat.adc file is present, it will be ignored.

-gnatb

Generate brief messages to stderr even if verbose mode set.

-gnatc

Check syntax and semantics only (no code generation attempted).

-gnatC

Compress debug information and external symbol name table entries.

-gnatD

Output expanded source files for source level debugging. This switch also suppress generation of cross-reference information (see -gnatx).

-gnatecpath

Specify a configuration pragma file. (see The Configuration Pragmas Files)

-gnatempath

Specify a mapping file. (see Units to Sources Mapping Files)

-gnatE

Full dynamic elaboration checks.

-gnatf

Full errors. Multiple errors per line, all undefined references.

-gnatF

Externals names are folded to all uppercase.

-gnatg

Internal GNAT implementation mode. This should not be used for applications programs, it is intended only for use by the compiler and its run-time library. For documentation, see the GNAT sources.

-gnatG

List generated expanded code in source form.

-gnatic

Identifier character set (c=1/2/3/4/8/9/p/f/n/w).

-gnath

Output usage information. The output is written to stdout.

-gnatkn

Limit file names to n (1-999) characters (k = krunch).

-gnatl

Output full source listing with embedded error messages.

-gnatmn

Limit number of detected errors to n (1-999).

-gnatn

Activate inlining across unit boundaries for subprograms for which pragma inline is specified.

-gnatN

Activate front end inlining.

-fno-inline

Suppresses all inlining, even if other optimization or inlining switches are set.

-fstack-check

Activates stack checking. See separate section on stack checking for details of the use of this option.

-gnato

Enable numeric overflow checking (which is not normally enabled by default). Not that division by zero is a separate check that is not controlled by this switch (division by zero checking is on by default).

-gnatp

Suppress all checks.

-gnatq

Don't quit; try semantics, even if parse errors.

-gnatQ

Don't quit; generate ali and tree files even if illegalities.

-gnatP

Enable polling. This is required on some systems (notably Windows NT) to obtain asynchronous abort and asynchronous transfer of control capability. See the description of pragma Polling in the GNAT Reference Manual for full details.

-gnatR[0/1/2/3][s]

Output representation information for declared types and objects.

-gnats

Syntax check only.

-gnatt

Tree output file to be generated.

-gnatT nnn

Set time slice to specified number of microseconds

-gnatu

List units for this compilation.

-gnatU

Tag all error messages with the unique string "error:"

-gnatv

Verbose mode. Full error output with source lines to stdout.

-gnatV

Control level of validity checking. See separate section describing this feature.

-gnatwxxxxxx

Warning mode where xxx is a string of options describing the exact warnings that are enabled or disabled. See separate section on warning control.

-gnatWe

Wide character encoding method (e=n/h/u/s/e/8).

-gnatx

Suppress generation of cross-reference information.

-gnaty

Enable built-in style checks. See separate section describing this feature.

-gnatzm

Distribution stub generation and compilation (m=r/c for receiver/caller stubs).

-gnat83

Enforce Ada 83 restrictions.

-pass-exit-codes

Catch exit codes from the compiler and use the most meaningful as exit status.

You may combine a sequence of GNAT switches into a single switch. For example, the combined switch

     -gnatofi3
     

is equivalent to specifying the following sequence of switches:

     -gnato -gnatf -gnati3
     

The following restrictions apply to the combination of switches in this manner:

• The switch -gnatc if combined with other switches must come first in the string.
• The switch -gnats if combined with other switches must come first in the string.
• Once a "y" appears in the string (that is a use of the -gnaty switch), then all further characters in the switch are interpreted as style modifiers (see description of -gnaty).
• Once a "d" appears in the string (that is a use of the -gnatd switch), then all further characters in the switch are interpreted as debug flags (see description of -gnatd).
• Once a "w" appears in the string (that is a use of the -gnatw switch), then all further characters in the switch are interpreted as warning mode modifiers (see description of -gnatw).
• Once a "V" appears in the string (that is a use of the -gnatV switch), then all further characters in the switch are interpreted as validity checking options (see description of -gnatV).

3.2.1 Output and Error Message Control

The standard default format for error messages is called "brief format." Brief format messages are written to stderr (the standard error file) and have the following form:

     e.adb:3:04: Incorrect spelling of keyword "function"
     e.adb:4:20: ";" should be "is"
     

The first integer after the file name is the line number in the file, and the second integer is the column number within the line. glide can parse the error messages and point to the referenced character. The following switches provide control over the error message format:

-gnatv

The v stands for verbose. The effect of this setting is to write long-format error messages to stdout (the standard output file. The same program compiled with the -gnatv switch would generate:

          

          3. funcion X (Q : Integer)
             |
          >>> Incorrect spelling of keyword "function"
          4. return Integer;
                           |
          >>> ";" should be "is"
          
          

The vertical bar indicates the location of the error, and the >>> prefix can be used to search for error messages. When this switch is used the only source lines output are those with errors.

-gnatl

The l stands for list. This switch causes a full listing of the file to be generated. The output might look as follows:

          

           1. procedure E is
           2.    V : Integer;
           3.    funcion X (Q : Integer)
                 |
              >>> Incorrect spelling of keyword "function"
           4.     return Integer;
                                |
              >>> ";" should be "is"
           5.    begin
           6.       return Q + Q;
           7.    end;
           8. begin
           9.    V := X + X;
          10.end E;
          
          

When you specify the -gnatv or -gnatl switches and standard output is redirected, a brief summary is written to stderr (standard error) giving the number of error messages and warning messages generated.

-gnatU

This switch forces all error messages to be preceded by the unique string "error:". This means that error messages take a few more characters in space, but allows easy searching for and identification of error messages.

-gnatb

The b stands for brief. This switch causes GNAT to generate the brief format error messages to stderr (the standard error file) as well as the verbose format message or full listing (which as usual is written to stdout (the standard output file).

-gnatmn

The m stands for maximum. n is a decimal integer in the range of 1 to 999 and limits the number of error messages to be generated. For example, using -gnatm2 might yield

          e.adb:3:04: Incorrect spelling of keyword "function"
          e.adb:5:35: missing ".."
          fatal error: maximum errors reached
          compilation abandoned
          

-gnatf

The f stands for full. Normally, the compiler suppresses error messages that are likely to be redundant. This switch causes all error messages to be generated. In particular, in the case of references to undefined variables. If a given variable is referenced several times, the normal format of messages is

          e.adb:7:07: "V" is undefined (more references follow)
          

where the parenthetical comment warns that there are additional references to the variable V. Compiling the same program with the -gnatf switch yields

          e.adb:7:07: "V" is undefined
          e.adb:8:07: "V" is undefined
          e.adb:8:12: "V" is undefined
          e.adb:8:16: "V" is undefined
          e.adb:9:07: "V" is undefined
          e.adb:9:12: "V" is undefined
          

-gnatq

The q stands for quit (really "don't quit"). In normal operation mode, the compiler first parses the program and determines if there are any syntax errors. If there are, appropriate error messages are generated and compilation is immediately terminated. This switch tells GNAT to continue with semantic analysis even if syntax errors have been found. This may enable the detection of more errors in a single run. On the other hand, the semantic analyzer is more likely to encounter some internal fatal error when given a syntactically invalid tree.

-gnatQ

In normal operation mode, the ali file is not generated if any illegalities are detected in the program. The use of -gnatQ forces generation of the ali file. This file is marked as being in error, so it cannot be used for binding purposes, but it does contain reasonably complete cross-reference information, and thus may be useful for use by tools (e.g. semantic browsing tools or integrated development environments) that are driven from the ali file.

In addition, if -gnatt is also specified, then the tree file is generated even if there are illegalities. It may be useful in this case to also specify -gnatq to ensure that full semantic processing occurs. The resulting tree file can be processed by ASIS, for the purpose of providing partial information about illegal units, but if the error causes the tree to be badly malformed, then ASIS may crash during the analysis.

In addition to error messages, which correspond to illegalities as defined in the Ada 95 Reference Manual, the compiler detects two kinds of warning situations.

First, the compiler considers some constructs suspicious and generates a warning message to alert you to a possible error. Second, if the compiler detects a situation that is sure to raise an exception at run time, it generates a warning message. The following shows an example of warning messages:

     e.adb:4:24: warning: creation of object may raise Storage_Error
     e.adb:10:17: warning: static value out of range
     e.adb:10:17: warning: "Constraint_Error" will be raised at run time
     
     

GNAT considers a large number of situations as appropriate for the generation of warning messages. As always, warnings are not definite indications of errors. For example, if you do an out-of-range assignment with the deliberate intention of raising a Constraint_Error exception, then the warning that may be issued does not indicate an error. Some of the situations for which GNAT issues warnings (at least some of the time) are given in the following list, which is not necessarily complete.

• Possible infinitely recursive calls
• Out-of-range values being assigned
• Possible order of elaboration problems
• Unreachable code
• Fixed-point type declarations with a null range
• Variables that are never assigned a value
• Variables that are referenced before being initialized
• Task entries with no corresponding accept statement
• Duplicate accepts for the same task entry in a select
• Objects that take too much storage
• Unchecked conversion between types of differing sizes
• Missing return statements along some execution paths in a function
• Incorrect (unrecognized) pragmas
• Incorrect external names
• Allocation from empty storage pool
• Potentially blocking operations in protected types
• Suspicious parenthesization of expressions
• Mismatching bounds in an aggregate
• Attempt to return local value by reference
• Unrecognized pragmas
• Premature instantiation of a generic body
• Attempt to pack aliased components
• Out of bounds array subscripts
• Wrong length on string assignment
• Violations of style rules if style checking is enabled
• Unused with clauses
• Bit_Order usage that does not have any effect
• Compile time biased rounding of floating-point constant
• Standard.Duration used to resolve universal fixed expression
• Dereference of possibly null value
• Declaration that is likely to cause storage error
• Internal GNAT unit with'ed by application unit
• Values known to be out of range at compile time
• Unreferenced labels and variables
• Address overlays that could clobber memory
• Unexpected initialization when address clause present
• Bad alignment for address clause
• Useless type conversions
• Redundant assignment statements
• Accidental hiding of name by child unit
• Unreachable code
• Access before elaboration detected at compile time
• A range in a for loop that is known to be null or might be null

The following switches are available to control the handling of warning messages:

-gnatwa (activate all optional errors)

This switch activates most optional warning messages, see remaining list in this section for details on optional warning messages that can be individually controlled. The warnings that are not turned on by this switch are -gnatwb (biased rounding), -gnatwd (implicit dereferencing), and -gnatwh (hiding). All other optional warnings are turned on.

-gnatwA (suppress all optional errors)

This switch suppresses all optional warning messages, see remaining list in this section for details on optional warning messages that can be individually controlled.

-gnatwb (activate warnings on biased rounding)

If a static floating-point expression has a value that is exactly half way between two adjacent machine numbers, then the rules of Ada (Ada Reference Manual, section 4.9(38)) require that this rounding be done away from zero, even if the normal unbiased rounding rules at run time would require rounding towards zero. This warning message alerts you to such instances where compile-time rounding and run-time rounding are not equivalent. If it is important to get proper run-time rounding, then you can force this by making one of the operands into a variable. The default is that such warnings are not generated. Note that -gnatwa does not affect the setting of this warning option.

-gnatwB (suppress warnings on biased rounding)

This switch disables warnings on biased rounding.

-gnatwc (activate warnings on conditionals)

This switch activates warnings for conditional expressions used in tests that are known to be True or False at compile time. The default is that such warnings are not generated. This warning can also be turned on using -gnatwa.

-gnatwC (suppress warnings on conditionals)

This switch suppresses warnings for conditional expressions used in tests that are known to be True or False at compile time.

-gnatwd (activate warnings on implicit dereferencing)

If this switch is set, then the use of a prefix of an access type in an indexed component, slice, or selected component without an explicit .all will generate a warning. With this warning enabled, access checks occur only at points where an explicit .all appears in the source code (assuming no warnings are generated as a result of this switch). The default is that such warnings are not generated. Note that -gnatwa does not affect the setting of this warning option.

-gnatwD (suppress warnings on implicit dereferencing)

This switch suppresses warnings for implicit deferences in indexed components, slices, and selected components.

-gnatwe (treat warnings as errors)

This switch causes warning messages to be treated as errors. The warning string still appears, but the warning messages are counted as errors, and prevent the generation of an object file.

-gnatwf (activate warnings on unreferenced formals)

This switch causes a warning to be generated if a formal parameter is not referenced in the body of the subprogram. This warning can also be turned on using -gnatwa or -gnatwu.

-gnatwF (suppress warnings on unreferenced formals)

This switch suppresses warnings for unreferenced formal parameters. Note that the combination -gnatwu followed by -gnatwF has the effect of warning on unreferenced entities other than subprogram formals.

-gnatwh (activate warnings on hiding)

This switch activates warnings on hiding declarations. A declaration is considered hiding if it is for a non-overloadable entity, and it declares an entity with the same name as some other entity that is directly or use-visible. The default is that such warnings are not generated. Note that -gnatwa does not affect the setting of this warning option.

-gnatwH (suppress warnings on hiding)

This switch suppresses warnings on hiding declarations.

-gnatwi (activate warnings on implementation units).

This switch activates warnings for a with of an internal GNAT implementation unit, defined as any unit from the Ada, Interfaces, GNAT, or System hierarchies that is not documented in either the Ada Reference Manual or the GNAT Programmer's Reference Manual. Such units are intended only for internal implementation purposes and should not be with'ed by user programs. The default is that such warnings are generated This warning can also be turned on using -gnatwa.

-gnatwI (disable warnings on implementation units).

This switch disables warnings for a with of an internal GNAT implementation unit.

-gnatwl (activate warnings on elaboration pragmas)

This switch activates warnings on missing pragma Elaborate_All statements. See the section in this guide on elaboration checking for details on when such pragma should be used. The default is that such warnings are not generated. This warning can also be turned on using -gnatwa.

-gnatwL (suppress warnings on elaboration pragmas)

This switch suppresses warnings on missing pragma Elaborate_All statements. See the section in this guide on elaboration checking for details on when such pragma should be used.

-gnatwo (activate warnings on address clause overlays)

This switch activates warnings for possibly unintended initialization effects of defining address clauses that cause one variable to overlap another. The default is that such warnings are generated. This warning can also be turned on using -gnatwa.

-gnatwO (suppress warnings on address clause overlays)

This switch suppresses warnings on possibly unintended initialization effects of defining address clauses that cause one variable to overlap another.

-gnatwp (activate warnings on ineffective pragma Inlines)

This switch activates warnings for failure of front end inlining (activated by -gnatN) to inline a particular call. There are many reasons for not being able to inline a call, including most commonly that the call is too complex to inline. This warning can also be turned on using -gnatwa.

-gnatwP (suppress warnings on ineffective pragma Inlines)

This switch suppresses warnings on ineffective pragma Inlines. If the inlining mechanism cannot inline a call, it will simply ignore the request silently.

-gnatwr (activate warnings on redundant constructs)

This switch activates warnings for redundant constructs. The following is the current list of constructs regarded as redundant: This warning can also be turned on using -gnatwa.

• Assignment of an item to itself.
• Type conversion that converts an expression to its own type.
• Use of the attribute Base where typ'Base is the same as typ.
• Use of pragma Pack when all components are placed by a record representation clause.

-gnatwR (suppress warnings on redundant constructs)

This switch suppresses warnings for redundant constructs.

-gnatws (suppress all warnings)

This switch completely suppresses the output of all warning messages from the GNAT front end. Note that it does not suppress warnings from the gcc back end. To suppress these back end warnings as well, use the switch -w in addition to -gnatws.

-gnatwu (activate warnings on unused entities)

This switch activates warnings to be generated for entities that are defined but not referenced, and for units that are with'ed and not referenced. In the case of packages, a warning is also generated if no entities in the package are referenced. This means that if the package is referenced but the only references are in use clauses or renames declarations, a warning is still generated. A warning is also generated for a generic package that is with'ed but never instantiated. In the case where a package or subprogram body is compiled, and there is a with on the corresponding spec that is only referenced in the body, a warning is also generated, noting that the with can be moved to the body. The default is that such warnings are not generated. This switch also activates warnings on unreferenced formals (it is includes the effect of -gnatwf). This warning can also be turned on using -gnatwa.

-gnatwU (suppress warnings on unused entities)

This switch suppresses warnings for unused entities and packages. It also turns off warnings on unreferenced formals (and thus includes the effect of -gnatwF).

A string of warning parameters can be used in the same parameter. For example:

          -gnatwaLe
          

Would turn on all optional warnings except for elaboration pragma warnings, and also specify that warnings should be treated as errors.

-w

This switch suppresses warnings from the gcc backend. It may be used in conjunction with -gnatws to ensure that all warnings are suppressed during the entire compilation process.

3.2.2 Debugging and Assertion Control

-gnata

The pragmas Assert and Debug normally have no effect and are ignored. This switch, where a stands for assert, causes Assert and Debug pragmas to be activated.

The pragmas have the form:

          

             pragma Assert (Boolean-expression [,
                                static-string-expression])
             pragma Debug (procedure call)
          
          

The Assert pragma causes Boolean-expression to be tested. If the result is True, the pragma has no effect (other than possible side effects from evaluating the expression). If the result is False, the exception Assert_Failure declared in the package System.Assertions is raised (passing static-string-expression, if present, as the message associated with the exception). If no string expression is given the default is a string giving the file name and line number of the pragma.

The Debug pragma causes procedure to be called. Note that pragma Debug may appear within a declaration sequence, allowing debugging procedures to be called between declarations.

3.2.3 Validity Checking

The Ada 95 Reference Manual has specific requirements for checking for invalid values. In particular, RM 13.9.1 requires that the evaluation of invalid values (for example from unchecked conversions), not result in erroneous execution. In GNAT, the result of such an evaluation in normal default mode is to either use the value unmodified, or to raise Constraint_Error in those cases where use of the unmodified value would cause erroneous execution. The cases where unmodified values might lead to erroneous execution are case statements (where a wild jump might result from an invalid value), and subscripts on the left hand side (where memory corruption could occur as a result of an invalid value).

The -gnatVx switch allows more control over the validity checking mode. The x argument here is a string of letters which control which validity checks are performed in addition to the default checks described above.

• -gnatVc Validity checks for copies

  The right hand side of assignments, and the initializing values of object declarations are validity checked.

• -gnatVd Default (RM) validity checks

  Some validity checks are done by default following normal Ada semantics (RM 13.9.1 (9-11)). A check is done in case statements that the expression is within the range of the subtype. If it is not, Constraint_Error is raised. For assignments to array components, a check is done that the expression used as index is within the range. If it is not, Constraint_Error is raised. Both these validity checks may be turned off using switch -gnatVD. They are turned on by default. If -gnatVD is specified, a subsequent switch -gnatVd will leave the checks turned on. Switch -gnatVD should be used only if you are sure that all such expressions have valid values. If you use this switch and invalid values are present, then the program is erroneous, and wild jumps or memory overwriting may occur.

• -gnatVi Validity checks for in mode parameters

  Arguments for parameters of mode in are validity checked in function and procedure calls at the point of call.

• -gnatVm Validity checks for in out mode parameters

  Arguments for parameters of mode in out are validity checked in procedure calls at the point of call. The 'm' here stands for modify, since this concerns parameters that can be modified by the call. Note that there is no specific option to test out parameters, but any reference within the subprogram will be tested in the usual manner, and if an invalid value is copied back, any reference to it will be subject to validity checking.

• -gnatVo Validity checks for operator and attribute operands

  Arguments for predefined operators and attributes are validity checked. This includes all operators in package Standard, the shift operators defined as intrinsic in package Interfaces and operands for attributes such as Pos.

• -gnatVr Validity checks for function returns

  The expression in return statements in functions is validity checked.

• -gnatVs Validity checks for subscripts

  All subscripts expressions are checked for validity, whether they appear on the right side or left side (in default mode only left side subscripts are validity checked).

• -gnatVt Validity checks for tests

  Expressions used as conditions in if, while or exit statements are checked, as well as guard expressions in entry calls.

• -gnatVf Validity checks for floating-point values

  In the absence of this switch, validity checking occurs only for discrete values. If -gnatVf is specified, then validity checking also applies for floating-point values, and NaN's and infinities are considered invalid, as well as out of range values for constrained types. Note that this means that standard IEEE infinity mode is not allowed. The exact contexts in which floating-point values are checked depends on the setting of other options. For example -gnatVif or -gnatVfi (the order does not matter) specifies that floating-point parameters of mode in should be validity checked.

• -gnatVa All validity checks

  All the above validity checks are turned on. That is -gnatVa is equivalent to gnatVcdfimorst.

• -gnatVn No validity checks

  This switch turns off all validity checking, including the default checking for case statements and left hand side subscripts. Note that the use of the switch -gnatp supresses all run-time checks, including validity checks, and thus implies -gnatVn.

The -gnatV switch may be followed by a string of letters to turn on a series of validity checking options. For example, -gnatVcr specifies that in addition to the default validity checking, copies and function return expressions be validity checked. In order to make it easier to specify a set of options, the upper case letters CDFIMORST may be used to turn off the corresponding lower case option, so for example -gnatVaM turns on all validity checking options except for checking of in out procedure arguments.

The specification of additional validity checking generates extra code (and in the case of -gnatva the code expansion can be substantial. However, these additional checks can be very useful in smoking out cases of uninitialized variables, incorrect use of unchecked conversion, and other errors leading to invalid values. The use of pragma Initialize_Scalars is useful in conjunction with the extra validity checking, since this ensures that wherever possible uninitialized variables have invalid values.

See also the pragma Validity_Checks which allows modification of the validity checking mode at the program source level, and also allows for temporary disabling of validity checks.

3.2.4 Style Checking

The -gnatyx switch causes the compiler to enforce specified style rules. A limited set of style rules has been used in writing the GNAT sources themselves. This switch allows user programs to activate all or some of these checks. If the source program fails a specified style check, an appropriate warning message is given, preceded by the character sequence "(style)". The string x is a sequence of letters or digits indicating the particular style checks to be performed. The following checks are defined:

1-9 (specify indentation level)

If a digit from 1-9 appears in the string after -gnaty then proper indentation is checked, with the digit indicating the indentation level required. The general style of required indentation is as specified by the examples in the Ada Reference Manual. Full line comments must be aligned with the -- starting on a column that is a multiple of the alignment level.

a (check attribute casing)

If the letter a appears in the string after -gnaty then attribute names, including the case of keywords such as digits used as attributes names, must be written in mixed case, that is, the initial letter and any letter following an underscore must be uppercase. All other letters must be lowercase.

b (blanks not allowed at statement end)

If the letter b appears in the string after -gnaty then trailing blanks are not allowed at the end of statements. The purpose of this rule, together with h (no horizontal tabs), is to enforce a canonical format for the use of blanks to separate source tokens.

c (check comments)

If the letter c appears in the string after -gnaty then comments must meet the following set of rules:

• The "-" that starts the column must either start in column one, or else at least one blank must precede this sequence.
• Comments that follow other tokens on a line must have at least one blank following the "-" at the start of the comment.
• Full line comments must have two blanks following the "-" that starts the comment, with the following exceptions.
• A line consisting only of the "-" characters, possibly preceded by blanks is permitted.
• A comment starting with "-x" where x is a special character is permitted. This alows proper processing of the output generated by specialized tools including gnatprep (where -! is used) and the SPARK annnotation language (where -# is used). For the purposes of this rule, a special character is defined as being in one of the ASCII ranges 16#21#..16#2F# or 16#3A#..16#3F#.
• A line consisting entirely of minus signs, possibly preceded by blanks, is permitted. This allows the construction of box comments where lines of minus signs are used to form the top and bottom of the box.
• If a comment starts and ends with "-" is permitted as long as at least one blank follows the initial "-". Together with the preceding rule, this allows the construction of box comments, as shown in the following example:

                 ---------------------------
                 -- This is a box comment --
                 -- with two text lines.  --
                 ---------------------------
                 

e (check end/exit labels)

If the letter e appears in the string after -gnaty then optional labels on end statements ending subprograms and on exit statements exiting named loops, are required to be present.

f (no form feeds or vertical tabs)

If the letter f appears in the string after -gnaty then neither form feeds nor vertical tab characters are not permitted in the source text.

h (no horizontal tabs)

If the letter h appears in the string after -gnaty then horizontal tab characters are not permitted in the source text. Together with the b (no blanks at end of line) check, this enforces a canonical form for the use of blanks to separate source tokens.

i (check if-then layout)

If the letter i appears in the string after -gnaty, then the keyword then must appear either on the same line as corresponding if, or on a line on its own, lined up under the if with at least one non-blank line in between containing all or part of the condition to be tested.

k (check keyword casing)

If the letter k appears in the string after -gnaty then all keywords must be in lower case (with the exception of keywords such as digits used as attribute names to which this check does not apply).

l (check layout)

If the letter l appears in the string after -gnaty then layout of statement and declaration constructs must follow the recommendations in the Ada Reference Manual, as indicated by the form of the syntax rules. For example an else keyword must be lined up with the corresponding if keyword.

There are two respects in which the style rule enforced by this check option are more liberal than those in the Ada Reference Manual. First in the case of record declarations, it is permissible to put the record keyword on the same line as the type keyword, and then the end in end record must line up under type. For example, either of the following two layouts is acceptable:

          

          type q is record
             a : integer;
             b : integer;
          end record;
          
          type q is
             record
                a : integer;
                b : integer;
             end record;
          
          

Second, in the case of a block statement, a permitted alternative is to put the block label on the same line as the declare or begin keyword, and then line the end keyword up under the block label. For example both the following are permitted:

          

          Block : declare
             A : Integer := 3;
          begin
             Proc (A, A);
          end Block;
          
          Block :
             declare
                A : Integer := 3;
             begin
                Proc (A, A);
             end Block;
          
          

The same alternative format is allowed for loops. For example, both of the following are permitted:

          

          Clear : while J < 10 loop
             A (J) := 0;
          end loop Clear;
          
          Clear :
             while J < 10 loop
                A (J) := 0;
             end loop Clear;
          
          

m (check maximum line length)

If the letter m appears in the string after -gnaty then the length of source lines must not exceed 79 characters, including any trailing blanks. The value of 79 allows convenient display on an 80 character wide device or window, allowing for possible special treatment of 80 character lines.

Mnnn (set maximum line length)

If the sequence Mnnn, where nnn is a decimal number, appears in the string after -gnaty then the length of lines must not exceed the given value.

n (check casing of entities in Standard)

If the letter n appears in the string after -gnaty then any identifier from Standard must be cased to match the presentation in the Ada Reference Manual (for example, Integer and ASCII.NUL).

o (check order of subprogram bodies)

If the letter o appears in the string after -gnaty then all subprogram bodies in a given scope (e.g. a package body) must be in alphabetical order. The ordering rule uses normal Ada rules for comparing strings, ignoring casing of letters, except that if there is a trailing numeric suffix, then the value of this suffix is used in the ordering (e.g. Junk2 comes before Junk10).

p (check pragma casing)

If the letter p appears in the string after -gnaty then pragma names must be written in mixed case, that is, the initial letter and any letter following an underscore must be uppercase. All other letters must be lowercase.

r (check references)

If the letter r appears in the string after -gnaty then all identifier references must be cased in the same way as the corresponding declaration. No specific casing style is imposed on identifiers. The only requirement is for consistency of references with declarations.

s (check separate specs)

If the letter s appears in the string after -gnaty then separate declarations ("specs") are required for subprograms (a body is not allowed to serve as its own declaration). The only exception is that parameterless library level procedures are not required to have a separate declaration. This exception covers the most frequent form of main program procedures.

t (check token spacing)

If the letter t appears in the string after -gnaty then the following token spacing rules are enforced:

• The keywords abs and not must be followed by a space.
• The token => must be surrounded by spaces.
• The token <> must be preceded by a space or a left parenthesis.
• Binary operators other than ** must be surrounded by spaces. There is no restriction on the layout of the ** binary operator.
• Colon must be surrounded by spaces.
• Colon-equal (assignment) must be surrounded by spaces.
• Comma must be the first non-blank character on the line, or be immediately preceded by a non-blank character, and must be followed by a space.
• If the token preceding a left paren ends with a letter or digit, then a space must separate the two tokens.
• A right parenthesis must either be the first non-blank character on a line, or it must be preceded by a non-blank character.
• A semicolon must not be preceded by a space, and must not be followed by a non-blank character.
• A unary plus or minus may not be followed by a space.
• A vertical bar must be surrounded by spaces.

In the above rules, appearing in column one is always permitted, that is, counts as meeting either a requirement for a required preceding space, or as meeting a requirement for no preceding space.

Appearing at the end of a line is also always permitted, that is, counts as meeting either a requirement for a following space, or as meeting a requirement for no following space.

If any of these style rules is violated, a message is generated giving details on the violation. The initial characters of such messages are always "(style)". Note that these messages are treated as warning messages, so they normally do not prevent the generation of an object file. The -gnatwe switch can be used to treat warning messages, including style messages, as fatal errors.

The switch -gnaty on its own (that is not followed by any letters or digits), is equivalent to gnaty3abcefhiklmprst, that is all checking options are enabled with the exception of -gnatyo, with an indentation level of 3. This is the standard checking option that is used for the GNAT sources.

3.2.5 Run-Time Checks

If you compile with the default options, GNAT will insert many run-time checks into the compiled code, including code that performs range checking against constraints, but not arithmetic overflow checking for integer operations (including division by zero) or checks for access before elaboration on subprogram calls. All other run-time checks, as required by the Ada 95 Reference Manual, are generated by default. The following gcc switches refine this default behavior:

-gnatp

Suppress all run-time checks as though pragma Suppress (all_checks) had been present in the source. Validity checks are also suppressed (in other words -gnatp also implies -gnatVn. Use this switch to improve the performance of the code at the expense of safety in the presence of invalid data or program bugs.

-gnato

Enables overflow checking for integer operations. This causes GNAT to generate slower and larger executable programs by adding code to check for overflow (resulting in raising Constraint_Error as required by standard Ada semantics). These overflow checks correspond to situations in which the true value of the result of an operation may be outside the base range of the result type. The following example shows the distinction:

          X1 : Integer := Integer'Last;
          X2 : Integer range 1 .. 5 := 5;
          ...
          X1 := X1 + 1;   -- -gnato required to catch the Constraint_Error
          X2 := X2 + 1;   -- range check, -gnato has no effect here
          

Here the first addition results in a value that is outside the base range of Integer, and hence requires an overflow check for detection of the constraint error. The second increment operation results in a violation of the explicit range constraint, and such range checks are always performed. Basically the compiler can assume that in the absence of the -gnato switch that any value of type xxx is in range of the base type of xxx.

Note that the -gnato switch does not affect the code generated for any floating-point operations; it applies only to integer semantics). For floating-point, GNAT has the Machine_Overflows attribute set to False and the normal mode of operation is to generate IEEE NaN and infinite values on overflow or invalid operations (such as dividing 0.0 by 0.0).

The reason that we distinguish overflow checking from other kinds of range constraint checking is that a failure of an overflow check can generate an incorrect value, but cannot cause erroneous behavior. This is unlike the situation with a constraint check on an array subscript, where failure to perform the check can result in random memory description, or the range check on a case statement, where failure to perform the check can cause a wild jump.

Note again that -gnato is off by default, so overflow checking is not performed in default mode. This means that out of the box, with the default settings, GNAT does not do all the checks expected from the language description in the Ada Reference Manual. If you want all constraint checks to be performed, as described in this Manual, then you must explicitly use the -gnato switch either on the gnatmake or gcc command.

-gnatE

Enables dynamic checks for access-before-elaboration on subprogram calls and generic instantiations. For full details of the effect and use of this switch, See Compiling Using gcc.

The setting of these switches only controls the default setting of the checks. You may modify them using either Suppress (to remove checks) or Unsuppress (to add back suppressed checks) pragmas in the program source.

3.2.6 Stack Overflow Checking

For most operating systems, gcc does not perform stack overflow checking by default. This means that if the main environment task or some other task exceeds the available stack space, then unpredictable behavior will occur.

To activate stack checking, compile all units with the gcc option -fstack-check. For example:

     gcc -c -fstack-check package1.adb
     

Units compiled with this option will generate extra instructions to check that any use of the stack (for procedure calls or for declaring local variables in declare blocks) do not exceed the available stack space. If the space is exceeded, then a Storage_Error exception is raised.

For declared tasks, the stack size is always controlled by the size given in an applicable Storage_Size pragma (or is set to the default size if no pragma is used.

For the environment task, the stack size depends on system defaults and is unknown to the compiler. The stack may even dynamically grow on some systems, precluding the normal Ada semantics for stack overflow. In the worst case, unbounded stack usage, causes unbounded stack expansion resulting in the system running out of virtual memory.

The stack checking may still work correctly if a fixed size stack is allocated, but this cannot be guaranteed. To ensure that a clean exception is signalled for stack overflow, set the environment variable GNAT_STACK_LIMIT to indicate the maximum stack area that can be used, as in:

     SET GNAT_STACK_LIMIT 1600
     

The limit is given in kilobytes, so the above declaration would set the stack limit of the environment task to 1.6 megabytes. Note that the only purpose of this usage is to limit the amount of stack used by the environment task. If it is necessary to increase the amount of stack for the environment task, then this is an operating systems issue, and must be addressed with the appropriate operating systems commands.

3.2.7 Run-Time Control

-gnatT nnn

The gnatT switch can be used to specify the time-slicing value to be used for task switching between equal priority tasks. The value nnn is given in microseconds as a decimal integer.

Setting the time-slicing value is only effective if the underlying thread control system can accommodate time slicing. Check the documentation of your operating system for details. Note that the time-slicing value can also be set by use of pragma Time_Slice or by use of the t switch in the gnatbind step. The pragma overrides a command line argument if both are present, and the t switch for gnatbind overrides both the pragma and the gcc command line switch.

3.2.8 Using gcc for Syntax Checking

-gnats

The s stands for syntax.

Run GNAT in syntax checking only mode. For example, the command

          $ gcc -c -gnats x.adb
          

compiles file x.adb in syntax-check-only mode. You can check a series of files in a single command , and can use wild cards to specify such a group of files. Note that you must specify the -c (compile only) flag in addition to the -gnats flag. .

You may use other switches in conjunction with -gnats. In particular, -gnatl and -gnatv are useful to control the format of any generated error messages.

The output is simply the error messages, if any. No object file or ALI file is generated by a syntax-only compilation. Also, no units other than the one specified are accessed. For example, if a unit X with's a unit Y, compiling unit X in syntax check only mode does not access the source file containing unit Y.

Normally, GNAT allows only a single unit in a source file. However, this restriction does not apply in syntax-check-only mode, and it is possible to check a file containing multiple compilation units concatenated together. This is primarily used by the gnatchop utility (see Renaming Files Using gnatchop).

3.2.9 Using gcc for Semantic Checking

-gnatc

The c stands for check. Causes the compiler to operate in semantic check mode, with full checking for all illegalities specified in the Ada 95 Reference Manual, but without generation of any object code (no object file is generated).

Because dependent files must be accessed, you must follow the GNAT semantic restrictions on file structuring to operate in this mode:

• The needed source files must be accessible (see Search Paths and the Run-Time Library (RTL)).
• Each file must contain only one compilation unit.
• The file name and unit name must match (see File Naming Rules).

The output consists of error messages as appropriate. No object file is generated. An ALI file is generated for use in the context of cross-reference tools, but this file is marked as not being suitable for binding (since no object file is generated). The checking corresponds exactly to the notion of legality in the Ada 95 Reference Manual.

Any unit can be compiled in semantics-checking-only mode, including units that would not normally be compiled (subunits, and specifications where a separate body is present).

3.2.10 Compiling Ada 83 Programs

-gnat83

Although GNAT is primarily an Ada 95 compiler, it accepts this switch to specify that an Ada 83 program is to be compiled in Ada83 mode. If you specify this switch, GNAT rejects most Ada 95 extensions and applies Ada 83 semantics where this can be done easily. It is not possible to guarantee this switch does a perfect job; for example, some subtle tests, such as are found in earlier ACVC tests (that have been removed from the ACVC suite for Ada 95), may not compile correctly. However, for most purposes, using this switch should help to ensure that programs that compile correctly under the -gnat83 switch can be ported easily to an Ada 83 compiler. This is the main use of the switch.

With few exceptions (most notably the need to use <> on unconstrained generic formal parameters, the use of the new Ada 95 keywords, and the use of packages with optional bodies), it is not necessary to use the -gnat83 switch when compiling Ada 83 programs, because, with rare exceptions, Ada 95 is upwardly compatible with Ada 83. This means that a correct Ada 83 program is usually also a correct Ada 95 program.

3.2.11 Character Set Control

-gnatic

Normally GNAT recognizes the Latin-1 character set in source program identifiers, as described in the Ada 95 Reference Manual. This switch causes GNAT to recognize alternate character sets in identifiers. c is a single character indicating the character set, as follows:

1

Latin-1 identifiers

2

Latin-2 letters allowed in identifiers

3

Latin-3 letters allowed in identifiers

4

Latin-4 letters allowed in identifiers

5

Latin-5 (Cyrillic) letters allowed in identifiers

9

Latin-9 letters allowed in identifiers

p

IBM PC letters (code page 437) allowed in identifiers

8

IBM PC letters (code page 850) allowed in identifiers

f

Full upper-half codes allowed in identifiers

n

No upper-half codes allowed in identifiers

w

Wide-character codes (that is, codes greater than 255) allowed in identifiers

See Foreign Language Representation, for full details on the implementation of these character sets.

-gnatWe

Specify the method of encoding for wide characters. e is one of the following:

h

Hex encoding (brackets coding also recognized)

u

Upper half encoding (brackets encoding also recognized)

s

Shift/JIS encoding (brackets encoding also recognized)

e

EUC encoding (brackets encoding also recognized)

8

UTF-8 encoding (brackets encoding also recognized)

b

Brackets encoding only (default value)

For full details on the these encoding methods see See Wide Character Encodings. Note that brackets coding is always accepted, even if one of the other options is specified, so for example -gnatW8 specifies that both brackets and UTF-8 encodings will be recognized. The units that are with'ed directly or indirectly will be scanned using the specified representation scheme, and so if one of the non-brackets scheme is used, it must be used consistently throughout the program. However, since brackets encoding is always recognized, it may be conveniently used in standard libraries, allowing these libraries to be used with any of the available coding schemes. scheme. If no -gnatW? parameter is present, then the default representation is Brackets encoding only.

Note that the wide character representation that is specified (explicitly or by default) for the main program also acts as the default encoding used for Wide_Text_IO files if not specifically overridden by a WCEM form parameter.

3.2.12 File Naming Control

-gnatkn

Activates file name "krunching". n, a decimal integer in the range 1-999, indicates the maximum allowable length of a file name (not including the .ads or .adb extension). The default is not to enable file name krunching.

For the source file naming rules, See File Naming Rules.

3.2.13 Subprogram Inlining Control

-gnatn

The n here is intended to suggest the first syllable of the word "inline". GNAT recognizes and processes Inline pragmas. However, for the inlining to actually occur, optimization must be enabled. To enable inlining across unit boundaries, this is, inlining a call in one unit of a subprogram declared in a with'ed unit, you must also specify this switch. In the absence of this switch, GNAT does not attempt inlining across units and does not need to access the bodies of subprograms for which pragma Inline is specified if they are not in the current unit.

If you specify this switch the compiler will access these bodies, creating an extra source dependency for the resulting object file, and where possible, the call will be inlined. For further details on when inlining is possible see See Inlining of Subprograms.

-gnatN

The front end inlining activated by this switch is generally more extensive, and quite often more effective than the standard -gnatn inlining mode. It will also generate additional dependencies.

3.2.14 Auxiliary Output Control

-gnatt

Causes GNAT to write the internal tree for a unit to a file (with the extension .adt. This not normally required, but is used by separate analysis tools. Typically these tools do the necessary compilations automatically, so you should not have to specify this switch in normal operation.

-gnatu

Print a list of units required by this compilation on stdout. The listing includes all units on which the unit being compiled depends either directly or indirectly.

-pass-exit-codes

If this switch is not used, the exit code returned by gcc when compiling multiple files indicates whether all source files have been successfully used to generate object files or not.

When -pass-exit-codes is used, gcc exits with an extended exit status and allows an integrated development environment to better react to a compilation failure. Those exit status are:

5

There was an error in at least one source file.

3

At least one source file did not generate an object file.

2

The compiler died unexpectedly (internal error for example).

0

An object file has been generated for every source file.

3.2.15 Debugging Control

-gnatdx

Activate internal debugging switches. x is a letter or digit, or string of letters or digits, which specifies the type of debugging outputs desired. Normally these are used only for internal development or system debugging purposes. You can find full documentation for these switches in the body of the Debug unit in the compiler source file debug.adb.

-gnatG

This switch causes the compiler to generate auxiliary output containing a pseudo-source listing of the generated expanded code. Like most Ada compilers, GNAT works by first transforming the high level Ada code into lower level constructs. For example, tasking operations are transformed into calls to the tasking run-time routines. A unique capability of GNAT is to list this expanded code in a form very close to normal Ada source. This is very useful in understanding the implications of various Ada usage on the efficiency of the generated code. There are many cases in Ada (e.g. the use of controlled types), where simple Ada statements can generate a lot of run-time code. By using -gnatG you can identify these cases, and consider whether it may be desirable to modify the coding approach to improve efficiency.

The format of the output is very similar to standard Ada source, and is easily understood by an Ada programmer. The following special syntactic additions correspond to low level features used in the generated code that do not have any exact analogies in pure Ada source form. The following is a partial list of these special constructions. See the specification of package Sprint in file sprint.ads for a full list.

new xxx [storage_pool = yyy]

Shows the storage pool being used for an allocator.

at end procedure-name;

Shows the finalization (cleanup) procedure for a scope.

(if expr then expr else expr)

Conditional expression equivalent to the x?y:z construction in C.

target^(source)

A conversion with floating-point truncation instead of rounding.

target?(source)

A conversion that bypasses normal Ada semantic checking. In particular enumeration types and fixed-point types are treated simply as integers.

target?^(source)

Combines the above two cases.

x #/ y

x #mod y

x #* y

x #rem y

A division or multiplication of fixed-point values which are treated as integers without any kind of scaling.

free expr [storage_pool = xxx]

Shows the storage pool associated with a free statement.

freeze typename [actions]

Shows the point at which typename is frozen, with possible associated actions to be performed at the freeze point.

reference itype

Reference (and hence definition) to internal type itype.

function-name! (arg, arg, arg)

Intrinsic function call.

labelname : label

Declaration of label labelname.

expr && expr && expr ... && expr

A multiple concatenation (same effect as expr & expr & expr, but handled more efficiently).

[constraint_error]

Raise the Constraint_Error exception.

expression'reference

A pointer to the result of evaluating expression.

target-type!(source-expression)

An unchecked conversion of source-expression to target-type.

[numerator/denominator]

Used to represent internal real literals (that) have no exact representation in base 2-16 (for example, the result of compile time evaluation of the expression 1.0/27.0).

-gnatD

This switch is used in conjunction with -gnatG to cause the expanded source, as described above to be written to files with names xxx.dg, where xxx is the normal file name, for example, if the source file name is hello.adb, then a file hello.adb.dg will be written. The debugging information generated by the gcc -g switch will refer to the generated xxx.dg file. This allows you to do source level debugging using the generated code which is sometimes useful for complex code, for example to find out exactly which part of a complex construction raised an exception. This switch also suppress generation of cross-reference information (see -gnatx).

-gnatC

In the generated debugging information, and also in the case of long external names, the compiler uses a compression mechanism if the name is very long. This compression method uses a checksum, and avoids trouble on some operating systems which have difficulty with very long names. The -gnatC switch forces this compression approach to be used on all external names and names in the debugging information tables. This reduces the size of the generated executable, at the expense of making the naming scheme more complex. The compression only affects the qualification of the name. Thus a name in the source:

               Very_Long_Package.Very_Long_Inner_Package.Var
               

would normally appear in these tables as:

               very_long_package__very_long_inner_package__var
               

but if the -gnatC switch is used, then the name appears as

               XCb7e0c705__var
               

Here b7e0c705 is a compressed encoding of the qualification prefix. The GNAT Ada aware version of GDB understands these encoded prefixes, so if this debugger is used, the encoding is largely hidden from the user of the compiler.

-gnatR[0|1|2|3][s]

This switch controls output from the compiler of a listing showing representation information for declared types and objects. For -gnatR0, no information is output (equivalent to omitting the -gnatR switch). For -gnatR1 (which is the default, so -gnatR with no parameter has the same effect), size and alignment information is listed for declared array and record types. For -gnatR2, size and alignment information is listed for all expression information for values that are computed at run time for variant records. These symbolic expressions have a mostly obvious format with #n being used to represent the value of the n'th discriminant. See source files repinfo.ads/adb in the GNAT sources for full detalis on the format of -gnatR3 output. If the switch is followed by an s (e.g. -gnatR2s), then the output is to a file with the name file.rep where file is the name of the corresponding source file.

-gnatx

Normally the compiler generates full cross-referencing information in the ALI file. This information is used by a number of tools, including gnatfind and gnatxref. The -gnatx switch suppresses this information. This saves some space and may slightly speed up compilation, but means that these tools cannot be used.

3.2.16 Units to Sources Mapping Files

-gnatempath

A mapping file is a way to communicate to the compiler two mappings: from unit names to file names (without any directory information) and from file names to path names (with full directory information). These mappings are used by the compiler to short-circuit the path search.

A mapping file is a sequence of sets of three lines. In each set, the first line is the unit name, in lower case, with "%s" appended for specifications and "%b" appended for bodies; the second line is the file name; and the third line is the path name.

Example:

             main%b
             main.2.ada
             /gnat/project1/sources/main.2.ada
          

When the switch -gnatem is specified, the compiler will create in memory the two mappings from the specified file. If there is any problem (non existent file, truncated file or duplicate entries), no mapping will be created.

Several -gnatem switches may be specified; however, only the last one on the command line will be taken into account.

When using a project file, gnatmake create a temporary mapping file and communicates it to the compiler using this switch.

3.3 Search Paths and the Run-Time Library (RTL)

With the GNAT source-based library system, the compiler must be able to find source files for units that are needed by the unit being compiled. Search paths are used to guide this process.

The compiler compiles one source file whose name must be given explicitly on the command line. In other words, no searching is done for this file. To find all other source files that are needed (the most common being the specs of units), the compiler examines the following directories, in the following order:

1. The directory containing the source file of the main unit being compiled (the file name on the command line).
2. Each directory named by an -I switch given on the gcc command line, in the order given.
3. Each of the directories listed in the value of the ADA_INCLUDE_PATH environment variable. Construct this value exactly as the PATH environment variable: a list of directory names separated by colons (semicolons when working with the NT version).
4. The content of the "ada_source_path" file which is part of the GNAT installation tree and is used to store standard libraries such as the GNAT Run Time Library (RTL) source files. Installing an Ada Library

Specifying the switch -I- inhibits the use of the directory containing the source file named in the command line. You can still have this directory on your search path, but in this case it must be explicitly requested with a -I switch.

Specifying the switch -nostdinc inhibits the search of the default location for the GNAT Run Time Library (RTL) source files.

The compiler outputs its object files and ALI files in the current working directory. Caution: The object file can be redirected with the -o switch; however, gcc and gnat1 have not been coordinated on this so the ALI file will not go to the right place. Therefore, you should avoid using the -o switch.

The packages Ada, System, and Interfaces and their children make up the GNAT RTL, together with the simple System.IO package used in the "Hello World" example. The sources for these units are needed by the compiler and are kept together in one directory. Not all of the bodies are needed, but all of the sources are kept together anyway. In a normal installation, you need not specify these directory names when compiling or binding. Either the environment variables or the built-in defaults cause these files to be found.

In addition to the language-defined hierarchies (System, Ada and Interfaces), the GNAT distribution provides a fourth hierarchy, consisting of child units of GNAT. This is a collection of generally useful routines. See the GNAT Reference Manual for further details.

Besides simplifying access to the RTL, a major use of search paths is in compiling sources from multiple directories. This can make development environments much more flexible.

3.4 Order of Compilation Issues

If, in our earlier example, there was a spec for the hello procedure, it would be contained in the file hello.ads; yet this file would not have to be explicitly compiled. This is the result of the model we chose to implement library management. Some of the consequences of this model are as follows:

• There is no point in compiling specs (except for package specs with no bodies) because these are compiled as needed by clients. If you attempt a useless compilation, you will receive an error message. It is also useless to compile subunits because they are compiled as needed by the parent.
• There are no order of compilation requirements: performing a compilation never obsoletes anything. The only way you can obsolete something and require recompilations is to modify one of the source files on which it depends.
• There is no library as such, apart from the ALI files (see The Ada Library Information Files, for information on the format of these files). For now we find it convenient to create separate ALI files, but eventually the information therein may be incorporated into the object file directly.
• When you compile a unit, the source files for the specs of all units that it with's, all its subunits, and the bodies of any generics it instantiates must be available (reachable by the search-paths mechanism described above), or you will receive a fatal error message.

3.5 Examples

The following are some typical Ada compilation command line examples:

$ gcc -c xyz.adb

Compile body in file xyz.adb with all default options.

$ gcc -c -O2 -gnata xyz-def.adb

Compile the child unit package in file xyz-def.adb with extensive optimizations, and pragma Assert/Debug statements enabled.

$ gcc -c -gnatc abc-def.adb

Compile the subunit in file abc-def.adb in semantic-checking-only mode.

4 Binding Using gnatbind

• Running gnatbind:
• Generating the Binder Program in C:
• Consistency-Checking Modes:
• Binder Error Message Control:
• Elaboration Control:
• Output Control:
• Binding with Non-Ada Main Programs:
• Binding Programs with No Main Subprogram:
• Summary of Binder Switches:
• Command-Line Access:
• Search Paths for gnatbind:
• Examples of gnatbind Usage:

This chapter describes the GNAT binder, gnatbind, which is used to bind compiled GNAT objects. The gnatbind program performs four separate functions:

1. Checks that a program is consistent, in accordance with the rules in Chapter 10 of the Ada 95 Reference Manual. In particular, error messages are generated if a program uses inconsistent versions of a given unit.
2. Checks that an acceptable order of elaboration exists for the program and issues an error message if it cannot find an order of elaboration that satisfies the rules in Chapter 10 of the Ada 95 Language Manual.
3. Generates a main program incorporating the given elaboration order. This program is a small Ada package (body and spec) that must be subsequently compiled using the GNAT compiler. The necessary compilation step is usually performed automatically by gnatlink. The two most important functions of this program are to call the elaboration routines of units in an appropriate order and to call the main program.
4. Determines the set of object files required by the given main program. This information is output in the forms of comments in the generated program, to be read by the gnatlink utility used to link the Ada application.

4.1 Running gnatbind

The form of the gnatbind command is

     $ gnatbind [switches] mainprog[.ali] [switches]
     

where mainprog.adb is the Ada file containing the main program unit body. If no switches are specified, gnatbind constructs an Ada package in two files which names are b~ada_main.ads, and b~ada_main.adb. For example, if given the parameter hello.ali, for a main program contained in file hello.adb, the binder output files would be b~hello.ads and b~hello.adb.

When doing consistency checking, the binder takes into consideration any source files it can locate. For example, if the binder determines that the given main program requires the package Pack, whose .ali file is pack.ali and whose corresponding source spec file is pack.ads, it attempts to locate the source file pack.ads (using the same search path conventions as previously described for the gcc command). If it can locate this source file, it checks that the time stamps or source checksums of the source and its references to in ali files match. In other words, any ali files that mentions this spec must have resulted from compiling this version of the source file (or in the case where the source checksums match, a version close enough that the difference does not matter).

The effect of this consistency checking, which includes source files, is that the binder ensures that the program is consistent with the latest version of the source files that can be located at bind time. Editing a source file without compiling files that depend on the source file cause error messages to be generated by the binder.

For example, suppose you have a main program hello.adb and a package P, from file p.ads and you perform the following steps:

1. Enter gcc -c hello.adb to compile the main program.
2. Enter gcc -c p.ads to compile package P.
3. Edit file p.ads.
4. Enter gnatbind hello.

At this point, the file p.ali contains an out-of-date time stamp because the file p.ads has been edited. The attempt at binding fails, and the binder generates the following error messages:

     error: "hello.adb" must be recompiled ("p.ads" has been modified)
     error: "p.ads" has been modified and must be recompiled
     

Now both files must be recompiled as indicated, and then the bind can succeed, generating a main program. You need not normally be concerned with the contents of this file, but it is similar to the following which is the binder file generated for a simple "hello world" program.

     --  The package is called Ada_Main unless this name is actually used
     --  as a unit name in the partition, in which case some other unique
     --  name is used.
     
     with System;
     package ada_main is
     
        Elab_Final_Code : Integer;
        pragma Import (C, Elab_Final_Code, "__gnat_inside_elab_final_code");
     
        --  The main program saves the parameters (argument count,
        --  argument values, environment pointer) in global variables
        --  for later access by other units including
        --  Ada.Command_Line.
     
        gnat_argc : Integer;
        gnat_argv : System.Address;
        gnat_envp : System.Address;
     
        --  The actual variables are stored in a library routine. This
        --  is useful for some shared library situations, where there
        --  are problems if variables are not in the library.
     
        pragma Import (C, gnat_argc);
        pragma Import (C, gnat_argv);
        pragma Import (C, gnat_envp);
     
        --  The exit status is similarly an external location
     
        gnat_exit_status : Integer;
        pragma Import (C, gnat_exit_status);
     
        GNAT_Version : constant String :=
                         "GNAT Version: 3.15w (20010315)";
        pragma Export (C, GNAT_Version, "__gnat_version");
     
        --  This is the generated adafinal routine that performs
        --  finalization at the end of execution. In the case where
        --  Ada is the main program, this main program makes a call
        --  to adafinal at program termination.
     
        procedure adafinal;
        pragma Export (C, adafinal, "adafinal");
     
        --  This is the generated adainit routine that performs
        --  initialization at the start of execution. In the case
        --  where Ada is the main program, this main program makes
        --  a call to adainit at program startup.
     
        procedure adainit;
        pragma Export (C, adainit, "adainit");
     
        --  This routine is called at the start of execution. It is
        --  a dummy routine that is used by the debugger to breakpoint
        --  at the start of execution.
     
        procedure Break_Start;
        pragma Import (C, Break_Start, "__gnat_break_start");
     
        --  This is the actual generated main program (it would be
        --  suppressed if the no main program switch were used). As
        --  required by standard system conventions, this program has
        --  the external name main.
     
        function main
          (argc : Integer;
           argv : System.Address;
           envp : System.Address)
           return Integer;
        pragma Export (C, main, "main");
     
        --  The following set of constants give the version
        --  identification values for every unit in the bound
        --  partition. This identification is computed from all
        --  dependent semantic units, and corresponds to the
        --  string that would be returned by use of the
        --  Body_Version or Version attributes.
     
        type Version_32 is mod 2 ** 32;
        u00001 : constant Version_32 := 16#7880BEB3#;
        u00002 : constant Version_32 := 16#0D24CBD0#;
        u00003 : constant Version_32 := 16#3283DBEB#;
        u00004 : constant Version_32 := 16#2359F9ED#;
        u00005 : constant Version_32 := 16#664FB847#;
        u00006 : constant Version_32 := 16#68E803DF#;
        u00007 : constant Version_32 := 16#5572E604#;
        u00008 : constant Version_32 := 16#46B173D8#;
        u00009 : constant Version_32 := 16#156A40CF#;
        u00010 : constant Version_32 := 16#033DABE0#;
        u00011 : constant Version_32 := 16#6AB38FEA#;
        u00012 : constant Version_32 := 16#22B6217D#;
        u00013 : constant Version_32 := 16#68A22947#;
        u00014 : constant Version_32 := 16#18CC4A56#;
        u00015 : constant Version_32 := 16#08258E1B#;
        u00016 : constant Version_32 := 16#367D5222#;
        u00017 : constant Version_32 := 16#20C9ECA4#;
        u00018 : constant Version_32 := 16#50D32CB6#;
        u00019 : constant Version_32 := 16#39A8BB77#;
        u00020 : constant Version_32 := 16#5CF8FA2B#;
        u00021 : constant Version_32 := 16#2F1EB794#;
        u00022 : constant Version_32 := 16#31AB6444#;
        u00023 : constant Version_32 := 16#1574B6E9#;
        u00024 : constant Version_32 := 16#5109C189#;
        u00025 : constant Version_32 := 16#56D770CD#;
        u00026 : constant Version_32 := 16#02F9DE3D#;
        u00027 : constant Version_32 := 16#08AB6B2C#;
        u00028 : constant Version_32 := 16#3FA37670#;
        u00029 : constant Version_32 := 16#476457A0#;
        u00030 : constant Version_32 := 16#731E1B6E#;
        u00031 : constant Version_32 := 16#23C2E789#;
        u00032 : constant Version_32 := 16#0F1BD6A1#;
        u00033 : constant Version_32 := 16#7C25DE96#;
        u00034 : constant Version_32 := 16#39ADFFA2#;
        u00035 : constant Version_32 := 16#571DE3E7#;
        u00036 : constant Version_32 := 16#5EB646AB#;
        u00037 : constant Version_32 := 16#4249379B#;
        u00038 : constant Version_32 := 16#0357E00A#;
        u00039 : constant Version_32 := 16#3784FB72#;
        u00040 : constant Version_32 := 16#2E723019#;
        u00041 : constant Version_32 := 16#623358EA#;
        u00042 : constant Version_32 := 16#107F9465#;
        u00043 : constant Version_32 := 16#6843F68A#;
        u00044 : constant Version_32 := 16#63305874#;
        u00045 : constant Version_32 := 16#31E56CE1#;
        u00046 : constant Version_32 := 16#02917970#;
        u00047 : constant Version_32 := 16#6CCBA70E#;
        u00048 : constant Version_32 := 16#41CD4204#;
        u00049 : constant Version_32 := 16#572E3F58#;
        u00050 : constant Version_32 := 16#20729FF5#;
        u00051 : constant Version_32 := 16#1D4F93E8#;
        u00052 : constant Version_32 := 16#30B2EC3D#;
        u00053 : constant Version_32 := 16#34054F96#;
        u00054 : constant Version_32 := 16#5A199860#;
        u00055 : constant Version_32 := 16#0E7F912B#;
        u00056 : constant Version_32 := 16#5760634A#;
        u00057 : constant Version_32 := 16#5D851835#;
     
        --  The following Export pragmas export the version numbers
        --  with symbolic names ending in B (for body) or S
        --  (for spec) so that they can be located in a link. The
        --  information provided here is sufficient to track down
        --  the exact versions of units used in a given build.
     
        pragma Export (C, u00001, "helloB");
        pragma Export (C, u00002, "system__standard_libraryB");
        pragma Export (C, u00003, "system__standard_libraryS");
        pragma Export (C, u00004, "adaS");
        pragma Export (C, u00005, "ada__text_ioB");
        pragma Export (C, u00006, "ada__text_ioS");
        pragma Export (C, u00007, "ada__exceptionsB");
        pragma Export (C, u00008, "ada__exceptionsS");
        pragma Export (C, u00009, "gnatS");
        pragma Export (C, u00010, "gnat__heap_sort_aB");
        pragma Export (C, u00011, "gnat__heap_sort_aS");
        pragma Export (C, u00012, "systemS");
        pragma Export (C, u00013, "system__exception_tableB");
        pragma Export (C, u00014, "system__exception_tableS");
        pragma Export (C, u00015, "gnat__htableB");
        pragma Export (C, u00016, "gnat__htableS");
        pragma Export (C, u00017, "system__exceptionsS");
        pragma Export (C, u00018, "system__machine_state_operationsB");
        pragma Export (C, u00019, "system__machine_state_operationsS");
        pragma Export (C, u00020, "system__machine_codeS");
        pragma Export (C, u00021, "system__storage_elementsB");
        pragma Export (C, u00022, "system__storage_elementsS");
        pragma Export (C, u00023, "system__secondary_stackB");
        pragma Export (C, u00024, "system__secondary_stackS");
        pragma Export (C, u00025, "system__parametersB");
        pragma Export (C, u00026, "system__parametersS");
        pragma Export (C, u00027, "system__soft_linksB");
        pragma Export (C, u00028, "system__soft_linksS");
        pragma Export (C, u00029, "system__stack_checkingB");
        pragma Export (C, u00030, "system__stack_checkingS");
        pragma Export (C, u00031, "system__tracebackB");
        pragma Export (C, u00032, "system__tracebackS");
        pragma Export (C, u00033, "ada__streamsS");
        pragma Export (C, u00034, "ada__tagsB");
        pragma Export (C, u00035, "ada__tagsS");
        pragma Export (C, u00036, "system__string_opsB");
        pragma Export (C, u00037, "system__string_opsS");
        pragma Export (C, u00038, "interfacesS");
        pragma Export (C, u00039, "interfaces__c_streamsB");
        pragma Export (C, u00040, "interfaces__c_streamsS");
        pragma Export (C, u00041, "system__file_ioB");
        pragma Export (C, u00042, "system__file_ioS");
        pragma Export (C, u00043, "ada__finalizationB");
        pragma Export (C, u00044, "ada__finalizationS");
        pragma Export (C, u00045, "system__finalization_rootB");
        pragma Export (C, u00046, "system__finalization_rootS");
        pragma Export (C, u00047, "system__finalization_implementationB");
        pragma Export (C, u00048, "system__finalization_implementationS");
        pragma Export (C, u00049, "system__string_ops_concat_3B");
        pragma Export (C, u00050, "system__string_ops_concat_3S");
        pragma Export (C, u00051, "system__stream_attributesB");
        pragma Export (C, u00052, "system__stream_attributesS");
        pragma Export (C, u00053, "ada__io_exceptionsS");
        pragma Export (C, u00054, "system__unsigned_typesS");
        pragma Export (C, u00055, "system__file_control_blockS");
        pragma Export (C, u00056, "ada__finalization__list_controllerB");
        pragma Export (C, u00057, "ada__finalization__list_controllerS");
     
        -- BEGIN ELABORATION ORDER
        -- ada (spec)
        -- gnat (spec)
        -- gnat.heap_sort_a (spec)
        -- gnat.heap_sort_a (body)
        -- gnat.htable (spec)
        -- gnat.htable (body)
        -- interfaces (spec)
        -- system (spec)
        -- system.machine_code (spec)
        -- system.parameters (spec)
        -- system.parameters (body)
        -- interfaces.c_streams (spec)
        -- interfaces.c_streams (body)
        -- system.standard_library (spec)
        -- ada.exceptions (spec)
        -- system.exception_table (spec)
        -- system.exception_table (body)
        -- ada.io_exceptions (spec)
        -- system.exceptions (spec)
        -- system.storage_elements (spec)
        -- system.storage_elements (body)
        -- system.machine_state_operations (spec)
        -- system.machine_state_operations (body)
        -- system.secondary_stack (spec)
        -- system.stack_checking (spec)
        -- system.soft_links (spec)
        -- system.soft_links (body)
        -- system.stack_checking (body)
        -- system.secondary_stack (body)
        -- system.standard_library (body)
        -- system.string_ops (spec)
        -- system.string_ops (body)
        -- ada.tags (spec)
        -- ada.tags (body)
        -- ada.streams (spec)
        -- system.finalization_root (spec)
        -- system.finalization_root (body)
        -- system.string_ops_concat_3 (spec)
        -- system.string_ops_concat_3 (body)
        -- system.traceback (spec)
        -- system.traceback (body)
        -- ada.exceptions (body)
        -- system.unsigned_types (spec)
        -- system.stream_attributes (spec)
        -- system.stream_attributes (body)
        -- system.finalization_implementation (spec)
        -- system.finalization_implementation (body)
        -- ada.finalization (spec)
        -- ada.finalization (body)
        -- ada.finalization.list_controller (spec)
        -- ada.finalization.list_controller (body)
        -- system.file_control_block (spec)
        -- system.file_io (spec)
        -- system.file_io (body)
        -- ada.text_io (spec)
        -- ada.text_io (body)
        -- hello (body)
        -- END ELABORATION ORDER
     
     end ada_main;
     
     --  The following source file name pragmas allow the generated file
     --  names to be unique for different main programs. They are needed
     --  since the package name will always be Ada_Main.
     
     pragma Source_File_Name (ada_main, Spec_File_Name => "b~hello.ads");
     pragma Source_File_Name (ada_main, Body_File_Name => "b~hello.adb");
     
     --  Generated package body for Ada_Main starts here
     
     package body ada_main is
     
        --  The actual finalization is performed by calling the
        --  library routine in System.Standard_Library.Adafinal
     
        procedure Do_Finalize;
        pragma Import (C, Do_Finalize, "system__standard_library__adafinal");
     
        -------------
        -- adainit --
        -------------
     
        procedure adainit is
     
           --  These booleans are set to True once the associated unit has
           --  been elaborated. It is also used to avoid elaborating the
           --  same unit twice.
     
           E040 : Boolean; pragma Import (Ada, E040, "interfaces__c_streams_E");
           E008 : Boolean; pragma Import (Ada, E008, "ada__exceptions_E");
           E014 : Boolean; pragma Import (Ada, E014, "system__exception_table_E");
           E053 : Boolean; pragma Import (Ada, E053, "ada__io_exceptions_E");
           E017 : Boolean; pragma Import (Ada, E017, "system__exceptions_E");
           E024 : Boolean; pragma Import (Ada, E024, "system__secondary_stack_E");
           E030 : Boolean; pragma Import (Ada, E030, "system__stack_checking_E");
           E028 : Boolean; pragma Import (Ada, E028, "system__soft_links_E");
           E035 : Boolean; pragma Import (Ada, E035, "ada__tags_E");
           E033 : Boolean; pragma Import (Ada, E033, "ada__streams_E");
           E046 : Boolean; pragma Import (Ada, E046, "system__finalization_root_E");
           E048 : Boolean; pragma Import (Ada, E048, "system__finalization_implementation_E");
           E044 : Boolean; pragma Import (Ada, E044, "ada__finalization_E");
           E057 : Boolean; pragma Import (Ada, E057, "ada__finalization__list_controller_E");
           E055 : Boolean; pragma Import (Ada, E055, "system__file_control_block_E");
           E042 : Boolean; pragma Import (Ada, E042, "system__file_io_E");
           E006 : Boolean; pragma Import (Ada, E006, "ada__text_io_E");
     
           --  Set_Globals is a library routine that stores away the
           --  value of the indicated set of global values in global
           --  variables within the library.
     
           procedure Set_Globals
             (Main_Priority            : Integer;
              Time_Slice_Value         : Integer;
              WC_Encoding              : Character;
              Locking_Policy           : Character;
              Queuing_Policy           : Character;
              Task_Dispatching_Policy  : Character;
              Adafinal                 : System.Address;
              Unreserve_All_Interrupts : Integer;
              Exception_Tracebacks     : Integer);
           pragma Import (C, Set_Globals, "__gnat_set_globals");
     
           --  SDP_Table_Build is a library routine used to build the
           --  exception tables. See unit Ada.Exceptions in files
           --  a-except.ads/adb for full details of how zero cost
           --  exception handling works. This procedure, the call to
           --  it, and the two following tables are all omitted if the
           --  build is in longjmp/setjump exception mode.
     
           procedure SDP_Table_Build
             (SDP_Addresses   : System.Address;
              SDP_Count       : Natural;
              Elab_Addresses  : System.Address;
              Elab_Addr_Count : Natural);
           pragma Import (C, SDP_Table_Build, "__gnat_SDP_Table_Build");
     
           --  Table of Unit_Exception_Table addresses. Used for zero
           --  cost exception handling to build the top level table.
     
           ST : aliased constant array (1 .. 23) of System.Address := (
             Hello'UET_Address,
             Ada.Text_Io'UET_Address,
             Ada.Exceptions'UET_Address,
             Gnat.Heap_Sort_A'UET_Address,
             System.Exception_Table'UET_Address,
             System.Machine_State_Operations'UET_Address,
             System.Secondary_Stack'UET_Address,
             System.Parameters'UET_Address,
             System.Soft_Links'UET_Address,
             System.Stack_Checking'UET_Address,
             System.Traceback'UET_Address,
             Ada.Streams'UET_Address,
             Ada.Tags'UET_Address,
             System.String_Ops'UET_Address,
             Interfaces.C_Streams'UET_Address,
             System.File_Io'UET_Address,
             Ada.Finalization'UET_Address,
             System.Finalization_Root'UET_Address,
             System.Finalization_Implementation'UET_Address,
             System.String_Ops_Concat_3'UET_Address,
             System.Stream_Attributes'UET_Address,
             System.File_Control_Block'UET_Address,
             Ada.Finalization.List_Controller'UET_Address);
     
           --  Table of addresses of elaboration routines. Used for
           --  zero cost exception handling to make sure these
           --  addresses are included in the top level procedure
           --  address table.
     
           EA : aliased constant array (1 .. 23) of System.Address := (
             adainit'Code_Address,
             Do_Finalize'Code_Address,
             Ada.Exceptions'Elab_Spec'Address,
             System.Exceptions'Elab_Spec'Address,
             Interfaces.C_Streams'Elab_Spec'Address,
             System.Exception_Table'Elab_Body'Address,
             Ada.Io_Exceptions'Elab_Spec'Address,
             System.Stack_Checking'Elab_Spec'Address,
             System.Soft_Links'Elab_Body'Address,
             System.Secondary_Stack'Elab_Body'Address,
             Ada.Tags'Elab_Spec'Address,
             Ada.Tags'Elab_Body'Address,
             Ada.Streams'Elab_Spec'Address,
             System.Finalization_Root'Elab_Spec'Address,
             Ada.Exceptions'Elab_Body'Address,
             System.Finalization_Implementation'Elab_Spec'Address,
             System.Finalization_Implementation'Elab_Body'Address,
             Ada.Finalization'Elab_Spec'Address,
             Ada.Finalization.List_Controller'Elab_Spec'Address,
             System.File_Control_Block'Elab_Spec'Address,
             System.File_Io'Elab_Body'Address,
             Ada.Text_Io'Elab_Spec'Address,
             Ada.Text_Io'Elab_Body'Address);
     
        --  Start of processing for adainit
     
        begin
     
           --  Call SDP_Table_Build to build the top level procedure
           --  table for zero cost exception handling (omitted in
           --  longjmp/setjump mode).
     
           SDP_Table_Build (ST'Address, 23, EA'Address, 23);
     
           --  Call Set_Globals to record various information for
           --  this partition.  The values are derived by the binder
           --  from information stored in the ali files by the compiler.
     
           Set_Globals
             (Main_Priority            => -1,
              --  Priority of main program, -1 if no pragma Priority used
     
              Time_Slice_Value         => -1,
              --  Time slice from Time_Slice pragma, -1 if none used
     
              WC_Encoding              => 'b',
              --  Wide_Character encoding used, default is brackets
     
              Locking_Policy           => ' ',
              --  Locking_Policy used, default of space means not
              --  specified, otherwise it is the first character of
              --  the policy name.
     
              Queuing_Policy           => ' ',
              --  Queuing_Policy used, default of space means not
              --  specified, otherwise it is the first character of
              --  the policy name.
     
              Task_Dispatching_Policy  => ' ',
              --  Task_Dispatching_Policy used, default of space means
              --  not specified, otherwise first character of the
              --  policy name.
     
              Adafinal                 => System.Null_Address,
              --  Address of Adafinal routine, not used anymore
     
              Unreserve_All_Interrupts => 0,
              --  Set true if pragma Unreserve_All_Interrupts was used
     
              Exception_Tracebacks     => 0);
              --  Indicates if exception tracebacks are enabled
     
           Elab_Final_Code := 1;
     
           --  Now we have the elaboration calls for all units in the partition.
           --  The Elab_Spec and Elab_Body attributes generate references to the
           --  implicit elaboration procedures generated by the compiler for
           --  each unit that requires elaboration.
     
           if not E040 then
              Interfaces.C_Streams'Elab_Spec;
           end if;
           E040 := True;
           if not E008 then
              Ada.Exceptions'Elab_Spec;
           end if;
           if not E014 then
              System.Exception_Table'Elab_Body;
              E014 := True;
           end if;
           if not E053 then
              Ada.Io_Exceptions'Elab_Spec;
              E053 := True;
           end if;
           if not E017 then
              System.Exceptions'Elab_Spec;
              E017 := True;
           end if;
           if not E030 then
              System.Stack_Checking'Elab_Spec;
           end if;
           if not E028 then
              System.Soft_Links'Elab_Body;
              E028 := True;
           end if;
           E030 := True;
           if not E024 then
              System.Secondary_Stack'Elab_Body;
              E024 := True;
           end if;
           if not E035 then
              Ada.Tags'Elab_Spec;
           end if;
           if not E035 then
              Ada.Tags'Elab_Body;
              E035 := True;
           end if;
           if not E033 then
              Ada.Streams'Elab_Spec;
              E033 := True;
           end if;
           if not E046 then
              System.Finalization_Root'Elab_Spec;
           end if;
           E046 := True;
           if not E008 then
              Ada.Exceptions'Elab_Body;
              E008 := True;
           end if;
           if not E048 then
              System.Finalization_Implementation'Elab_Spec;
           end if;
           if not E048 then
              System.Finalization_Implementation'Elab_Body;
              E048 := True;
           end if;
           if not E044 then
              Ada.Finalization'Elab_Spec;
           end if;
           E044 := True;
           if not E057 then
              Ada.Finalization.List_Controller'Elab_Spec;
           end if;
           E057 := True;
           if not E055 then
              System.File_Control_Block'Elab_Spec;
              E055 := True;
           end if;
           if not E042 then
              System.File_Io'Elab_Body;
              E042 := True;
           end if;
           if not E006 then
              Ada.Text_Io'Elab_Spec;
           end if;
           if not E006 then
              Ada.Text_Io'Elab_Body;
              E006 := True;
           end if;
     
           Elab_Final_Code := 0;
        end adainit;
     
        --------------
        -- adafinal --
        --------------
     
        procedure adafinal is
        begin
           Do_Finalize;
        end adafinal;
     
        ----------
        -- main --
        ----------
     
        --  main is actually a function, as in the ANSI C standard,
        --  defined to return the exit status. The three parameters
        --  are the argument count, argument values and environment
        --  pointer.
     
        function main
          (argc : Integer;
           argv : System.Address;
           envp : System.Address)
           return Integer
        is
           --  The initialize routine performs low level system
           --  initialization using a standard library routine which
           --  sets up signal handling and performs any other
           --  required setup. The routine can be found in file
           --  a-init.c.
     
           procedure initialize;
           pragma Import (C, initialize, "__gnat_initialize");
     
           --  The finalize routine performs low level system
           --  finalization using a standard library routine. The
           --  routine is found in file a-final.c and in the standard
           --  distribution is a dummy routine that does nothing, so
           --  really this is a hook for special user finalization.
     
           procedure finalize;
           pragma Import (C, finalize, "__gnat_finalize");
     
           --  We get to the main program of the partition by using
           --  pragma Import because if we try to with the unit and
           --  call it Ada style, then not only do we waste time
           --  recompiling it, but also, we don't really know the right
           --  switches (e.g. identifier character set) to be used
           --  to compile it.
     
           procedure Ada_Main_Program;
           pragma Import (Ada, Ada_Main_Program, "_ada_hello");
     
        --  Start of processing for main
     
        begin
           --  Save global variables
     
           gnat_argc := argc;
           gnat_argv := argv;
           gnat_envp := envp;
     
           --  Call low level system initialization
     
           Initialize;
     
           --  Call our generated Ada initialization routine
     
           adainit;
     
           --  This is the point at which we want the debugger to get
           --  control
     
           Break_Start;
     
           --  Now we call the main program of the partition
     
           Ada_Main_Program;
     
           --  Perform Ada finalization
     
           adafinal;
     
           --  Perform low level system finalization
     
           Finalize;
     
           --  Return the proper exit status
           return (gnat_exit_status);
        end;
     
     --  This section is entirely comments, so it has no effect on the
     --  compilation of the Ada_Main package. It provides the list of
     --  object files and linker options, as well as some standard
     --  libraries needed for the link. The gnatlink utility parses
     --  this b~hello.adb file to read these comment lines to generate
     --  the appropriate command line arguments for the call to the
     --  system linker. The BEGIN/END lines are used for sentinels for
     --  this parsing operation.
     
     --  The exact file names will of course depend on the environment,
     --  host/target and location of files on the host system.
     
     -- BEGIN Object file/option list
        --   ./hello.o
        --   -L./
        --   -L/usr/local/gnat/lib/gcc-lib/i686-pc-linux-gnu/2.8.1/adalib/
        --   /usr/local/gnat/lib/gcc-lib/i686-pc-linux-gnu/2.8.1/adalib/libgnat.a
     -- END Object file/option list
     
     end ada_main;
     
     

The Ada code in the above example is exactly what is generated by the binder. We have added comments to more clearly indicate the function of each part of the generated Ada_Main package.

The code is standard Ada in all respects, and can be processed by any tools that handle Ada. In particular, it is possible to use the debugger in Ada mode to debug the generated Ada_Main package. For example, suppose that for reasons that you do not understand, your program is blowing up during elaboration of the body of Ada.Text_IO. To chase this bug down, you can place a breakpoint on the call:

     Ada.Text_Io'Elab_Body;
     

and trace the elaboration routine for this package to find out where the problem might be (more usually of course you would be debugging elaboration code in your own application).

4.2 Generating the Binder Program in C

In most normal usage, the default mode of gnatbind which is to generate the main package in Ada, as described in the previous section. In particular, this means that any Ada programmer can read and understand the generated main program. It can also be debugged just like any other Ada code provided the -g switch is used for gnatbind and gnatlink.

However for some purposes it may be convenient to generate the main program in C rather than Ada. This may for example be helpful when you are generating a mixed language program with the main program in C. The GNAT compiler itself is an example. The use of the -C switch for both gnatbind and gnatlink will cause the program to be generated in C (and compiled using the gnu C compiler). The following shows the C code generated for the same "Hello World" program:

     
     #ifdef __STDC__
     #define PARAMS(paramlist) paramlist
     #else
     #define PARAMS(paramlist) ()
     #endif
     
     extern void __gnat_set_globals
      PARAMS ((int, int, int, int, int, int,
               void (*) PARAMS ((void)), int, int));
     extern void adafinal PARAMS ((void));
     extern void adainit PARAMS ((void));
     extern void system__standard_library__adafinal PARAMS ((void));
     extern int main PARAMS ((int, char **, char **));
     extern void exit PARAMS ((int));
     extern void __gnat_break_start PARAMS ((void));
     extern void _ada_hello PARAMS ((void));
     extern void __gnat_initialize PARAMS ((void));
     extern void __gnat_finalize PARAMS ((void));
     
     extern void ada__exceptions___elabs PARAMS ((void));
     extern void system__exceptions___elabs PARAMS ((void));
     extern void interfaces__c_streams___elabs PARAMS ((void));
     extern void system__exception_table___elabb PARAMS ((void));
     extern void ada__io_exceptions___elabs PARAMS ((void));
     extern void system__stack_checking___elabs PARAMS ((void));
     extern void system__soft_links___elabb PARAMS ((void));
     extern void system__secondary_stack___elabb PARAMS ((void));
     extern void ada__tags___elabs PARAMS ((void));
     extern void ada__tags___elabb PARAMS ((void));
     extern void ada__streams___elabs PARAMS ((void));
     extern void system__finalization_root___elabs PARAMS ((void));
     extern void ada__exceptions___elabb PARAMS ((void));
     extern void system__finalization_implementation___elabs PARAMS ((void));
     extern void system__finalization_implementation___elabb PARAMS ((void));
     extern void ada__finalization___elabs PARAMS ((void));
     extern void ada__finalization__list_controller___elabs PARAMS ((void));
     extern void system__file_control_block___elabs PARAMS ((void));
     extern void system__file_io___elabb PARAMS ((void));
     extern void ada__text_io___elabs PARAMS ((void));
     extern void ada__text_io___elabb PARAMS ((void));
     
     extern int __gnat_inside_elab_final_code;
     
     extern int gnat_argc;
     extern char **gnat_argv;
     extern char **gnat_envp;
     extern int gnat_exit_status;
     
     char __gnat_version[] = "GNAT Version: 3.15w (20010315)";
     void adafinal () {
        system__standard_library__adafinal ();
     }
     
     void adainit ()
     {
        extern char ada__exceptions_E;
        extern char system__exceptions_E;
        extern char interfaces__c_streams_E;
        extern char system__exception_table_E;
        extern char ada__io_exceptions_E;
        extern char system__secondary_stack_E;
        extern char system__stack_checking_E;
        extern char system__soft_links_E;
        extern char ada__tags_E;
        extern char ada__streams_E;
        extern char system__finalization_root_E;
        extern char system__finalization_implementation_E;
        extern char ada__finalization_E;
        extern char ada__finalization__list_controller_E;
        extern char system__file_control_block_E;
        extern char system__file_io_E;
        extern char ada__text_io_E;
     
        extern void *__gnat_hello__SDP;
        extern void *__gnat_ada__text_io__SDP;
        extern void *__gnat_ada__exceptions__SDP;
        extern void *__gnat_gnat__heap_sort_a__SDP;
        extern void *__gnat_system__exception_table__SDP;
        extern void *__gnat_system__machine_state_operations__SDP;
        extern void *__gnat_system__secondary_stack__SDP;
        extern void *__gnat_system__parameters__SDP;
        extern void *__gnat_system__soft_links__SDP;
        extern void *__gnat_system__stack_checking__SDP;
        extern void *__gnat_system__traceback__SDP;
        extern void *__gnat_ada__streams__SDP;
        extern void *__gnat_ada__tags__SDP;
        extern void *__gnat_system__string_ops__SDP;
        extern void *__gnat_interfaces__c_streams__SDP;
        extern void *__gnat_system__file_io__SDP;
        extern void *__gnat_ada__finalization__SDP;
        extern void *__gnat_system__finalization_root__SDP;
        extern void *__gnat_system__finalization_implementation__SDP;
        extern void *__gnat_system__string_ops_concat_3__SDP;
        extern void *__gnat_system__stream_attributes__SDP;
        extern void *__gnat_system__file_control_block__SDP;
        extern void *__gnat_ada__finalization__list_controller__SDP;
     
        void **st[23] = {
          &__gnat_hello__SDP,
          &__gnat_ada__text_io__SDP,
          &__gnat_ada__exceptions__SDP,
          &__gnat_gnat__heap_sort_a__SDP,
          &__gnat_system__exception_table__SDP,
          &__gnat_system__machine_state_operations__SDP,
          &__gnat_system__secondary_stack__SDP,
          &__gnat_system__parameters__SDP,
          &__gnat_system__soft_links__SDP,
          &__gnat_system__stack_checking__SDP,
          &__gnat_system__traceback__SDP,
          &__gnat_ada__streams__SDP,
          &__gnat_ada__tags__SDP,
          &__gnat_system__string_ops__SDP,
          &__gnat_interfaces__c_streams__SDP,
          &__gnat_system__file_io__SDP,
          &__gnat_ada__finalization__SDP,
          &__gnat_system__finalization_root__SDP,
          &__gnat_system__finalization_implementation__SDP,
          &__gnat_system__string_ops_concat_3__SDP,
          &__gnat_system__stream_attributes__SDP,
          &__gnat_system__file_control_block__SDP,
          &__gnat_ada__finalization__list_controller__SDP};
     
        extern void ada__exceptions___elabs ();
        extern void system__exceptions___elabs ();
        extern void interfaces__c_streams___elabs ();
        extern void system__exception_table___elabb ();
        extern void ada__io_exceptions___elabs ();
        extern void system__stack_checking___elabs ();
        extern void system__soft_links___elabb ();
        extern void system__secondary_stack___elabb ();
        extern void ada__tags___elabs ();
        extern void ada__tags___elabb ();
        extern void ada__streams___elabs ();
        extern void system__finalization_root___elabs ();
        extern void ada__exceptions___elabb ();
        extern void system__finalization_implementation___elabs ();
        extern void system__finalization_implementation___elabb ();
        extern void ada__finalization___elabs ();
        extern void ada__finalization__list_controller___elabs ();
        extern void system__file_control_block___elabs ();
        extern void system__file_io___elabb ();
        extern void ada__text_io___elabs ();
        extern void ada__text_io___elabb ();
     
        void (*ea[23]) () = {
          adainit,
          system__standard_library__adafinal,
          ada__exceptions___elabs,
          system__exceptions___elabs,
          interfaces__c_streams___elabs,
          system__exception_table___elabb,
          ada__io_exceptions___elabs,
          system__stack_checking___elabs,
          system__soft_links___elabb,
          system__secondary_stack___elabb,
          ada__tags___elabs,
          ada__tags___elabb,
          ada__streams___elabs,
          system__finalization_root___elabs,
          ada__exceptions___elabb,
          system__finalization_implementation___elabs,
          system__finalization_implementation___elabb,
          ada__finalization___elabs,
          ada__finalization__list_controller___elabs,
          system__file_control_block___elabs,
          system__file_io___elabb,
          ada__text_io___elabs,
          ada__text_io___elabb};
     
        __gnat_SDP_Table_Build (&st, 23, ea, 23);
        __gnat_set_globals (
           -1,      /* Main_Priority              */
           -1,      /* Time_Slice_Value           */
           'b',     /* WC_Encoding                */
           ' ',     /* Locking_Policy             */
           ' ',     /* Queuing_Policy             */
           ' ',     /* Tasking_Dispatching_Policy */
           0,       /* Finalization routine address, not used anymore */
           0,       /* Unreserve_All_Interrupts */
           0);      /* Exception_Tracebacks */
     
        __gnat_inside_elab_final_code = 1;
     
        if (ada__exceptions_E == 0) {
           ada__exceptions___elabs ();
        }
        if (system__exceptions_E == 0) {
           system__exceptions___elabs ();
           system__exceptions_E++;
        }
        if (interfaces__c_streams_E == 0) {
           interfaces__c_streams___elabs ();
        }
        interfaces__c_streams_E = 1;
        if (system__exception_table_E == 0) {
           system__exception_table___elabb ();
           system__exception_table_E++;
        }
        if (ada__io_exceptions_E == 0) {
           ada__io_exceptions___elabs ();
           ada__io_exceptions_E++;
        }
        if (system__stack_checking_E == 0) {
           system__stack_checking___elabs ();
        }
        if (system__soft_links_E == 0) {
           system__soft_links___elabb ();
           system__soft_links_E++;
        }
        system__stack_checking_E = 1;
        if (system__secondary_stack_E == 0) {
           system__secondary_stack___elabb ();
           system__secondary_stack_E++;
        }
        if (ada__tags_E == 0) {
           ada__tags___elabs ();
        }
        if (ada__tags_E == 0) {
           ada__tags___elabb ();
           ada__tags_E++;
        }
        if (ada__streams_E == 0) {
           ada__streams___elabs ();
           ada__streams_E++;
        }
        if (system__finalization_root_E == 0) {
           system__finalization_root___elabs ();
        }
        system__finalization_root_E = 1;
        if (ada__exceptions_E == 0) {
           ada__exceptions___elabb ();
           ada__exceptions_E++;
        }
        if (system__finalization_implementation_E == 0) {
           system__finalization_implementation___elabs ();
        }
        if (system__finalization_implementation_E == 0) {
           system__finalization_implementation___elabb ();
           system__finalization_implementation_E++;
        }
        if (ada__finalization_E == 0) {
           ada__finalization___elabs ();
        }
        ada__finalization_E = 1;
        if (ada__finalization__list_controller_E == 0) {
           ada__finalization__list_controller___elabs ();
        }
        ada__finalization__list_controller_E = 1;
        if (system__file_control_block_E == 0) {
           system__file_control_block___elabs ();
           system__file_control_block_E++;
        }
        if (system__file_io_E == 0) {
           system__file_io___elabb ();
           system__file_io_E++;
        }
        if (ada__text_io_E == 0) {
           ada__text_io___elabs ();
        }
        if (ada__text_io_E == 0) {
           ada__text_io___elabb ();
           ada__text_io_E++;
        }
     
        __gnat_inside_elab_final_code = 0;
     }
     int main (argc, argv, envp)
         int argc;
         char **argv;
         char **envp;
     {
        gnat_argc = argc;
        gnat_argv = argv;
        gnat_envp = envp;
     
        __gnat_initialize ();
        adainit ();
        __gnat_break_start ();
     
        _ada_hello ();
     
        system__standard_library__adafinal ();
        __gnat_finalize ();
        exit (gnat_exit_status);
     }
     unsigned helloB = 0x7880BEB3;
     unsigned system__standard_libraryB = 0x0D24CBD0;
     unsigned system__standard_libraryS = 0x3283DBEB;
     unsigned adaS = 0x2359F9ED;
     unsigned ada__text_ioB = 0x47C85FC4;
     unsigned ada__text_ioS = 0x496FE45C;
     unsigned ada__exceptionsB = 0x74F50187;
     unsigned ada__exceptionsS = 0x6736945B;
     unsigned gnatS = 0x156A40CF;
     unsigned gnat__heap_sort_aB = 0x033DABE0;
     unsigned gnat__heap_sort_aS = 0x6AB38FEA;
     unsigned systemS = 0x0331C6FE;
     unsigned system__exceptionsS = 0x20C9ECA4;
     unsigned system__exception_tableB = 0x68A22947;
     unsigned system__exception_tableS = 0x394BADD5;
     unsigned gnat__htableB = 0x08258E1B;
     unsigned gnat__htableS = 0x367D5222;
     unsigned system__machine_state_operationsB = 0x4F3B7492;
     unsigned system__machine_state_operationsS = 0x182F5CF4;
     unsigned system__storage_elementsB = 0x2F1EB794;
     unsigned system__storage_elementsS = 0x102C83C7;
     unsigned system__secondary_stackB = 0x1574B6E9;
     unsigned system__secondary_stackS = 0x708E260A;
     unsigned system__parametersB = 0x56D770CD;
     unsigned system__parametersS = 0x237E39BE;
     unsigned system__soft_linksB = 0x08AB6B2C;
     unsigned system__soft_linksS = 0x1E2491F3;
     unsigned system__stack_checkingB = 0x476457A0;
     unsigned system__stack_checkingS = 0x5299FCED;
     unsigned system__tracebackB = 0x2971EBDE;
     unsigned system__tracebackS = 0x2E9C3122;
     unsigned ada__streamsS = 0x7C25DE96;
     unsigned ada__tagsB = 0x39ADFFA2;
     unsigned ada__tagsS = 0x769A0464;
     unsigned system__string_opsB = 0x5EB646AB;
     unsigned system__string_opsS = 0x63CED018;
     unsigned interfacesS = 0x0357E00A;
     unsigned interfaces__c_streamsB = 0x3784FB72;
     unsigned interfaces__c_streamsS = 0x2E723019;
     unsigned system__file_ioB = 0x623358EA;
     unsigned system__file_ioS = 0x31F873E6;
     unsigned ada__finalizationB = 0x6843F68A;
     unsigned ada__finalizationS = 0x63305874;
     unsigned system__finalization_rootB = 0x31E56CE1;
     unsigned system__finalization_rootS = 0x23169EF3;
     unsigned system__finalization_implementationB = 0x6CCBA70E;
     unsigned system__finalization_implementationS = 0x604AA587;
     unsigned system__string_ops_concat_3B = 0x572E3F58;
     unsigned system__string_ops_concat_3S = 0x01F57876;
     unsigned system__stream_attributesB = 0x1D4F93E8;
     unsigned system__stream_attributesS = 0x30B2EC3D;
     unsigned ada__io_exceptionsS = 0x34054F96;
     unsigned system__unsigned_typesS = 0x7B9E7FE3;
     unsigned system__file_control_blockS = 0x2FF876A8;
     unsigned ada__finalization__list_controllerB = 0x5760634A;
     unsigned ada__finalization__list_controllerS = 0x5D851835;
     
     /* BEGIN ELABORATION ORDER
     ada (spec)
     gnat (spec)
     gnat.heap_sort_a (spec)
     gnat.htable (spec)
     gnat.htable (body)
     interfaces (spec)
     system (spec)
     system.parameters (spec)
     system.standard_library (spec)
     ada.exceptions (spec)
     system.exceptions (spec)
     system.parameters (body)
     gnat.heap_sort_a (body)
     interfaces.c_streams (spec)
     interfaces.c_streams (body)
     system.exception_table (spec)
     system.exception_table (body)
     ada.io_exceptions (spec)
     system.storage_elements (spec)
     system.storage_elements (body)
     system.machine_state_operations (spec)
     system.machine_state_operations (body)
     system.secondary_stack (spec)
     system.stack_checking (spec)
     system.soft_links (spec)
     system.soft_links (body)
     system.stack_checking (body)
     system.secondary_stack (body)
     system.standard_library (body)
     system.string_ops (spec)
     system.string_ops (body)
     ada.tags (spec)
     ada.tags (body)
     ada.streams (spec)
     system.finalization_root (spec)
     system.finalization_root (body)
     system.string_ops_concat_3 (spec)
     system.string_ops_concat_3 (body)
     system.traceback (spec)
     system.traceback (body)
     ada.exceptions (body)
     system.unsigned_types (spec)
     system.stream_attributes (spec)
     system.stream_attributes (body)
     system.finalization_implementation (spec)
     system.finalization_implementation (body)
     ada.finalization (spec)
     ada.finalization (body)
     ada.finalization.list_controller (spec)
     ada.finalization.list_controller (body)
     system.file_control_block (spec)
     system.file_io (spec)
     system.file_io (body)
     ada.text_io (spec)
     ada.text_io (body)
     hello (body)
        END ELABORATION ORDER */
     
     /* BEGIN Object file/option list
     ./hello.o
     -L./
     -L/usr/local/gnat/lib/gcc-lib/alpha-dec-osf5.1/2.8.1/adalib/
     /usr/local/gnat/lib/gcc-lib/alpha-dec-osf5.1/2.8.1/adalib/libgnat.a
     -lexc
        END Object file/option list */
     
     

Here again, the C code is exactly what is generated by the binder. The functions of the various parts of this code correspond in an obvious manner with the commented Ada code shown in the example in the previous section.

4.3 Consistency-Checking Modes

As described in the previous section, by default gnatbind checks that object files are consistent with one another and are consistent with any source files it can locate. The following switches control binder access to sources.

-s

Require source files to be present. In this mode, the binder must be able to locate all source files that are referenced, in order to check their consistency. In normal mode, if a source file cannot be located it is simply ignored. If you specify this switch, a missing source file is an error.

-x

Exclude source files. In this mode, the binder only checks that ALI files are consistent with one another. Source files are not accessed. The binder runs faster in this mode, and there is still a guarantee that the resulting program is self-consistent. If a source file has been edited since it was last compiled, and you specify this switch, the binder will not detect that the object file is out of date with respect to the source file. Note that this is the mode that is automatically used by gnatmake because in this case the checking against sources has already been performed by gnatmake in the course of compilation (i.e. before binding).

4.4 Binder Error Message Control

The following switches provide control over the generation of error messages from the binder:

-v

Verbose mode. In the normal mode, brief error messages are generated to stderr. If this switch is present, a header is written to stdout and any error messages are directed to stdout. All that is written to stderr is a brief summary message.

-b

Generate brief error messages to stderr even if verbose mode is specified. This is relevant only when used with the -v switch.

-mn

Limits the number of error messages to n, a decimal integer in the range 1-999. The binder terminates immediately if this limit is reached.

-Mxxx

Renames the generated main program from main to xxx. This is useful in the case of some cross-building environments, where the actual main program is separate from the one generated by gnatbind.

-ws

Suppress all warning messages.

-we

Treat any warning messages as fatal errors.

-t

The binder performs a number of consistency checks including:

• Check that time stamps of a given source unit are consistent
• Check that checksums of a given source unit are consistent
• Check that consistent versions of GNAT were used for compilation
• Check consistency of configuration pragmas as required

Normally failure of such checks, in accordance with the consistency requirements of the Ada Reference Manual, causes error messages to be generated which abort the binder and prevent the output of a binder file and subsequent link to obtain an executable.

The -t switch converts these error messages into warnings, so that binding and linking can continue to completion even in the presence of such errors. The result may be a failed link (due to missing symbols), or a non-functional executable which has undefined semantics. This means that -t should be used only in unusual situations, with extreme care.

4.5 Elaboration Control

The following switches provide additional control over the elaboration order. For full details see See Elaboration Order Handling in GNAT.

-p

Normally the binder attempts to choose an elaboration order that is likely to minimize the likelihood of an elaboration order error resulting in raising a Program_Error exception. This switch reverses the action of the binder, and requests that it deliberately choose an order that is likely to maximize the likelihood of an elaboration error. This is useful in ensuring portability and avoiding dependence on accidental fortuitous elaboration ordering.

Normally it only makes sense to use the -p switch if dynamic elaboration checking is used (-gnatE switch used for compilation). This is because in the default static elaboration mode, all necessary Elaborate_All pragmas are implicitly inserted. These implicit pragmas are still respected by the binder in -p mode, so a safe elaboration order is assured.

4.6 Output Control

The following switches allow additional control over the output generated by the binder.

-A

Generate binder program in Ada (default). The binder program is named b~mainprog.adb by default. This can be changed with -o gnatbind option.

-c

Check only. Do not generate the binder output file. In this mode the binder performs all error checks but does not generate an output file.

-C

Generate binder program in C. The binder program is named b_mainprog.c. This can be changed with -o gnatbind option.

-e

Output complete list of elaboration-order dependencies, showing the reason for each dependency. This output can be rather extensive but may be useful in diagnosing problems with elaboration order. The output is written to stdout.

-h

Output usage information. The output is written to stdout.

-K

Output linker options to stdout. Includes library search paths, contents of pragmas Ident and Linker_Options, and libraries added by gnatbind.

-l

Output chosen elaboration order. The output is written to stdout.

-O

Output full names of all the object files that must be linked to provide the Ada component of the program. The output is written to stdout. This list includes the files explicitly supplied and referenced by the user as well as implicitly referenced run-time unit files. The latter are omitted if the corresponding units reside in shared libraries. The directory names for the run-time units depend on the system configuration.

-o file

Set name of output file to file instead of the normal b~mainprog.adb default. Note that file denote the Ada binder generated body filename. In C mode you would normally give file an extension of .c because it will be a C source program. Note that if this option is used, then linking must be done manually. It is not possible to use gnatlink in this case, since it cannot locate the binder file.

-r

Generate list of pragma Rerstrictions that could be applied to the current unit. This is useful for code audit purposes, and also may be used to improve code generation in some cases.

4.7 Binding with Non-Ada Main Programs

In our description so far we have assumed that the main program is in Ada, and that the task of the binder is to generate a corresponding function main that invokes this Ada main program. GNAT also supports the building of executable programs where the main program is not in Ada, but some of the called routines are written in Ada and compiled using GNAT (see Mixed Language Programming). The following switch is used in this situation:

-n

No main program. The main program is not in Ada.

In this case, most of the functions of the binder are still required, but instead of generating a main program, the binder generates a file containing the following callable routines:

adainit

You must call this routine to initialize the Ada part of the program by calling the necessary elaboration routines. A call to adainit is required before the first call to an Ada subprogram.

Note that it is assumed that the basic execution environment must be setup to be appropriate for Ada execution at the point where the first Ada subprogram is called. In particular, if the Ada code will do any floating-point operations, then the FPU must be setup in an appropriate manner. For the case of the x86, for example, full precision mode is required. The procedure GNAT.Float_Control.Reset may be used to ensure that the FPU is in the right state.

adafinal

You must call this routine to perform any library-level finalization required by the Ada subprograms. A call to adafinal is required after the last call to an Ada subprogram, and before the program terminates.

If the -n switch is given, more than one ALI file may appear on the command line for gnatbind. The normal closure calculation is performed for each of the specified units. Calculating the closure means finding out the set of units involved by tracing with references. The reason it is necessary to be able to specify more than one ALI file is that a given program may invoke two or more quite separate groups of Ada units.

The binder takes the name of its output file from the last specified ALI file, unless overridden by the use of the -o file. The output is an Ada unit in source form that can be compiled with GNAT unless the -C switch is used in which case the output is a C source file, which must be compiled using the C compiler. This compilation occurs automatically as part of the gnatlink processing.

Currently the GNAT run time requires a FPU using 80 bits mode precision. Under targets where this is not the default it is required to call GNAT.Float_Control.Reset before using floating point numbers (this include float computation, float input and output) in the Ada code. A side effect is that this could be the wrong mode for the foreign code where floating point computation could be broken after this call.

4.8 Binding Programs with No Main Subprogram

It is possible to have an Ada program which does not have a main subprogram. This program will call the elaboration routines of all the packages, then the finalization routines.

The following switch is used to bind programs organized in this manner:

-z

Normally the binder checks that the unit name given on the command line corresponds to a suitable main subprogram. When this switch is used, a list of ALI files can be given, and the execution of the program consists of elaboration of these units in an appropriate order.

4.9 Summary of Binder Switches

The following are the switches available with gnatbind:

-aO

Specify directory to be searched for ALI files.

-aI

Specify directory to be searched for source file.

-A

Generate binder program in Ada (default)

-b

Generate brief messages to stderr even if verbose mode set.

-c

Check only, no generation of binder output file.

-C

Generate binder program in C

-e

Output complete list of elaboration-order dependencies.

-E

Store tracebacks in exception occurrences when the target supports it. This is the default with the zero cost exception mechanism. This option is currently supported on the following targets: all x86 ports, Solaris, Windows, HP-UX, AIX, PowerPC VxWorks and Alpha VxWorks. See also the packages GNAT.Traceback and GNAT.Traceback.Symbolic for more information. Note that on x86 ports, you must not use -fomit-frame-pointer gcc option.

-h

Output usage (help) information

-I

Specify directory to be searched for source and ALI files.

-I-

Do not look for sources in the current directory where gnatbind was invoked, and do not look for ALI files in the directory containing the ALI file named in the gnatbind command line.

-l

Output chosen elaboration order.

-Lxxx

Binds the units for library building. In this case the adainit and adafinal procedures (See see Binding with Non-Ada Main Programs) are renamed to xxxinit and xxxfinal. Implies -n. See see GNAT and Libraries for more details.

-Mxyz

Rename generated main program from main to xyz

-mn

Limit number of detected errors to n (1-999).

-n

No main program.

-nostdinc

Do not look for sources in the system default directory.

-nostdlib

Do not look for library files in the system default directory.

--RTS=rts-path

Specifies the default location of the runtime library. Same meaning as the equivalent gnatmake flag (see Switches for gnatmake).

-o file

Name the output file file (default is b~xxx.adb). Note that if this option is used, then linking must be done manually, gnatlink cannot be used.

-O

Output object list.

-p

Pessimistic (worst-case) elaboration order

-s

Require all source files to be present.

-static

Link against a static GNAT run time.

-shared

Link against a shared GNAT run time when available.

-t

Tolerate time stamp and other consistency errors

-Tn

Set the time slice value to n microseconds. A value of zero means no time slicing and also indicates to the tasking run time to match as close as possible to the annex D requirements of the RM.

-v

Verbose mode. Write error messages, header, summary output to stdout.

-wx

Warning mode (x=s/e for suppress/treat as error)

-x

Exclude source files (check object consistency only).

-z

No main subprogram.

You may obtain this listing by running the program gnatbind with no arguments.

4.10 Command-Line Access

The package Ada.Command_Line provides access to the command-line arguments and program name. In order for this interface to operate correctly, the two variables

     

     int gnat_argc;
     char **gnat_argv;
     
     

are declared in one of the GNAT library routines. These variables must be set from the actual argc and argv values passed to the main program. With no n present, gnatbind generates the C main program to automatically set these variables. If the n switch is used, there is no automatic way to set these variables. If they are not set, the procedures in Ada.Command_Line will not be available, and any attempt to use them will raise Constraint_Error. If command line access is required, your main program must set gnat_argc and gnat_argv from the argc and argv values passed to it.

4.11 Search Paths for gnatbind

The binder takes the name of an ALI file as its argument and needs to locate source files as well as other ALI files to verify object consistency.

For source files, it follows exactly the same search rules as gcc (see Search Paths and the Run-Time Library (RTL)). For ALI files the directories searched are:

1. The directory containing the ALI file named in the command line, unless the switch -I- is specified.
2. All directories specified by -I switches on the gnatbind command line, in the order given.
3. Each of the directories listed in the value of the ADA_OBJECTS_PATH environment variable. Construct this value exactly as the PATH environment variable: a list of directory names separated by colons (semicolons when working with the NT version of GNAT).
4. The content of the "ada_object_path" file which is part of the GNAT installation tree and is used to store standard libraries such as the GNAT Run Time Library (RTL) unless the switch -nostdlib is specified. Installing an Ada Library

In the binder the switch -I is used to specify both source and library file paths. Use -aI instead if you want to specify source paths only, and -aO if you want to specify library paths only. This means that for the binder -Idir is equivalent to -aIdir -aOdir. The binder generates the bind file (a C language source file) in the current working directory.

The packages Ada, System, and Interfaces and their children make up the GNAT Run-Time Library, together with the package GNAT and its children, which contain a set of useful additional library functions provided by GNAT. The sources for these units are needed by the compiler and are kept together in one directory. The ALI files and object files generated by compiling the RTL are needed by the binder and the linker and are kept together in one directory, typically different from the directory containing the sources. In a normal installation, you need not specify these directory names when compiling or binding. Either the environment variables or the built-in defaults cause these files to be found.

Besides simplifying access to the RTL, a major use of search paths is in compiling sources from multiple directories. This can make development environments much more flexible.

4.12 Examples of gnatbind Usage

This section contains a number of examples of using the GNAT binding utility gnatbind.

gnatbind hello

The main program Hello (source program in hello.adb) is bound using the standard switch settings. The generated main program is b~hello.adb. This is the normal, default use of the binder.

gnatbind hello -o mainprog.adb

The main program Hello (source program in hello.adb) is bound using the standard switch settings. The generated main program is mainprog.adb with the associated spec in mainprog.ads. Note that you must specify the body here not the spec, in the case where the output is in Ada. Note that if this option is used, then linking must be done manually, since gnatlink will not be able to find the generated file.

gnatbind main -C -o mainprog.c -x

The main program Main (source program in main.adb) is bound, excluding source files from the consistency checking, generating the file mainprog.c.

gnatbind -x main_program -C -o mainprog.c

This command is exactly the same as the previous example. Switches may appear anywhere in the command line, and single letter switches may be combined into a single switch.

gnatbind -n math dbase -C -o ada-control.c

The main program is in a language other than Ada, but calls to subprograms in packages Math and Dbase appear. This call to gnatbind generates the file ada-control.c containing the adainit and adafinal routines to be called before and after accessing the Ada units.

5 Linking Using gnatlink

This chapter discusses gnatlink, a utility program used to link Ada programs and build an executable file. This is a simple program that invokes the Unix linker (via the gcc command) with a correct list of object files and library references. gnatlink automatically determines the list of files and references for the Ada part of a program. It uses the binder file generated by the binder to determine this list.

• Running gnatlink:
• Switches for gnatlink:
• Setting Stack Size from gnatlink:
• Setting Heap Size from gnatlink:

5.1 Running gnatlink

The form of the gnatlink command is

     $ gnatlink [switches] mainprog[.ali] [non-Ada objects]
           [linker options]
     

mainprog.ali references the ALI file of the main program. The .ali extension of this file can be omitted. From this reference, gnatlink locates the corresponding binder file b~mainprog.adb and, using the information in this file along with the list of non-Ada objects and linker options, constructs a Unix linker command file to create the executable.

The arguments following mainprog.ali are passed to the linker uninterpreted. They typically include the names of object files for units written in other languages than Ada and any library references required to resolve references in any of these foreign language units, or in pragma Import statements in any Ada units.

linker options is an optional list of linker specific switches. The default linker called by gnatlink is gcc which in turn calls the appropriate system linker usually called ld. Standard options for the linker such as -lmy_lib or -Ldir can be added as is. For options that are not recognized by gcc as linker options, the gcc switches -Xlinker or -Wl, shall be used. Refer to the GCC documentation for details. Here is an example showing how to generate a linker map assuming that the underlying linker is GNU ld:

     $ gnatlink my_prog -Wl,-Map,MAPFILE
     

Using linker options it is possible to set the program stack and heap size. See see Setting Stack Size from gnatlink and see Setting Heap Size from gnatlink.

gnatlink determines the list of objects required by the Ada program and prepends them to the list of objects passed to the linker. gnatlink also gathers any arguments set by the use of pragma Linker_Options and adds them to the list of arguments presented to the linker.

5.2 Switches for gnatlink

The following switches are available with the gnatlink utility:

-A

The binder has generated code in Ada. This is the default.

-C

If instead of generating a file in Ada, the binder has generated one in C, then the linker needs to know about it. Use this switch to signal to gnatlink that the binder has generated C code rather than Ada code.

-f

On some targets, the command line length is limited, and gnatlink will generate a separate file for the linker if the list of object files is too long. The -f flag forces this file to be generated even if the limit is not exceeded. This is useful in some cases to deal with special situations where the command line length is exceeded.

-g

The option to include debugging information causes the Ada bind file (in other words, b~mainprog.adb) to be compiled with -g. In addition, the binder does not delete the b~mainprog.adb, b~mainprog.o and b~mainprog.ali files. Without -g, the binder removes these files by default. The same procedure apply if a C bind file was generated using -C gnatbind option, in this case the filenames are b_mainprog.c and b_mainprog.o.

-n

Do not compile the file generated by the binder. This may be used when a link is rerun with different options, but there is no need to recompile the binder file.

-v

Causes additional information to be output, including a full list of the included object files. This switch option is most useful when you want to see what set of object files are being used in the link step.

-v -v

Very verbose mode. Requests that the compiler operate in verbose mode when it compiles the binder file, and that the system linker run in verbose mode.

-o exec-name

exec-name specifies an alternate name for the generated executable program. If this switch is omitted, the executable has the same name as the main unit. For example, gnatlink try.ali creates an executable called try.

-b target

Compile your program to run on target, which is the name of a system configuration. You must have a GNAT cross-compiler built if target is not the same as your host system.

-Bdir

Load compiler executables (for example, gnat1, the Ada compiler) from dir instead of the default location. Only use this switch when multiple versions of the GNAT compiler are available. See the gcc manual page for further details. You would normally use the -b or -V switch instead.

--GCC=compiler_name

Program used for compiling the binder file. The default is `gcc'. You need to use quotes around compiler_name if compiler_name contains spaces or other separator characters. As an example --GCC="foo -x -y" will instruct gnatlink to use foo -x -y as your compiler. Note that switch -c is always inserted after your command name. Thus in the above example the compiler command that will be used by gnatlink will be foo -c -x -y. If several --GCC=compiler_name are used, only the last compiler_name is taken into account. However, all the additional switches are also taken into account. Thus, --GCC="foo -x -y" --GCC="bar -z -t" is equivalent to --GCC="bar -x -y -z -t".

--LINK=name

name is the name of the linker to be invoked. This is especially useful in mixed language programs since languages such as c++ require their own linker to be used. When this switch is omitted, the default name for the linker is (gcc). When this switch is used, the specified linker is called instead of (gcc) with exactly the same parameters that would have been passed to (gcc) so if the desired linker requires different parameters it is necessary to use a wrapper script that massages the parameters before invoking the real linker. It may be useful to control the exact invocation by using the verbose switch.

5.3 Setting Stack Size from gnatlink

It is possible to specify the program stack size from gnatlink. Assuming that the underlying linker is GNU ld there is two ways to do so:

• using -Xlinker linker option

            $ gnatlink hello -Xlinker --stack=0x10000,0x1000
            

  This set the stack reserve size to 0x10000 bytes and the stack commit size to 0x1000 bytes.

• using -Wl linker option

            $ gnatlink hello -Wl,--stack=0x1000000
            

  This set the stack reserve size to 0x1000000 bytes. Note that with -Wl option it is not possible to set the stack commit size because the coma is a separator for this option.

5.4 Setting Heap Size from gnatlink

It is possible to specify the program heap size from gnatlink. Assuming that the underlying linker is GNU ld there is two ways to do so:

• using -Xlinker linker option

            $ gnatlink hello -Xlinker --heap=0x10000,0x1000
            

  This set the heap reserve size to 0x10000 bytes and the heap commit size to 0x1000 bytes.

• using -Wl linker option

            $ gnatlink hello -Wl,--heap=0x1000000
            

  This set the heap reserve size to 0x1000000 bytes. Note that with -Wl option it is not possible to set the heap commit size because the coma is a separator for this option.

6 The GNAT Make Program gnatmake

• Running gnatmake:
• Switches for gnatmake:
• Mode Switches for gnatmake:
• Notes on the Command Line:
• How gnatmake Works:
• Examples of gnatmake Usage:

A typical development cycle when working on an Ada program consists of the following steps:

1. Edit some sources to fix bugs.
2. Add enhancements.
3. Compile all sources affected.
4. Rebind and relink.
5. Test.

The third step can be tricky, because not only do the modified files have to be compiled, but any files depending on these files must also be recompiled. The dependency rules in Ada can be quite complex, especially in the presence of overloading, use clauses, generics and inlined subprograms.

gnatmake automatically takes care of the third and fourth steps of this process. It determines which sources need to be compiled, compiles them, and binds and links the resulting object files.

Unlike some other Ada make programs, the dependencies are always accurately recomputed from the new sources. The source based approach of the GNAT compilation model makes this possible. This means that if changes to the source program cause corresponding changes in dependencies, they will always be tracked exactly correctly by gnatmake.

6.1 Running gnatmake

The usual form of the gnatmake command is

     $ gnatmake [switches] file_name [file_names] [mode_switches]
     

The only required argument is one file_name, which specifies a compilation unit that is a main program. Several file_names can be specified: this will result in several executables being built. If switches are present, they can be placed before the first file_name, between file_names or after the last file_name. If mode_switches are present, they must always be placed after the last file_name and all switches.

If you are using standard file extensions (.adb and .ads), then the extension may be omitted from the file_name arguments. However, if you are using non-standard extensions, then it is required that the extension be given. A relative or absolute directory path can be specified in a file_name, in which case, the input source file will be searched for in the specified directory only. Otherwise, the input source file will first be searched in the directory where gnatmake was invoked and if it is not found, it will be search on the source path of the compiler as described in Search Paths and the Run-Time Library (RTL).

When several file_names are specified, if an executable needs to be rebuilt and relinked, all subsequent executables will be rebuilt and relinked, even if this would not be absolutely necessary.

All gnatmake output (except when you specify -M) is to stderr. The output produced by the -M switch is send to stdout.

6.2 Switches for gnatmake

You may specify any of the following switches to gnatmake:

--GCC=compiler_name

Program used for compiling. The default is `gcc'. You need to use quotes around compiler_name if compiler_name contains spaces or other separator characters. As an example --GCC="foo -x -y" will instruct gnatmake to use foo -x -y as your compiler. Note that switch -c is always inserted after your command name. Thus in the above example the compiler command that will be used by gnatmake will be foo -c -x -y. If several --GCC=compiler_name are used, only the last compiler_name is taken into account. However, all the additional switches are also taken into account. Thus, --GCC="foo -x -y" --GCC="bar -z -t" is equivalent to --GCC="bar -x -y -z -t".

--GNATBIND=binder_name

Program used for binding. The default is `gnatbind'. You need to use quotes around binder_name if binder_name contains spaces or other separator characters. As an example --GNATBIND="bar -x -y" will instruct gnatmake to use bar -x -y as your binder. Binder switches that are normally appended by gnatmake to `gnatbind' are now appended to the end of bar -x -y.

--GNATLINK=linker_name

Program used for linking. The default is `gnatlink'. You need to use quotes around linker_name if linker_name contains spaces or other separator characters. As an example --GNATLINK="lan -x -y" will instruct gnatmake to use lan -x -y as your linker. Linker switches that are normally appended by gnatmake to `gnatlink' are now appended to the end of lan -x -y.

-a

Consider all files in the make process, even the GNAT internal system files (for example, the predefined Ada library files), as well as any locked files. Locked files are files whose ALI file is write-protected. By default, gnatmake does not check these files, because the assumption is that the GNAT internal files are properly up to date, and also that any write protected ALI files have been properly installed. Note that if there is an installation problem, such that one of these files is not up to date, it will be properly caught by the binder. You may have to specify this switch if you are working on GNAT itself. -a is also useful in conjunction with -f if you need to recompile an entire application, including run-time files, using special configuration pragma settings, such as a non-standard Float_Representation pragma. By default gnatmake -a compiles all GNAT internal files with gcc -c -gnatpg rather than gcc -c.

-b

Bind only. Can be combined with -c to do compilation and binding, but no link. Can be combined with -l to do binding and linking. When not combined with -c all the units in the closure of the main program must have been previously compiled and must be up to date. The root unit specified by file_name may be given without extension, with the source extension or, if no GNAT Project File is specified, with the ALI file extension.

-c

Compile only. Do not perform binding, except when -b is also specified. Do not perform linking, except if both -b and -l are also specified. If the root unit specified by file_name is not a main unit, this is the default. Otherwise gnatmake will attempt binding and linking unless all objects are up to date and the executable is more recent than the objects.

-C

Use a mapping file. A mapping file is a way to communicate to the compiler two mappings: from unit names to file names (without any directory information) and from file names to path names (with full directory information). These mappings are used by the compiler to short-circuit the path search. When gnatmake is invoked with this switch, it will create a mapping file, initially populated by the project manager, if -P is used, otherwise initially empty. Each invocation of the compiler will add the newly accessed sources to the mapping file. This will improve the source search during the next invocation of the compiler.

-f

Force recompilations. Recompile all sources, even though some object files may be up to date, but don't recompile predefined or GNAT internal files or locked files (files with a write-protected ALI file), unless the -a switch is also specified.

-i

In normal mode, gnatmake compiles all object files and ALI files into the current directory. If the -i switch is used, then instead object files and ALI files that already exist are overwritten in place. This means that once a large project is organized into separate directories in the desired manner, then gnatmake will automatically maintain and update this organization. If no ALI files are found on the Ada object path (Search Paths and the Run-Time Library (RTL)), the new object and ALI files are created in the directory containing the source being compiled. If another organization is desired, where objects and sources are kept in different directories, a useful technique is to create dummy ALI files in the desired directories. When detecting such a dummy file, gnatmake will be forced to recompile the corresponding source file, and it will be put the resulting object and ALI files in the directory where it found the dummy file.

-jn

Use n processes to carry out the (re)compilations. On a multiprocessor machine compilations will occur in parallel. In the event of compilation errors, messages from various compilations might get interspersed (but gnatmake will give you the full ordered list of failing compiles at the end). If this is problematic, rerun the make process with n set to 1 to get a clean list of messages.

-k

Keep going. Continue as much as possible after a compilation error. To ease the programmer's task in case of compilation errors, the list of sources for which the compile fails is given when gnatmake terminates.

If gnatmake is invoked with several file_names and with this switch, if there are compilation errors when building an executable, gnatmake will not attempt to build the following executables.

-l

Link only. Can be combined with -b to binding and linking. Linking will not be performed if combined with -c but not with -b. When not combined with -b all the units in the closure of the main program must have been previously compiled and must be up to date, and the main program need to have been bound. The root unit specified by file_name may be given without extension, with the source extension or, if no GNAT Project File is specified, with the ALI file extension.

-m

Specifies that the minimum necessary amount of recompilations be performed. In this mode gnatmake ignores time stamp differences when the only modifications to a source file consist in adding/removing comments, empty lines, spaces or tabs. This means that if you have changed the comments in a source file or have simply reformatted it, using this switch will tell gnatmake not to recompile files that depend on it (provided other sources on which these files depend have undergone no semantic modifications). Note that the debugging information may be out of date with respect to the sources if the -m switch causes a compilation to be switched, so the use of this switch represents a trade-off between compilation time and accurate debugging information.

-M

Check if all objects are up to date. If they are, output the object dependences to stdout in a form that can be directly exploited in a Makefile. By default, each source file is prefixed with its (relative or absolute) directory name. This name is whatever you specified in the various -aI and -I switches. If you use gnatmake -M -q (see below), only the source file names, without relative paths, are output. If you just specify the -M switch, dependencies of the GNAT internal system files are omitted. This is typically what you want. If you also specify the -a switch, dependencies of the GNAT internal files are also listed. Note that dependencies of the objects in external Ada libraries (see switch -aLdir in the following list) are never reported.

-n

Don't compile, bind, or link. Checks if all objects are up to date. If they are not, the full name of the first file that needs to be recompiled is printed. Repeated use of this option, followed by compiling the indicated source file, will eventually result in recompiling all required units.

-o exec_name

Output executable name. The name of the final executable program will be exec_name. If the -o switch is omitted the default name for the executable will be the name of the input file in appropriate form for an executable file on the host system.

This switch cannot be used when invoking gnatmake with several file_names.

-q

Quiet. When this flag is not set, the commands carried out by gnatmake are displayed.

-s

Recompile if compiler switches have changed since last compilation. All compiler switches but -I and -o are taken into account in the following way: orders between different "first letter" switches are ignored, but orders between same switches are taken into account. For example, -O -O2 is different than -O2 -O, but -g -O is equivalent to -O -g.

-u

Unique. Recompile at most the main file. It implies -c. Combined with -f, it is equivalent to calling the compiler directly.

-v

Verbose. Displays the reason for all recompilations gnatmake decides are necessary.

-z

No main subprogram. Bind and link the program even if the unit name given on the command line is a package name. The resulting executable will execute the elaboration routines of the package and its closure, then the finalization routines.

gcc switches

The switch -g or any uppercase switch (other than -A, -L or -S) or any switch that is more than one character is passed to gcc (e.g. -O, -gnato, etc.)

Source and library search path switches:

-aIdir

When looking for source files also look in directory dir. The order in which source files search is undertaken is described in Search Paths and the Run-Time Library (RTL).

-aLdir

Consider dir as being an externally provided Ada library. Instructs gnatmake to skip compilation units whose .ali files have been located in directory dir. This allows you to have missing bodies for the units in dir and to ignore out of date bodies for the same units. You still need to specify the location of the specs for these units by using the switches -aIdir or -Idir. Note: this switch is provided for compatibility with previous versions of gnatmake. The easier method of causing standard libraries to be excluded from consideration is to write-protect the corresponding ALI files.

-aOdir

When searching for library and object files, look in directory dir. The order in which library files are searched is described in Search Paths for gnatbind.

-Adir

Equivalent to -aLdir -aIdir.

-Idir

Equivalent to -aOdir -aIdir.

-I-

Do not look for source files in the directory containing the source file named in the command line. Do not look for ALI or object files in the directory where gnatmake was invoked.

-Ldir

Add directory dir to the list of directories in which the linker will search for libraries. This is equivalent to -largs -Ldir.

-nostdinc

Do not look for source files in the system default directory.

-nostdlib

Do not look for library files in the system default directory.

--RTS=rts-path

Specifies the default location of the runtime library. We look for the runtime in the following directories, and stop as soon as a valid runtime is found ("adainclude" or "ada_source_path", and "adalib" or "ada_object_path" present):

• <current directory>/$rts_path
• <default-search-dir>/$rts_path
• <default-search-dir>/rts-$rts_path

The selected path is handled like a normal RTS path.

6.3 Mode Switches for gnatmake

The mode switches (referred to as mode_switches) allow the inclusion of switches that are to be passed to the compiler itself, the binder or the linker. The effect of a mode switch is to cause all subsequent switches up to the end of the switch list, or up to the next mode switch, to be interpreted as switches to be passed on to the designated component of GNAT.

-cargs switches

Compiler switches. Here switches is a list of switches that are valid switches for gcc. They will be passed on to all compile steps performed by gnatmake.

-bargs switches

Binder switches. Here switches is a list of switches that are valid switches for gcc. They will be passed on to all bind steps performed by gnatmake.

-largs switches

Linker switches. Here switches is a list of switches that are valid switches for gcc. They will be passed on to all link steps performed by gnatmake.

6.4 Notes on the Command Line

This section contains some additional useful notes on the operation of the gnatmake command.

• If gnatmake finds no ALI files, it recompiles the main program and all other units required by the main program. This means that gnatmake can be used for the initial compile, as well as during subsequent steps of the development cycle.
• If you enter gnatmake file.adb, where file.adb is a subunit or body of a generic unit, gnatmake recompiles file.adb (because it finds no ALI) and stops, issuing a warning.
• In gnatmake the switch -I is used to specify both source and library file paths. Use -aI instead if you just want to specify source paths only and -aO if you want to specify library paths only.
• gnatmake examines both an ALI file and its corresponding object file for consistency. If an ALI is more recent than its corresponding object, or if the object file is missing, the corresponding source will be recompiled. Note that gnatmake expects an ALI and the corresponding object file to be in the same directory.
• gnatmake will ignore any files whose ALI file is write-protected. This may conveniently be used to exclude standard libraries from consideration and in particular it means that the use of the -f switch will not recompile these files unless -a is also specified.
• gnatmake has been designed to make the use of Ada libraries particularly convenient. Assume you have an Ada library organized as follows: obj-dir contains the objects and ALI files for of your Ada compilation units, whereas include-dir contains the specs of these units, but no bodies. Then to compile a unit stored in main.adb, which uses this Ada library you would just type

            $ gnatmake -aIinclude-dir  -aLobj-dir  main
            

• Using gnatmake along with the -m (minimal recompilation) switch provides a mechanism for avoiding unnecessary rcompilations. Using this switch, you can update the comments/format of your source files without having to recompile everything. Note, however, that adding or deleting lines in a source files may render its debugging info obsolete. If the file in question is a spec, the impact is rather limited, as that debugging info will only be useful during the elaboration phase of your program. For bodies the impact can be more significant. In all events, your debugger will warn you if a source file is more recent than the corresponding object, and alert you to the fact that the debugging information may be out of date.

6.5 How gnatmake Works

Generally gnatmake automatically performs all necessary recompilations and you don't need to worry about how it works. However, it may be useful to have some basic understanding of the gnatmake approach and in particular to understand how it uses the results of previous compilations without incorrectly depending on them.

First a definition: an object file is considered up to date if the corresponding ALI file exists and its time stamp predates that of the object file and if all the source files listed in the dependency section of this ALI file have time stamps matching those in the ALI file. This means that neither the source file itself nor any files that it depends on have been modified, and hence there is no need to recompile this file.

gnatmake works by first checking if the specified main unit is up to date. If so, no compilations are required for the main unit. If not, gnatmake compiles the main program to build a new ALI file that reflects the latest sources. Then the ALI file of the main unit is examined to find all the source files on which the main program depends, and gnatmake recursively applies the above procedure on all these files.

This process ensures that gnatmake only trusts the dependencies in an existing ALI file if they are known to be correct. Otherwise it always recompiles to determine a new, guaranteed accurate set of dependencies. As a result the program is compiled "upside down" from what may be more familiar as the required order of compilation in some other Ada systems. In particular, clients are compiled before the units on which they depend. The ability of GNAT to compile in any order is critical in allowing an order of compilation to be chosen that guarantees that gnatmake will recompute a correct set of new dependencies if necessary.

When invoking gnatmake with several file_names, if a unit is imported by several of the executables, it will be recompiled at most once.

6.6 Examples of gnatmake Usage

gnatmake hello.adb

Compile all files necessary to bind and link the main program hello.adb (containing unit Hello) and bind and link the resulting object files to generate an executable file hello.

gnatmake main1 main2 main3

Compile all files necessary to bind and link the main programs main1.adb (containing unit Main1), main2.adb (containing unit Main2) and main3.adb (containing unit Main3) and bind and link the resulting object files to generate three executable files main1, main2 and main3.

gnatmake -q Main_Unit -cargs -O2 -bargs -l

Compile all files necessary to bind and link the main program unit Main_Unit (from file main_unit.adb). All compilations will be done with optimization level 2 and the order of elaboration will be listed by the binder. gnatmake will operate in quiet mode, not displaying commands it is executing.

7 Renaming Files Using gnatchop

This chapter discusses how to handle files with multiple units by using the gnatchop utility. This utility is also useful in renaming files to meet the standard GNAT default file naming conventions.

• Handling Files with Multiple Units:
• Operating gnatchop in Compilation Mode:
• Command Line for gnatchop:
• Switches for gnatchop:
• Examples of gnatchop Usage:

7.1 Handling Files with Multiple Units

The basic compilation model of GNAT requires that a file submitted to the compiler have only one unit and there be a strict correspondence between the file name and the unit name.

The gnatchop utility allows both of these rules to be relaxed, allowing GNAT to process files which contain multiple compilation units and files with arbitrary file names. gnatchop reads the specified file and generates one or more output files, containing one unit per file. The unit and the file name correspond, as required by GNAT.

If you want to permanently restructure a set of "foreign" files so that they match the GNAT rules, and do the remaining development using the GNAT structure, you can simply use gnatchop once, generate the new set of files and work with them from that point on.

Alternatively, if you want to keep your files in the "foreign" format, perhaps to maintain compatibility with some other Ada compilation system, you can set up a procedure where you use gnatchop each time you compile, regarding the source files that it writes as temporary files that you throw away.

7.2 Operating gnatchop in Compilation Mode

The basic function of gnatchop is to take a file with multiple units and split it into separate files. The boundary between files is reasonably clear, except for the issue of comments and pragmas. In default mode, the rule is that any pragmas between units belong to the previous unit, except that configuration pragmas always belong to the following unit. Any comments belong to the following unit. These rules almost always result in the right choice of the split point without needing to mark it explicitly and most users will find this default to be what they want. In this default mode it is incorrect to submit a file containing only configuration pragmas, or one that ends in configuration pragmas, to gnatchop.

However, using a special option to activate "compilation mode", gnatchop can perform another function, which is to provide exactly the semantics required by the RM for handling of configuration pragmas in a compilation. In the absence of configuration pragmas (at the main file level), this option has no effect, but it causes such configuration pragmas to be handled in a quite different manner.

First, in compilation mode, if gnatchop is given a file that consists of only configuration pragmas, then this file is appended to the gnat.adc file in the current directory. This behavior provides the required behavior described in the RM for the actions to be taken on submitting such a file to the compiler, namely that these pragmas should apply to all subsequent compilations in the same compilation environment. Using GNAT, the current directory, possibly containing a gnat.adc file is the representation of a compilation environment. For more information on the gnat.adc file, see the section on handling of configuration pragmas see Handling of Configuration Pragmas.

Second, in compilation mode, if gnatchop is given a file that starts with configuration pragmas, and contains one or more units, then these configuration pragmas are prepended to each of the chopped files. This behavior provides the required behavior described in the RM for the actions to be taken on compiling such a file, namely that the pragmas apply to all units in the compilation, but not to subsequently compiled units.

Finally, if configuration pragmas appear between units, they are appended to the previous unit. This results in the previous unit being illegal, since the compiler does not accept configuration pragmas that follow a unit. This provides the required RM behavior that forbids configuration pragmas other than those preceding the first compilation unit of a compilation.

For most purposes, gnatchop will be used in default mode. The compilation mode described above is used only if you need exactly accurate behavior with respect to compilations, and you have files that contain multiple units and configuration pragmas. In this circumstance the use of gnatchop with the compilation mode switch provides the required behavior, and is for example the mode in which GNAT processes the ACVC tests.

7.3 Command Line for gnatchop

The gnatchop command has the form:

     $ gnatchop switches file name [file name file name ...]
           [directory]
     

The only required argument is the file name of the file to be chopped. There are no restrictions on the form of this file name. The file itself contains one or more Ada units, in normal GNAT format, concatenated together. As shown, more than one file may be presented to be chopped.

When run in default mode, gnatchop generates one output file in the current directory for each unit in each of the files.

directory, if specified, gives the name of the directory to which the output files will be written. If it is not specified, all files are written to the current directory.

For example, given a file called hellofiles containing

     

     procedure hello;
     
     with Text_IO; use Text_IO;
     procedure hello is
     begin
        Put_Line ("Hello");
     end hello;
     
     

the command

     $ gnatchop hellofiles
     

generates two files in the current directory, one called hello.ads containing the single line that is the procedure spec, and the other called hello.adb containing the remaining text. The original file is not affected. The generated files can be compiled in the normal manner.

7.4 Switches for gnatchop

gnatchop recognizes the following switches:

-c

Causes gnatchop to operate in compilation mode, in which configuration pragmas are handled according to strict RM rules. See previous section for a full description of this mode.

-gnatxxx

This passes the given -gnatxxx switch to gnat which is used to parse the given file. Not all xxx options make sense, but for example, the use of -gnati2 allows gnatchop to process a source file that uses Latin-2 coding for identifiers.

-h

Causes gnatchop to generate a brief help summary to the standard output file showing usage information.

-kmm

Limit generated file names to the specified number mm of characters. This is useful if the resulting set of files is required to be interoperable with systems which limit the length of file names. No space is allowed between the -k and the numeric value. The numeric value may be omitted in which case a default of -k8, suitable for use with DOS-like file systems, is used. If no -k switch is present then there is no limit on the length of file names.

-p

Causes the file modification time stamp of the input file to be preserved and used for the time stamp of the output file(s). This may be useful for preserving coherency of time stamps in an enviroment where gnatchop is used as part of a standard build process.

-q

Causes output of informational messages indicating the set of generated files to be suppressed. Warnings and error messages are unaffected.

-r

Generate Source_Reference pragmas. Use this switch if the output files are regarded as temporary and development is to be done in terms of the original unchopped file. This switch causes Source_Reference pragmas to be inserted into each of the generated files to refers back to the original file name and line number. The result is that all error messages refer back to the original unchopped file. In addition, the debugging information placed into the object file (when the -g switch of gcc or gnatmake is specified) also refers back to this original file so that tools like profilers and debuggers will give information in terms of the original unchopped file.

If the original file to be chopped itself contains a Source_Reference pragma referencing a third file, then gnatchop respects this pragma, and the generated Source_Reference pragmas in the chopped file refer to the original file, with appropriate line numbers. This is particularly useful when gnatchop is used in conjunction with gnatprep to compile files that contain preprocessing statements and multiple units.

-v

Causes gnatchop to operate in verbose mode. The version number and copyright notice are output, as well as exact copies of the gnat1 commands spawned to obtain the chop control information.

-w

Overwrite existing file names. Normally gnatchop regards it as a fatal error if there is already a file with the same name as a file it would otherwise output, in other words if the files to be chopped contain duplicated units. This switch bypasses this check, and causes all but the last instance of such duplicated units to be skipped.

--GCC=xxxx

Specify the path of the GNAT parser to be used. When this switch is used, no attempt is made to add the prefix to the GNAT parser executable.

7.5 Examples of gnatchop Usage

gnatchop -w hello_s.ada ichbiah/files

Chops the source file hello_s.ada. The output files will be placed in the directory ichbiah/files, overwriting any files with matching names in that directory (no files in the current directory are modified).

gnatchop archive

Chops the source file archive into the current directory. One useful application of gnatchop is in sending sets of sources around, for example in email messages. The required sources are simply concatenated (for example, using a Unix cat command), and then gnatchop is used at the other end to reconstitute the original file names.

gnatchop file1 file2 file3 direc

Chops all units in files file1, file2, file3, placing the resulting files in the directory direc. Note that if any units occur more than once anywhere within this set of files, an error message is generated, and no files are written. To override this check, use the -w switch, in which case the last occurrence in the last file will be the one that is output, and earlier duplicate occurrences for a given unit will be skipped.

8 Configuration Pragmas

In Ada 95, configuration pragmas include those pragmas described as such in the Ada 95 Reference Manual, as well as implementation-dependent pragmas that are configuration pragmas. See the individual descriptions of pragmas in the GNAT Reference Manual for details on these additional GNAT-specific configuration pragmas. Most notably, the pragma Source_File_Name, which allows specifying non-default names for source files, is a configuration pragma. The following is a complete list of configuration pragmas recognized by GNAT:

        Ada_83
        Ada_95
        C_Pass_By_Copy
        Component_Alignment
        Discard_Names
        Elaboration_Checks
        Eliminate
        Extend_System
        Extensions_Allowed
        External_Name_Casing
        Float_Representation
        Initialize_Scalars
        License
        Locking_Policy
        Long_Float
        No_Run_Time
        Normalize_Scalars
        Polling
        Propagate_Exceptions
        Queuing_Policy
        Ravenscar
        Restricted_Run_Time
        Restrictions
        Reviewable
        Source_File_Name
        Style_Checks
        Suppress
        Task_Dispatching_Policy
        Unsuppress
        Use_VADS_Size
        Warnings
        Validity_Checks
     

• Handling of Configuration Pragmas:
• The Configuration Pragmas Files:

8.1 Handling of Configuration Pragmas

Configuration pragmas may either appear at the start of a compilation unit, in which case they apply only to that unit, or they may apply to all compilations performed in a given compilation environment.

GNAT also provides the gnatchop utility to provide an automatic way to handle configuration pragmas following the semantics for compilations (that is, files with multiple units), described in the RM. See section see Operating gnatchop in Compilation Mode for details. However, for most purposes, it will be more convenient to edit the gnat.adc file that contains configuration pragmas directly, as described in the following section.

8.2 The Configuration Pragmas Files

In GNAT a compilation environment is defined by the current directory at the time that a compile command is given. This current directory is searched for a file whose name is gnat.adc. If this file is present, it is expected to contain one or more configuration pragmas that will be applied to the current compilation. However, if the switch -gnatA is used, gnat.adc is not considered.

Configuration pragmas may be entered into the gnat.adc file either by running gnatchop on a source file that consists only of configuration pragmas, or more conveniently by direct editing of the gnat.adc file, which is a standard format source file.

In addition to gnat.adc, one additional file containing configuration pragmas may be applied to the current compilation using the switch -gnatecpath. path must designate an existing file that contains only configuration pragmas. These configuration pragmas are in addition to those found in gnat.adc (provided gnat.adc is present and switch -gnatA is not used).

It is allowed to specify several switches -gnatec, however only the last one on the command line will be taken into account.

9 Handling Arbitrary File Naming Conventions Using gnatname

• Arbitrary File Naming Conventions:
• Running gnatname:
• Switches for gnatname:
• Examples of gnatname Usage:

9.1 Arbitrary File Naming Conventions

The GNAT compiler must be able to know the source file name of a compilation unit. When using the standard GNAT default file naming conventions (.ads for specs, .adb for bodies), the GNAT compiler does not need additional information.

When the source file names do not follow the standard GNAT default file naming conventions, the GNAT compiler must be given additional information through a configuration pragmas file (see Configuration Pragmas) or a project file. When the non standard file naming conventions are well-defined, a small number of pragmas Source_File_Name specifying a naming pattern (see Alternative File Naming Schemes) may be sufficient. However, if the file naming conventions are irregular or arbitrary, a number of pragma Source_File_Name for individual compilation units must be defined. To help maintain the correspondence between compilation unit names and source file names within the compiler, GNAT provides a tool gnatname to generate the required pragmas for a set of files.

9.2 Running gnatname

The usual form of the gnatname command is

     $ gnatname [switches] naming_pattern [naming_patterns]
     

All of the arguments are optional. If invoked without any argument, gnatname will display its usage.

When used with at least one naming pattern, gnatname will attempt to find all the compilation units in files that follow at least one of the naming patterns. To find these compilation units, gnatname will use the GNAT compiler in syntax-check-only mode on all regular files.

One or several Naming Patterns may be given as arguments to gnatname. Each Naming Pattern is enclosed between double quotes. A Naming Pattern is a regular expression similar to the wildcard patterns used in file names by the Unix shells or the DOS prompt.

Examples of Naming Patterns are

        "*.[12].ada"
        "*.ad[sb]*"
        "body_*"    "spec_*"
     

For a more complete description of the syntax of Naming Patterns, see the second kind of regular expressions described in g-regexp.ads (the "Glob" regular expressions).

When invoked with no switches, gnatname will create a configuration pragmas file gnat.adc in the current working directory, with pragmas Source_File_Name for each file that contains a valid Ada unit.

9.3 Switches for gnatname

Switches for gnatname must precede any specified Naming Pattern.

You may specify any of the following switches to gnatname:

-cfile

Create a configuration pragmas file file (instead of the default gnat.adc). There may be zero, one or more space between -c and file. file may include directory information. file must be writeable. There may be only one switch -c. When a switch -c is specified, no switch -P may be specified (see below).

-ddir

Look for source files in directory dir. There may be zero, one or more spaces between -d and dir. When a switch -d is specified, the current working directory will not be searched for source files, unless it is explictly specified with a -d or -D switch. Several switches -d may be specified. If dir is a relative path, it is relative to the directory of the configuration pragmas file specified with switch -c, or to the directory of the project file specified with switch -P or, if neither switch -c nor switch -P are specified, it is relative to the current working directory. The directory specified with switch -c must exist and be readable.

-Dfile

Look for source files in all directories listed in text file file. There may be zero, one or more spaces between -d and dir. file must be an existing, readable text file. Each non empty line in file must be a directory. Specifying switch -D is equivalent to specifying as many switches -d as there are non empty lines in file.

-h

Output usage (help) information. The output is written to stdout.

-Pproj

Create or update project file proj. There may be zero, one or more space between -P and proj. proj may include directory information. proj must be writeable. There may be only one switch -P. When a switch -P is specified, no switch -c may be specified.

-v

Verbose mode. Output detailed explanation of behavior to stdout. This includes name of the file written, the name of the directories to search and, for each file in those directories whose name matches at least one of the Naming Patterns, an indication of whether the file contains a unit, and if so the name of the unit.

-v -v

Very Verbose mode. In addition to the output produced in verbose mode, for each file in the searched directories whose name matches none of the Naming Patterns, an indication is given that there is no match.

-xpattern

Excluded patterns. Using this switch, it is possible to exclude some files that would match the name patterns. For example, "gnatname -x "*_nt.ada" "*.ada" will look for Ada units in all files with the .ada extension, except those whose names end with _nt.ada.

9.4 Examples of gnatname Usage

     $ gnatname -c /home/me/names.adc -d sources "[a-z]*.ada*"
     

In this example, the directory /home/me must already exist and be writeable. In addition, the directory /home/me/sources (specified by -d sources) must exist and be readable. Note the optional spaces after -c and -d.

     $ gnatname -P/home/me/proj -x "*_nt_body.ada" -dsources -dsources/plus -Dcommon_dirs.txt "body_*" "spec_*"
     

Note that several switches -d may be used, even in conjunction with one or several switches -D. Several Naming Patterns and one excluded pattern are used in this example.

10 GNAT Project Manager

• Introduction:
• Examples of Project Files:
• Project File Syntax:
• Objects and Sources in Project Files:
• Importing Projects:
• Project Extension:
• External References in Project Files:
• Packages in Project Files:
• Variables from Imported Projects:
• Naming Schemes:
• Library Projects:
• Switches Related to Project Files:
• Tools Supporting Project Files:
• An Extended Example:
• Project File Complete Syntax:

10.1 Introduction

This chapter describes GNAT's Project Manager, a facility that lets you configure various properties for a collection of source files. In particular, you can specify:

• The directory or set of directories containing the source files, and/or the names of the specific source files themselves
• The directory in which the compiler's output (ALI files, object files, tree files) will be placed
• The directory in which the executable programs will be placed
• Switch settings for any of the project-enabled tools (gnatmake, compiler, binder, linker, gnatls, gnatxref, gnatfind); you can apply these settings either globally or to individual units
• The source files containing the main subprogram(s) to be built
• The source programming language(s) (currently Ada and/or C)
• Source file naming conventions; you can specify these either globally or for individual units

• Project Files:

10.1.1 Project Files

A project is a specific set of values for these properties. You can define a project's settings in a project file, a text file with an Ada-like syntax; a property value is either a string or a list of strings. Properties that are not explicitly set receive default values. A project file may interrogate the values of external variables (user-defined command-line switches or environment variables), and it may specify property settings conditionally, based on the value of such variables.

In simple cases, a project's source files depend only on other source files in the same project, or on the predefined libraries. ("Dependence" is in the technical sense; for example, one Ada unit "with"ing another.) However, the Project Manager also allows much more sophisticated arrangements, with the source files in one project depending on source files in other projects:

• One project can import other projects containing needed source files.
• You can organize GNAT projects in a hierarchy: a child project can extend a parent project, inheriting the parent's source files and optionally overriding any of them with alternative versions

More generally, the Project Manager lets you structure large development efforts into hierarchical subsystems, with build decisions deferred to the subsystem level and thus different compilation environments (switch settings) used for different subsystems.

The Project Manager is invoked through the -Pprojectfile switch to gnatmake or to the gnat front driver. If you want to define (on the command line) an external variable that is queried by the project file, additionally use the -Xvbl=value switch. The Project Manager parses and interprets the project file, and drives the invoked tool based on the project settings.

The Project Manager supports a wide range of development strategies, for systems of all sizes. Some typical practices that are easily handled:

• Using a common set of source files, but generating object files in different directories via different switch settings
• Using a mostly-shared set of source files, but with different versions of some unit or units

The destination of an executable can be controlled inside a project file using the -o switch. In the absence of such a switch either inside the project file or on the command line, any executable files generated by gnatmake will be placed in the directory Exec_Dir specified in the project file. If no Exec_Dir is specified, they will be placed in the object directory of the project.

You can use project files to achieve some of the effects of a source versioning system (for example, defining separate projects for the different sets of sources that comprise different releases) but the Project Manager is independent of any source configuration management tools that might be used by the developers.

The next section introduces the main features of GNAT's project facility through a sequence of examples; subsequent sections will present the syntax and semantics in more detail.

10.2 Examples of Project Files

This section illustrates some of the typical uses of project files and explains their basic structure and behavior.

• Common Sources with Different Switches and Different Output Directories:
• Using External Variables:
• Importing Other Projects:
• Extending a Project:

10.2.1 Common Sources with Different Switches and Different Output Directories

• Source Files:
• Specifying the Object Directory:
• Specifying the Exec Directory:
• Project File Packages:
• Specifying Switch Settings:
• Main Subprograms:
• Source File Naming Conventions:
• Source Language(s):

Assume that the Ada source files pack.ads, pack.adb, and proc.adb are in the /common directory. The file proc.adb contains an Ada main subprogram Proc that "with"s package Pack. We want to compile these source files under two sets of switches:

• When debugging, we want to pass the -g switch to gnatmake, and the -gnata, -gnato, and -gnatE switches to the compiler; the compiler's output is to appear in /common/debug
• When preparing a release version, we want to pass the -O2 switch to the compiler; the compiler's output is to appear in /common/release

The GNAT project files shown below, respectively debug.gpr and release.gpr in the /common directory, achieve these effects.

Diagrammatically:

     /common
       debug.gpr
       release.gpr
       pack.ads
       pack.adb
       proc.adb
     /common/debug {-g, -gnata, -gnato, -gnatE}
       proc.ali, proc.o
       pack.ali, pack.o
     /common/release {-O2}
       proc.ali, proc.o
       pack.ali, pack.o
     

     project Debug is
       for Object_Dir use "debug";
       for Main use ("proc");
     
       package Builder is
         for Default_Switches ("Ada") use ("-g");
       end Builder;
     
       package Compiler is
         for Default_Switches ("Ada")
            use ("-fstack-check", "-gnata", "-gnato", "-gnatE");
       end Compiler;
     end Debug;
     

     project Release is
       for Object_Dir use "release";
       for Exec_Dir use ".";
       for Main use ("proc");
     
       package Compiler is
         for Default_Switches ("Ada") use ("-O2");
       end Compiler;
     end Release;
     

The name of the project defined by debug.gpr is "Debug" (case insensitive), and analogously the project defined by release.gpr is "Release". For consistency the file should have the same name as the project, and the project file's extension should be "gpr". These conventions are not required, but a warning is issued if they are not followed.

If the current directory is /temp, then the command

     gnatmake -P/common/debug.gpr
     

generates object and ALI files in /common/debug, and the proc executable also in /common/debug, using the switch settings defined in the project file.

Likewise, the command

     gnatmake -P/common/release.gpr
     

generates object and ALI files in /common/release, and the proc executable in /common, using the switch settings from the project file.

Source Files

If a project file does not explicitly specify a set of source directories or a set of source files, then by default the project's source files are the Ada source files in the project file directory. Thus pack.ads, pack.adb, and proc.adb are the source files for both projects.

Specifying the Object Directory

Several project properties are modeled by Ada-style attributes; you define the property by supplying the equivalent of an Ada attribute definition clause in the project file. A project's object directory is such a property; the corresponding attribute is Object_Dir, and its value is a string expression. A directory may be specified either as absolute or as relative; in the latter case, it is relative to the project file directory. Thus the compiler's output is directed to /common/debug (for the Debug project) and to /common/release (for the Release project). If Object_Dir is not specified, then the default is the project file directory.

Specifying the Exec Directory

A project's exec directory is another property; the corresponding attribute is Exec_Dir, and its value is also a string expression, either specified as relative or absolute. If Exec_Dir is not specified, then the default is the object directory (which may also be the project file directory if attribute Object_Dir is not specified). Thus the executable is placed in /common/debug for the Debug project (attribute Exec_Dir not specified) and in /common for the Release project.

Project File Packages

A GNAT tool integrated with the Project Manager is modeled by a corresponding package in the project file. The Debug project defines the packages Builder (for gnatmake) and Compiler; the Release project defines only the Compiler package.

The Ada package syntax is not to be taken literally. Although packages in project files bear a surface resemblance to packages in Ada source code, the notation is simply a way to convey a grouping of properties for a named entity. Indeed, the package names permitted in project files are restricted to a predefined set, corresponding to the project-aware tools, and the contents of packages are limited to a small set of constructs. The packages in the example above contain attribute definitions.

Specifying Switch Settings

Switch settings for a project-aware tool can be specified through attributes in the package corresponding to the tool. The example above illustrates one of the relevant attributes, Default_Switches, defined in the packages in both project files. Unlike simple attributes like Source_Dirs, Default_Switches is known as an associative array. When you define this attribute, you must supply an "index" (a literal string), and the effect of the attribute definition is to set the value of the "array" at the specified "index". For the Default_Switches attribute, the index is a programming language (in our case, Ada) , and the value specified (after use) must be a list of string expressions.

The attributes permitted in project files are restricted to a predefined set. Some may appear at project level, others in packages. For any attribute that is an associate array, the index must always be a literal string, but the restrictions on this string (e.g., a file name or a language name) depend on the individual attribute. Also depending on the attribute, its specified value will need to be either a string or a string list.

In the Debug project, we set the switches for two tools, gnatmake and the compiler, and thus we include corresponding packages, with each package defining the Default_Switches attribute with index "Ada". Note that the package corresponding to gnatmake is named Builder. The Release project is similar, but with just the Compiler package.

In project Debug above the switches starting with -gnat that are specified in package Compiler could have been placed in package Builder, since gnatmake transmits all such switches to the compiler.

Main Subprograms

One of the properties of a project is its list of main subprograms (actually a list of names of source files containing main subprograms, with the file extension optional. This property is captured in the Main attribute, whose value is a list of strings. If a project defines the Main attribute, then you do not need to identify the main subprogram(s) when invoking gnatmake (see gnatmake and Project Files).

Source File Naming Conventions

Since the project files do not specify any source file naming conventions, the GNAT defaults are used. The mechanism for defining source file naming conventions - a package named Naming - will be described below (see Naming Schemes).

Source Language(s)

Since the project files do not specify a Languages attribute, by default the GNAT tools assume that the language of the project file is Ada. More generally, a project can comprise source files in Ada, C, and/or other languages.

10.2.2 Using External Variables

Instead of supplying different project files for debug and release, we can define a single project file that queries an external variable (set either on the command line or via an environment variable) in order to conditionally define the appropriate settings. Again, assume that the source files pack.ads, pack.adb, and proc.adb are located in directory /common. The following project file, build.gpr, queries the external variable named STYLE and defines an object directory and switch settings based on whether the value is "deb" (debug) or "rel" (release), where the default is "deb".

     project Build is
       for Main use ("proc");
     
       type Style_Type is ("deb", "rel");
       Style : Style_Type := external ("STYLE", "deb");
     
       case Style is
         when "deb" =>
           for Object_Dir use "debug";
     
         when "rel" =>
           for Object_Dir use "release";
           for Exec_Dir use ".";
       end case;
     
       package Builder is
     
         case Style is
           when "deb" =>
             for Default_Switches ("Ada") use ("-g");
         end case;
     
       end Builder;
     
       package Compiler is
     
         case Style is
           when "deb" =>
             for Default_Switches ("Ada") use ("-gnata", "-gnato", "-gnatE");
     
           when "rel" =>
             for Default_Switches ("Ada") use ("-O2");
         end case;
     
       end Compiler;
     
     end Build;
     

Style_Type is an example of a string type, which is the project file analog of an Ada enumeration type but containing string literals rather than identifiers. Style is declared as a variable of this type.

The form external("STYLE", "deb") is known as an external reference; its first argument is the name of an external variable, and the second argument is a default value to be used if the external variable doesn't exist. You can define an external variable on the command line via the -X switch, or you can use an environment variable as an external variable.

Each case construct is expanded by the Project Manager based on the value of Style. Thus the command

     gnatmake -P/common/build.gpr -XSTYLE=deb
     

is equivalent to the gnatmake invocation using the project file debug.gpr in the earlier example. So is the command

     gnatmake -P/common/build.gpr
     

since "deb" is the default for STYLE.

Analogously,

     gnatmake -P/common/build.gpr -XSTYLE=rel
     

is equivalent to the gnatmake invocation using the project file release.gpr in the earlier example.

10.2.3 Importing Other Projects

A compilation unit in a source file in one project may depend on compilation units in source files in other projects. To obtain this behavior, the dependent project must import the projects containing the needed source files. This effect is embodied in syntax similar to an Ada with clause, but the "with"ed entities are strings denoting project files.

As an example, suppose that the two projects GUI_Proj and Comm_Proj are defined in the project files gui_proj.gpr and comm_proj.gpr in directories /gui and /comm, respectively. Assume that the source files for GUI_Proj are gui.ads and gui.adb, and that the source files for Comm_Proj are comm.ads and comm.adb, with each set of files located in its respective project file directory. Diagrammatically:

     /gui
       gui_proj.gpr
       gui.ads
       gui.adb
     
     /comm
       comm_proj.gpr
       comm.ads
       comm.adb
     

We want to develop an application in directory /app that "with"s the packages GUI and Comm, using the properties of the corresponding project files (e.g. the switch settings and object directory). Skeletal code for a main procedure might be something like the following:

     with GUI, Comm;
     procedure App_Main is
        ...
     begin
        ...
     end App_Main;
     

Here is a project file, app_proj.gpr, that achieves the desired effect:

     with "/gui/gui_proj", "/comm/comm_proj";
     project App_Proj is
        for Main use ("app_main");
     end App_Proj;
     

Building an executable is achieved through the command:

     gnatmake -P/app/app_proj
     

which will generate the app_main executable in the directory where app_proj.gpr resides.

If an imported project file uses the standard extension (gpr) then (as illustrated above) the with clause can omit the extension.

Our example specified an absolute path for each imported project file. Alternatively, you can omit the directory if either

• The imported project file is in the same directory as the importing project file, or
• You have defined an environment variable ADA_PROJECT_PATH that includes the directory containing the needed project file.

Thus, if we define ADA_PROJECT_PATH to include /gui and /comm, then our project file app_proj.gpr could be written as follows:

     with "gui_proj", "comm_proj";
     project App_Proj is
        for Main use ("app_main");
     end App_Proj;
     

Importing other projects raises the possibility of ambiguities. For example, the same unit might be present in different imported projects, or it might be present in both the importing project and an imported project. Both of these conditions are errors. Note that in the current version of the Project Manager, it is illegal to have an ambiguous unit even if the unit is never referenced by the importing project. This restriction may be relaxed in a future release.

10.2.4 Extending a Project

A common situation in large software systems is to have multiple implementations for a common interface; in Ada terms, multiple versions of a package body for the same specification. For example, one implementation might be safe for use in tasking programs, while another might only be used in sequential applications. This can be modeled in GNAT using the concept of project extension. If one project (the "child") extends another project (the "parent") then by default all source files of the parent project are inherited by the child, but the child project can override any of the parent's source files with new versions, and can also add new files. This facility is the project analog of extension in Object-Oriented Programming. Project hierarchies are permitted (a child project may be the parent of yet another project), and a project that inherits one project can also import other projects.

As an example, suppose that directory /seq contains the project file seq_proj.gpr and the source files pack.ads, pack.adb, and proc.adb:

     /seq
       pack.ads
       pack.adb
       proc.adb
       seq_proj.gpr
     

Note that the project file can simply be empty (that is, no attribute or package is defined):

     project Seq_Proj is
     end Seq_Proj;
     

implying that its source files are all the Ada source files in the project directory.

Suppose we want to supply an alternate version of pack.adb, in directory /tasking, but use the existing versions of pack.ads and proc.adb. We can define a project Tasking_Proj that inherits Seq_Proj:

     /tasking
       pack.adb
       tasking_proj.gpr
     
     project Tasking_Proj extends "/seq/seq_proj" is
     end Tasking_Proj;
     

The version of pack.adb used in a build depends on which project file is specified.

Note that we could have designed this using project import rather than project inheritance; a base project would contain the sources for pack.ads and proc.adb, a sequential project would import base and add pack.adb, and likewise a tasking project would import base and add a different version of pack.adb. The choice depends on whether other sources in the original project need to be overridden. If they do, then project extension is necessary, otherwise, importing is sufficient.

10.3 Project File Syntax

• Basic Syntax:
• Packages:
• Expressions:
• String Types:
• Variables:
• Attributes:
• Associative Array Attributes:
• case Constructions:

This section describes the structure of project files.

A project may be an independent project, entirely defined by a single project file. Any Ada source file in an independent project depends only on the predefined library and other Ada source files in the same project.

A project may also depend on other projects, in either or both of the following ways:

• It may import any number of projects
• It may extend at most one other project

The dependence relation is a directed acyclic graph (the subgraph reflecting the "extends" relation is a tree).

A project's immediate sources are the source files directly defined by that project, either implicitly by residing in the project file's directory, or explicitly through any of the source-related attributes described below. More generally, a project proj's sources are the immediate sources of proj together with the immediate sources (unless overridden) of any project on which proj depends (either directly or indirectly).

10.3.1 Basic Syntax

As seen in the earlier examples, project files have an Ada-like syntax. The minimal project file is:

     project Empty is
     
     end Empty;
     

The identifier Empty is the name of the project. This project name must be present after the reserved word end at the end of the project file, followed by a semi-colon.

Any name in a project file, such as the project name or a variable name, has the same syntax as an Ada identifier.

The reserved words of project files are the Ada reserved words plus extends, external, and project. Note that the only Ada reserved words currently used in project file syntax are:

• case
• end
• for
• is
• others
• package
• renames
• type
• use
• when
• with

Comments in project files have the same syntax as in Ada, two consecutives hyphens through the end of the line.

10.3.2 Packages

A project file may contain packages. The name of a package must be one of the identifiers (case insensitive) from a predefined list, and a package with a given name may only appear once in a project file. The predefined list includes the following packages:

• Naming
• Builder
• Compiler
• Binder
• Linker
• Finder
• Cross_Reference
• gnatls

(The complete list of the package names and their attributes can be found in file prj-attr.adb).

In its simplest form, a package may be empty:

     project Simple is
       package Builder is
       end Builder;
     end Simple;
     

A package may contain attribute declarations, variable declarations and case constructions, as will be described below.

When there is ambiguity between a project name and a package name, the name always designates the project. To avoid possible confusion, it is always a good idea to avoid naming a project with one of the names allowed for packages or any name that starts with gnat.

10.3.3 Expressions

An expression is either a string expression or a string list expression.

A string expression is either a simple string expression or a compound string expression.

A simple string expression is one of the following:

• A literal string; e.g."comm/my_proj.gpr"
• A string-valued variable reference (see Variables)
• A string-valued attribute reference (see Attributes)
• An external reference (see External References in Project Files)

A compound string expression is a concatenation of string expressions, using "&"

            Path & "/" & File_Name & ".ads"
     

A string list expression is either a simple string list expression or a compound string list expression.

A simple string list expression is one of the following:

• A parenthesized list of zero or more string expressions, separated by commas

               File_Names := (File_Name, "gnat.adc", File_Name & ".orig");
               Empty_List := ();
            

• A string list-valued variable reference
• A string list-valued attribute reference

A compound string list expression is the concatenation (using "&") of a simple string list expression and an expression. Note that each term in a compound string list expression, except the first, may be either a string expression or a string list expression.

        File_Name_List := () & File_Name; --  One string in this list
        Extended_File_Name_List := File_Name_List & (File_Name & ".orig");
        --  Two strings
        Big_List := File_Name_List & Extended_File_Name_List;
        --  Concatenation of two string lists: three strings
        Illegal_List := "gnat.adc" & Extended_File_Name_List;
        --  Illegal: must start with a string list
     

10.3.4 String Types

The value of a variable may be restricted to a list of string literals. The restricted list of string literals is given in a string type declaration.

Here is an example of a string type declaration:

        type OS is ("NT, "nt", "Unix", "Linux", "other OS");
     

Variables of a string type are called typed variables; all other variables are called untyped variables. Typed variables are particularly useful in case constructions (see case Constructions).

A string type declaration starts with the reserved word type, followed by the name of the string type (case-insensitive), followed by the reserved word is, followed by a parenthesized list of one or more string literals separated by commas, followed by a semicolon.

The string literals in the list are case sensitive and must all be different. They may include any graphic characters allowed in Ada, including spaces.

A string type may only be declared at the project level, not inside a package.

A string type may be referenced by its name if it has been declared in the same project file, or by its project name, followed by a dot, followed by the string type name.

10.3.5 Variables

A variable may be declared at the project file level, or in a package. Here are some examples of variable declarations:

        This_OS : OS := external ("OS"); --  a typed variable declaration
        That_OS := "Linux";              --  an untyped variable declaration
     

A typed variable declaration includes the variable name, followed by a colon, followed by the name of a string type, followed by :=, followed by a simple string expression.

An untyped variable declaration includes the variable name, followed by :=, followed by an expression. Note that, despite the terminology, this form of "declaration" resembles more an assignment than a declaration in Ada. It is a declaration in several senses:

• The variable name does not need to be defined previously
• The declaration establishes the kind (string versus string list) of the variable, and later declarations of the same variable need to be consistent with this

A string variable declaration (typed or untyped) declares a variable whose value is a string. This variable may be used as a string expression.

        File_Name       := "readme.txt";
        Saved_File_Name := File_Name & ".saved";
     

A string list variable declaration declares a variable whose value is a list of strings. The list may contain any number (zero or more) of strings.

        Empty_List := ();
        List_With_One_Element := ("-gnaty");
        List_With_Two_Elements := List_With_One_Element & "-gnatg";
        Long_List := ("main.ada", "pack1_.ada", "pack1.ada", "pack2_.ada"
                      "pack2.ada", "util_.ada", "util.ada");
     

The same typed variable may not be declared more than once at project level, and it may not be declared more than once in any package; it is in effect a constant or a readonly variable.

The same untyped variable may be declared several times. In this case, the new value replaces the old one, and any subsequent reference to the variable uses the new value. However, as noted above, if a variable has been declared as a string, all subsequent declarations must give it a string value. Similarly, if a variable has been declared as a string list, all subsequent declarations must give it a string list value.

A variable reference may take several forms:

• The simple variable name, for a variable in the current package (if any) or in the current project
• A context name, followed by a dot, followed by the variable name.

A context may be one of the following:

• The name of an existing package in the current project
• The name of an imported project of the current project
• The name of an ancestor project (i.e., a project extended by the current project, either directly or indirectly)
• An imported/parent project name, followed by a dot, followed by a package name

A variable reference may be used in an expression.

10.3.6 Attributes

A project (and its packages) may have attributes that define the project's properties. Some attributes have values that are strings; others have values that are string lists.

There are two categories of attributes: simple attributes and associative arrays (see Associative Array Attributes).

The names of the attributes are restricted; there is a list of project attributes, and a list of package attributes for each package. The names are not case sensitive.

The project attributes are as follows (all are simple attributes):

Attribute Name	Value
Source_Files	string list
Source_Dirs	string list
Source_List_File	string
Object_Dir	string
Exec_Dir	string
Main	string list
Languages	string list
Library_Dir	string
Library_Name	string
Library_Kind	string
Library_Elaboration	string
Library_Version	string

The attributes for package Naming are as follows (see Naming Schemes):

Attribute Name	Category	Index	Value
Specification_Suffix	associative array	language name	string
Implementation_Suffix	associative array	language name	string
Separate_Suffix	simple attribute	n/a	string
Casing	simple attribute	n/a	string
Dot_Replacement	simple attribute	n/a	string
Specification	associative array	Ada unit name	string
Implementation	associative array	Ada unit name	string
Specification_Exceptions	associative array	language name	string list
Implementation_Exceptions	associative array	language name	string list

The attributes for package Builder, Compiler, Binder, Linker, Cross_Reference, and Finder are as follows (see Switches and Project Files).

Attribute Name	Category	Index	Value
Default_Switches	associative array	language name	string list
Switches	associative array	file name	string list

In addition, package Builder has a single string attribute Local_Configuration_Pragmas and package Builder has a single string attribute Global_Configuration_Pragmas.

The attribute for package Glide are not documented: they are for internal use only.

Each simple attribute has a default value: the empty string (for string-valued attributes) and the empty list (for string list-valued attributes).

Similar to variable declarations, an attribute declaration defines a new value for an attribute.

Examples of simple attribute declarations:

        for Object_Dir use "objects";
        for Source_Dirs use ("units", "test/drivers");
     

A simple attribute declaration starts with the reserved word for, followed by the name of the attribute, followed by the reserved word use, followed by an expression (whose kind depends on the attribute), followed by a semicolon.

Attributes may be referenced in expressions. The general form for such a reference is <entity>'<attribute>: the entity for which the attribute is defined, followed by an apostrophe, followed by the name of the attribute. For associative array attributes, a litteral string between parentheses need to be supplied as index.

Examples are:

       project'Object_Dir
       Naming'Dot_Replacement
       Imported_Project'Source_Dirs
       Imported_Project.Naming'Casing
       Builder'Default_Switches("Ada")
     

The entity may be:

• project for an attribute of the current project
• The name of an existing package of the current project
• The name of an imported project
• The name of a parent project (extended by the current project)
• An imported/parent project name, followed by a dot, followed by a package name

Example:

        project Prj is
          for Source_Dirs use project'Source_Dirs & "units";
          for Source_Dirs use project'Source_Dirs & "test/drivers"
        end Prj;
     

In the first attribute declaration, initially the attribute Source_Dirs has the default value: an empty string list. After this declaration, Source_Dirs is a string list of one element: "units". After the second attribute declaration Source_Dirs is a string list of two elements: "units" and "test/drivers".

Note: this example is for illustration only. In practice, the project file would contain only one attribute declaration:

        for Source_Dirs use ("units", "test/drivers");
     

10.3.7 Associative Array Attributes

Some attributes are defined as associative arrays. An associative array may be regarded as a function that takes a string as a parameter and delivers a string or string list value as its result.

Here are some examples of associative array attribute declarations:

        for Implementation ("main") use "Main.ada";
        for Switches ("main.ada") use ("-v", "-gnatv");
        for Switches ("main.ada") use Builder'Switches ("main.ada") & "-g";
     

Like untyped variables and simple attributes, associative array attributes may be declared several times. Each declaration supplies a new value for the attribute, replacing the previous setting.

10.3.8 case Constructions

A case construction is used in a project file to effect conditional behavior. Here is a typical example:

     project MyProj is
        type OS_Type is ("Linux", "Unix", "NT", "VMS");
     
        OS : OS_Type := external ("OS", "Linux");
     
        package Compiler is
          case OS is
            when "Linux" | "Unix" =>
              for Default_Switches ("Ada") use ("-gnath");
            when "NT" =>
              for Default_Switches ("Ada") use ("-gnatP");
            when others =>
          end case;
        end Compiler;
     end MyProj;
     

The syntax of a case construction is based on the Ada case statement (although there is no null construction for empty alternatives).

Following the reserved word case there is the case variable (a typed string variable), the reserved word is, and then a sequence of one or more alternatives. Each alternative comprises the reserved word when, either a list of literal strings separated by the "|" character or the reserved word others, and the "=>" token. Each literal string must belong to the string type that is the type of the case variable. An others alternative, if present, must occur last. The end case; sequence terminates the case construction.

After each =>, there are zero or more constructions. The only constructions allowed in a case construction are other case constructions and attribute declarations. String type declarations, variable declarations and package declarations are not allowed.

The value of the case variable is often given by an external reference (see External References in Project Files).

10.4 Objects and Sources in Project Files

• Object Directory:
• Exec Directory:
• Source Directories:
• Source File Names:

Each project has exactly one object directory and one or more source directories. The source directories must contain at least one source file, unless the project file explicitly specifies that no source files are present (see Source File Names).

10.4.1 Object Directory

The object directory for a project is the directory containing the compiler's output (such as ALI files and object files) for the project's immediate sources. Note that for inherited sources (when extending a parent project) the parent project's object directory is used.

The object directory is given by the value of the attribute Object_Dir in the project file.

        for Object_Dir use "objects";
     

The attribute Object_Dir has a string value, the path name of the object directory. The path name may be absolute or relative to the directory of the project file. This directory must already exist, and be readable and writable.

By default, when the attribute Object_Dir is not given an explicit value or when its value is the empty string, the object directory is the same as the directory containing the project file.

10.4.2 Exec Directory

The exec directory for a project is the directory containing the executables for the project's main subprograms.

The exec directory is given by the value of the attribute Exec_Dir in the project file.

        for Exec_Dir use "executables";
     

The attribute Exec_Dir has a string value, the path name of the exec directory. The path name may be absolute or relative to the directory of the project file. This directory must already exist, and be writable.

By default, when the attribute Exec_Dir is not given an explicit value or when its value is the empty string, the exec directory is the same as the object directory of the project file.

10.4.3 Source Directories

The source directories of a project are specified by the project file attribute Source_Dirs.

This attribute's value is a string list. If the attribute is not given an explicit value, then there is only one source directory, the one where the project file resides.

A Source_Dirs attribute that is explicitly defined to be the empty list, as in

         for Source_Dirs use ();
     

indicates that the project contains no source files.

Otherwise, each string in the string list designates one or more source directories.

        for Source_Dirs use ("sources", "test/drivers");
     

If a string in the list ends with "/**", then the directory whose path name precedes the two asterisks, as well as all its subdirectories (recursively), are source directories.

        for Source_Dirs use ("/system/sources/**");
     

Here the directory /system/sources and all of its subdirectories (recursively) are source directories.

To specify that the source directories are the directory of the project file and all of its subdirectories, you can declare Source_Dirs as follows:

        for Source_Dirs use ("./**");
     

Each of the source directories must exist and be readable.

10.4.4 Source File Names

In a project that contains source files, their names may be specified by the attributes Source_Files (a string list) or Source_List_File (a string). Source file names never include any directory information.

If the attribute Source_Files is given an explicit value, then each element of the list is a source file name.

        for Source_Files use ("main.adb");
        for Source_Files use ("main.adb", "pack1.ads", "pack2.adb");
     

If the attribute Source_Files is not given an explicit value, but the attribute Source_List_File is given a string value, then the source file names are contained in the text file whose path name (absolute or relative to the directory of the project file) is the value of the attribute Source_List_File.

Each line in the file that is not empty or is not a comment contains a source file name. A comment line starts with two hyphens.

        for Source_List_File use "source_list.txt";
     

By default, if neither the attribute Source_Files nor the attribute Source_List_File is given an explicit value, then each file in the source directories that conforms to the project's naming scheme (see Naming Schemes) is an immediate source of the project.

A warning is issued if both attributes Source_Files and Source_List_File are given explicit values. In this case, the attribute Source_Files prevails.

Each source file name must be the name of one and only one existing source file in one of the source directories.

A Source_Files attribute defined with an empty list as its value indicates that there are no source files in the project.

Except for projects that are clearly specified as containing no Ada source files (Source_Dirs or Source_Files specified as an empty list, or Languages specified without "Ada" in the list)

        for Source_Dirs use ();
        for Source_Files use ();
        for Languages use ("C", "C++");
     

a project must contain at least one immediate source.

Projects with no source files are useful as template packages (see Packages in Project Files) for other projects; in particular to define a package Naming (see Naming Schemes).

10.5 Importing Projects

An immediate source of a project P may depend on source files that are neither immediate sources of P nor in the predefined library. To get this effect, P must import the projects that contain the needed source files.

       with "project1", "utilities.gpr";
       with "/namings/apex.gpr";
       project Main is
         ...
     

As can be seen in this example, the syntax for importing projects is similar to the syntax for importing compilation units in Ada. However, project files use literal strings instead of names, and the with clause identifies project files rather than packages.

Each literal string is the file name or path name (absolute or relative) of a project file. If a string is simply a file name, with no path, then its location is determined by the project path:

• If the environment variable ADA_PROJECT_PATH exists, then the project path includes all the directories in this environment variable, plus the directory of the project file.
• If the environment variable ADA_PROJECT_PATH does not exist, then the project path contains only one directory, namely the one where the project file is located.

If a relative pathname is used as in

       with "tests/proj";
     

then the path is relative to the directory where the importing project file is located. Any symbolic link will be fully resolved in the directory of the importing project file before the imported project file is looked up.

When the with'ed project file name does not have an extension, the default is .gpr. If a file with this extension is not found, then the file name as specified in the with clause (no extension) will be used. In the above example, if a file project1.gpr is found, then it will be used; otherwise, if a file project1 exists then it will be used; if neither file exists, this is an error.

A warning is issued if the name of the project file does not match the name of the project; this check is case insensitive.

Any source file that is an immediate source of the imported project can be used by the immediate sources of the importing project, and recursively. Thus if A imports B, and B imports C, the immediate sources of A may depend on the immediate sources of C, even if A does not import C explicitly. However, this is not recommended, because if and when B ceases to import C, some sources in A will no longer compile.

A side effect of this capability is that cyclic dependences are not permitted: if A imports B (directly or indirectly) then B is not allowed to import A.

10.6 Project Extension

During development of a large system, it is sometimes necessary to use modified versions of some of the source files without changing the original sources. This can be achieved through a facility known as project extension.

        project Modified_Utilities extends "/baseline/utilities.gpr" is ...
     

The project file for the project being extended (the parent) is identified by the literal string that follows the reserved word extends, which itself follows the name of the extending project (the child).

By default, a child project inherits all the sources of its parent. However, inherited sources can be overridden: a unit with the same name as one in the parent will hide the original unit. Inherited sources are considered to be sources (but not immediate sources) of the child project; see Project File Syntax.

An inherited source file retains any switches specified in the parent project.

For example if the project Utilities contains the specification and the body of an Ada package Util_IO, then the project Modified_Utilities can contain a new body for package Util_IO. The original body of Util_IO will not be considered in program builds. However, the package specification will still be found in the project Utilities.

A child project can have only one parent but it may import any number of other projects.

A project is not allowed to import directly or indirectly at the same time a child project and any of its ancestors.

10.7 External References in Project Files

A project file may contain references to external variables; such references are called external references.

An external variable is either defined as part of the environment (an environment variable in Unix, for example) or else specified on the command line via the -Xvbl=value switch. If both, then the command line value is used.

An external reference is denoted by the built-in function external, which returns a string value. This function has two forms:

• external (external_variable_name)
• external (external_variable_name, default_value)

Each parameter must be a string literal. For example:

        external ("USER")
        external ("OS", "Linux")
     

In the form with one parameter, the function returns the value of the external variable given as parameter. If this name is not present in the environment, then the returned value is an empty string.

In the form with two string parameters, the second parameter is the value returned when the variable given as the first parameter is not present in the environment. In the example above, if "OS" is not the name of an environment variable and is not passed on the command line, then the returned value will be "Linux".

An external reference may be part of a string expression or of a string list expression, to define variables or attributes.

        type Mode_Type is ("Debug", "Release");
        Mode : Mode_Type := external ("MODE");
        case Mode is
          when "Debug" =>
             ...
     

10.8 Packages in Project Files

The package is the project file feature that defines the settings for project-aware tools. For each such tool you can declare a corresponding package; the names for these packages are preset (see Packages) but are not case sensitive. A package may contain variable declarations, attribute declarations, and case constructions.

        project Proj is
           package Builder is  -- used by gnatmake
              for Default_Switches ("Ada") use ("-v", "-g");
           end Builder;
        end Proj;
     

A package declaration starts with the reserved word package, followed by the package name (case insensitive), followed by the reserved word is. It ends with the reserved word end, followed by the package name, finally followed by a semi-colon.

Most of the packages have an attribute Default_Switches. This attribute is an associative array, and its value is a string list. The index of the associative array is the name of a programming language (case insensitive). This attribute indicates the switch or switches to be used with the corresponding tool.

Some packages also have another attribute, Switches, an associative array whose value is a string list. The index is the name of a source file. This attribute indicates the switch or switches to be used by the corresponding tool when dealing with this specific file.

Further information on these switch-related attributes is found in Switches and Project Files.

A package may be declared as a renaming of another package; e.g., from the project file for an imported project.

       with "/global/apex.gpr";
       project Example is
         package Naming renames Apex.Naming;
         ...
       end Example;
     

Packages that are renamed in other project files often come from project files that have no sources: they are just used as templates. Any modification in the template will be reflected automatically in all the project files that rename a package from the template.

In addition to the tool-oriented packages, you can also declare a package named Naming to establish specialized source file naming conventions (see Naming Schemes).

10.9 Variables from Imported Projects

An attribute or variable defined in an imported or parent project can be used in expressions in the importing / extending project. Such an attribute or variable is prefixed with the name of the project and (if relevant) the name of package where it is defined.

       with "imported";
       project Main extends "base" is
          Var1 := Imported.Var;
          Var2 := Base.Var & ".new";
     
          package Builder is
             for Default_Switches ("Ada") use Imported.Builder.Ada_Switches &
                              "-gnatg" & "-v";
          end Builder;
     
          package Compiler is
             for Default_Switches ("Ada") use Base.Compiler.Ada_Switches;
          end Compiler;
       end Main;
     

In this example:

• Var1 is a copy of the variable Var defined in the project file "imported.gpr"
• the value of Var2 is a copy of the value of variable Var defined in the project file base.gpr, concatenated with ".new"
• attribute Default_Switches ("Ada") in package Builder is a string list that includes in its value a copy of variable Ada_Switches defined in the Builder package in project file imported.gpr plus two new elements: "-gnatg" and "-v";
• attribute Default_Switches ("Ada") in package Compiler is a copy of the variable Ada_Switches defined in the Compiler package in project file base.gpr, the project being extended.

10.10 Naming Schemes

Sometimes an Ada software system is ported from a foreign compilation environment to GNAT, with file names that do not use the default GNAT conventions. Instead of changing all the file names (which for a variety of reasons might not be possible), you can define the relevant file naming scheme in the Naming package in your project file. For example, the following package models the Apex file naming rules:

       package Naming is
         for Casing                        use "lowercase";
         for Dot_Replacement               use ".";
         for Specification_Suffix ("Ada")  use ".1.ada";
         for Implementation_Suffix ("Ada") use ".2.ada";
       end Naming;
     

You can define the following attributes in package Naming:

Casing

This must be a string with one of the three values "lowercase", "uppercase" or "mixedcase"; these strings are case insensitive.

If Casing is not specified, then the default is "lowercase".

Dot_Replacement

This must be a string whose value satisfies the following conditions:

• It must not be empty
• It cannot start or end with an alphanumeric character
• It cannot be a single underscore
• It cannot start with an underscore followed by an alphanumeric
• It cannot contain a dot '.' except if it the entire string is "."

If Dot_Replacement is not specified, then the default is "-".

Specification_Suffix

This is an associative array (indexed by the programming language name, case insensitive) whose value is a string that must satisfy the following conditions:

• It must not be empty
• It cannot start with an alphanumeric character
• It cannot start with an underscore followed by an alphanumeric character

If Specification_Suffix ("Ada") is not specified, then the default is ".ads".

Implementation_Suffix

This is an associative array (indexed by the programming language name, case insensitive) whose value is a string that must satisfy the following conditions:

• It must not be empty
• It cannot start with an alphanumeric character
• It cannot start with an underscore followed by an alphanumeric character
• It cannot be a suffix of Specification_Suffix

If Implementation_Suffix ("Ada") is not specified, then the default is ".adb".

Separate_Suffix

This must be a string whose value satisfies the same conditions as Implementation_Suffix.

If Separate_Suffix ("Ada") is not specified, then it defaults to same value as Implementation_Suffix ("Ada").

Specification

You can use the Specification attribute, an associative array, to define the source file name for an individual Ada compilation unit's spec. The array index must be a string literal that identifies the Ada unit (case insensitive). The value of this attribute must be a string that identifies the file that contains this unit's spec (case sensitive or insensitive depending on the operating system).

             for Specification ("MyPack.MyChild") use "mypack.mychild.spec";
          

Implementation

You can use the Implementation attribute, an associative array, to define the source file name for an individual Ada compilation unit's body (possibly a subunit). The array index must be a string literal that identifies the Ada unit (case insensitive). The value of this attribute must be a string that identifies the file that contains this unit's body or subunit (case sensitive or insensitive depending on the operating system).

             for Implementation ("MyPack.MyChild") use "mypack.mychild.body";
          

10.11 Library Projects

Library projects are projects whose object code is placed in a library. (Note that this facility is not yet supported on all platforms)

To create a library project, you need to define in its project file two project-level attributes: Library_Name and Library_Dir. Additionally, you may define the library-related attributes Library_Kind, Library_Version and Library_Elaboration.

The Library_Name attribute has a string value that must start with a letter and include only letters and digits.

The Library_Dir attribute has a string value that designates the path (absolute or relative) of the directory where the library will reside. It must designate an existing directory, and this directory needs to be different from the project's object directory. It also needs to be writable.

If both Library_Name and Library_Dir are specified and are legal, then the project file defines a library project. The optional library-related attributes are checked only for such project files.

The Library_Kind attribute has a string value that must be one of the following (case insensitive): "static", "dynamic" or "relocatable". If this attribute is not specified, the library is a static library. Otherwise, the library may be dynamic or relocatable. Depending on the operating system, there may or may not be a distinction between dynamic and relocatable libraries. For example, on Unix there is no such distinction.

The Library_Version attribute has a string value whose interpretation is platform dependent. On Unix, it is used only for dynamic/relocatable libraries as the internal name of the library (the "soname"). If the library file name (built from the Library_Name) is different from the Library_Version, then the library file will be a symbolic link to the actual file whose name will be Library_Version.

Example (on Unix):

     project Plib is
     
        Version := "1";
     
        for Library_Dir use "lib_dir";
        for Library_Name use "dummy";
        for Library_Kind use "relocatable";
        for Library_Version use "libdummy.so." & Version;
     
     end Plib;
     

Directory lib_dir will contain the internal library file whose name will be libdummy.so.1, and libdummy.so will be a symbolic link to libdummy.so.1.

When gnatmake detects that a project file (not the main project file) is a library project file, it will check all immediate sources of the project and rebuild the library if any of the sources have been recompiled. All ALI files will also be copied from the object directory to the library directory. To build executables, gnatmake will use the library rather than the individual object files.

10.12 Switches Related to Project Files

The following switches are used by GNAT tools that support project files:

-Pproject

Indicates the name of a project file. This project file will be parsed with the verbosity indicated by -vPx, if any, and using the external references indicated by -X switches, if any.

There must be only one -P switch on the command line.

Since the Project Manager parses the project file only after all the switches on the command line are checked, the order of the switches -P, -Vpx or -X is not significant.

-Xname=value

Indicates that external variable name has the value value. The Project Manager will use this value for occurrences of external(name) when parsing the project file.

If name or value includes a space, then name=value should be put between quotes.

            -XOS=NT
            -X"user=John Doe"
          

Several -X switches can be used simultaneously. If several -X switches specify the same name, only the last one is used.

An external variable specified with a -X switch takes precedence over the value of the same name in the environment.

-vPx

Indicates the verbosity of the parsing of GNAT project files. -vP0 means Default (no output for syntactically correct project files); -vP1 means Medium; -vP2 means High.

The default is Default.

If several -vPx switches are present, only the last one is used.

10.13 Tools Supporting Project Files

• gnatmake and Project Files:
• The GNAT Driver and Project Files:
• Glide and Project Files:

10.13.1 gnatmake and Project Files

This section covers two topics related to gnatmake and project files: defining switches for gnatmake and for the tools that it invokes; and the use of the Main attribute.

• Switches and Project Files:
• Project Files and Main Subprograms:

10.13.1.1 Switches and Project Files

For each of the packages Builder, Compiler, Binder, and Linker, you can specify a Default_Switches attribute, a Switches attribute, or both; as their names imply, these switch-related attributes affect which switches are used for which files when gnatmake is invoked. As will be explained below, these package-contributed switches precede the switches passed on the gnatmake command line.

The Default_Switches attribute is an associative array indexed by language name (case insensitive) and returning a string list. For example:

     package Compiler is
       for Default_Switches ("Ada") use ("-gnaty", "-v");
     end Compiler;
     

The Switches attribute is also an associative array, indexed by a file name (which may or may not be case sensitive, depending on the operating system) and returning a string list. For example:

     package Builder is
        for Switches ("main1.adb") use ("-O2");
        for Switches ("main2.adb") use ("-g");
     end Builder;
     

For the Builder package, the file names should designate source files for main subprograms. For the Binder and Linker packages, the file names should designate ALI or source files for main subprograms. In each case just the file name (without explicit extension) is acceptable.

For each tool used in a program build (gnatmake, the compiler, the binder, and the linker), its corresponding package contributes a set of switches for each file on which the tool is invoked, based on the switch-related attributes defined in the package. In particular, the switches that each of these packages contributes for a given file f comprise:

• the value of attribute Switches (f), if it is specified in the package for the given file,
• otherwise, the value of Default_Switches ("Ada"), if it is specified in the package.

If neither of these attributes is defined in the package, then the package does not contribute any switches for the given file.

When gnatmake is invoked on a file, the switches comprise two sets, in the following order: those contributed for the file by the Builder package; and the switches passed on the command line.

When gnatmake invokes a tool (compiler, binder, linker) on a file, the switches passed to the tool comprise three sets, in the following order:

1. the applicable switches contributed for the file by the Builder package in the project file supplied on the command line;
2. those contributed for the file by the package (in the relevant project file - see below) corresponding to the tool; and
3. the applicable switches passed on the command line.

The term applicable switches reflects the fact that gnatmake switches may or may not be passed to individual tools, depending on the individual switch.

gnatmake may invoke the compiler on source files from different projects. The Project Manager will use the appropriate project file to determine the Compiler package for each source file being compiled. Likewise for the Binder and Linker packages.

As an example, consider the following package in a project file:

     project Proj1 is
        package Compiler is
           for Default_Switches ("Ada") use ("-g");
           for Switches ("a.adb") use ("-O1");
           for Switches ("b.adb") use ("-O2", "-gnaty");
        end Compiler;
     end Proj1;
     

If gnatmake is invoked with this project file, and it needs to compile, say, the files a.adb, b.adb, and c.adb, then a.adb will be compiled with the switch -O1, b.adb with switches -O2 and -gnaty, and c.adb with -g.

Another example illustrates the ordering of the switches contributed by different packages:

     project Proj2 is
        package Builder is
           for Switches ("main.adb") use ("-g", "-O1", "-f");
        end Builder;
     
        package Compiler is
           for Switches ("main.adb") use ("-O2");
        end Compiler;
     end Proj2;
     

If you issue the command:

         gnatmake -PProj2 -O0 main
     

then the compiler will be invoked on main.adb with the following sequence of switches

        -g -O1 -O2 -O0
     

with the last -O switch having precedence over the earlier ones; several other switches (such as -c) are added implicitly.

The switches -g and -O1 are contributed by package Builder, -O2 is contributed by the package Compiler and -O0 comes from the command line.

The -g switch will also be passed in the invocation of gnatlink.

A final example illustrates switch contributions from packages in different project files:

     project Proj3 is
        for Source_Files use ("pack.ads", "pack.adb");
        package Compiler is
           for Default_Switches ("Ada") use ("-gnata");
        end Compiler;
     end Proj3;
     
     with "Proj3";
     project Proj4 is
        for Source_Files use ("foo_main.adb", "bar_main.adb");
        package Builder is
           for Switches ("foo_main.adb") use ("-s", "-g");
        end Builder;
     end Proj4;
     
     -- Ada source file:
     with Pack;
     procedure Foo_Main is
        ...
     end Foo_Main;
     

If the command is

     gnatmake -PProj4 foo_main.adb -cargs -gnato
     

then the switches passed to the compiler for foo_main.adb are -g (contributed by the package Proj4.Builder) and -gnato (passed on the command line). When the imported package Pack is compiled, the switches used are -g from Proj4.Builder, -gnata (contributed from package Proj3.Compiler, and -gnato from the command line.

10.13.1.2 Project Files and Main Subprograms

When using a project file, you can invoke gnatmake with several main subprograms, by specifying their source files on the command line. Each of these needs to be an immediate source file of the project.

         gnatmake -Pprj main1 main2 main3
     

When using a project file, you can also invoke gnatmake without explicitly specifying any main, and the effect depends on whether you have defined the Main attribute. This attribute has a string list value, where each element in the list is the name of a source file (the file extension is optional) containing a main subprogram.

If the Main attribute is defined in a project file as a non-empty string list and the switch -u is not used on the command line, then invoking gnatmake with this project file but without any main on the command line is equivalent to invoking gnatmake with all the file names in the Main attribute on the command line.

Example:

        project Prj is
           for Main use ("main1", "main2", "main3");
        end Prj;
     

With this project file, "gnatmake -Pprj" is equivalent to "gnatmake -Pprj main1 main2 main3".

When the project attribute Main is not specified, or is specified as an empty string list, or when the switch -u is used on the command line, then invoking gnatmake with no main on the command line will result in all immediate sources of the project file being checked, and potentially recompiled. Depending on the presence of the switch -u, sources from other project files on which the immediate sources of the main project file depend are also checked and potentially recompiled. In other words, the -u switch is applied to all of the immediate sources of themain project file.

10.13.2 The GNAT Driver and Project Files

A number of GNAT tools, other than gnatmake are project-aware: gnatbind, gnatfind, gnatlink, gnatls and gnatxref. However, none of these tools can be invoked directly with a project file switch (-P). They need to be invoke through the gnat driver.

The gnat driver is a front-end that accepts a number of commands and call the corresponding tool. It has been designed initially for VMS to convert VMS style qualifiers to Unix style switches, but it is now available to all the GNAT supported platforms.

On non VMS platforms, the gnat driver accepts the following commands (case insensitive):

• BIND to invoke gnatbind
• CHOP to invoke gnatchop
• COMP or COMPILE to invoke the compiler
• ELIM to invoke gnatelim
• FIND to invoke gnatfind
• KR or KRUNCH to invoke gnatkr
• LINK to invoke gnatlink
• LS or LIST to invoke gnatls
• MAKE to invoke gnatmake
• NAME to invoke gnatname
• PREP or PREPROCESS to invoke gnatprep
• PSTA or STANDARD to invoke gnatpsta
• STUB to invoke gnatstub
• XREF to invoke gnatxref

Note that the compiler is invoked using the command gnatmake -f -u.

Following the command, you may put switches and arguments for the invoked tool.

       gnat bind -C main.ali
       gnat ls -a main
       gnat chop foo.txt
     

In addition, for command BIND, FIND, LS or LIST, LINK and XREF, the project file related switches (-P, -X and -vPx) may be used in addition to the switches of the invoking tool.

For each of these command, there is possibly a package in the main project that corresponds to the invoked tool.

• package Binder for command BIND (invoking gnatbind)
• package Finder for command FIND (invoking gnatfind)
• package Gnatls for command LS or LIST (invoking gnatls)
• package Linker for command LINK (invoking gnatlink)
• package Cross_Reference for command XREF (invoking gnatlink)

Package Gnatls has a unique attribute Switches, a simple variable with a string list value. It contains switches for the invocation of gnatls.

     project Proj1 is
        package gnatls is
           for Switches use ("-a", "-v");
        end gnatls;
     end Proj1;
     

All other packages contains a switch Default_Switches, an associative array, indexed by the programming language (case insensitive) and having a string list value. Default_Switches ("Ada") contains the switches for the invocation of the tool corresponding to the package.

     project Proj is
     
        for Source_Dirs use ("./**");
     
        package gnatls is
           for Switches use ("-a", "-v");
        end gnatls;
     
        package Binder is
           for Default_Switches ("Ada") use ("-C", "-e");
        end Binder;
     
        package Linker is
           for Default_Switches ("Ada") use ("-C");
        end Linker;
     
        package Finder is
           for Default_Switches ("Ada") use ("-a", "-f");
        end Finder;
     
        package Cross_Reference is
           for Default_Switches ("Ada") use ("-a", "-f", "-d", "-u");
        end Cross_Reference;
     end Proj;
     

With the above project file, commands such as

        gnat ls -Pproj main
        gnat xref -Pproj main
        gnat bind -Pproj main.ali
     

will set up the environment properly and invoke the tool with the switches found in the package corresponding to the tool.

10.13.3 Glide and Project Files

Glide will automatically recognize the .gpr extension for project files, and will convert them to its own internal format automatically. However, it doesn't provide a syntax-oriented editor for modifying these files. The project file will be loaded as text when you select the menu item Ada => Project => Edit. You can edit this text and save the gpr file; when you next select this project file in Glide it will be automatically reloaded.

10.14 An Extended Example

Suppose that we have two programs, prog1 and prog2, with the sources in the respective directories. We would like to build them with a single gnatmake command, and we would like to place their object files into .build subdirectories of the source directories. Furthermore, we would like to have to have two separate subdirectories in .build - release and debug - which will contain the object files compiled with different set of compilation flags.

In other words, we have the following structure:

        main
          |- prog1
          |    |- .build
          |         | debug
          |         | release
          |- prog2
               |- .build
                    | debug
                    | release
     

Here are the project files that we need to create in a directory main to maintain this structure:

1. We create a Common project with a package Compiler that specifies the compilation switches:

             File "common.gpr":
             project Common is
             
                for Source_Dirs use (); -- No source files
             
                type Build_Type is ("release", "debug");
                Build : Build_Type := External ("BUILD", "debug");
                package Compiler is
                   case Build is
                      when "release" =>
                        for Default_Switches ("Ada") use ("-O2");
                      when "debug"   =>
                        for Default_Switches ("Ada") use ("-g");
                   end case;
                end Compiler;
             
             end Common;
             

2. We create separate projects for the two programs:

             File "prog1.gpr":
             
             with "common";
             project Prog1 is
             
                 for Source_Dirs use ("prog1");
                 for Object_Dir  use "prog1/.build/" & Common.Build;
             
                 package Compiler renames Common.Compiler;
             
             end Prog1;
             

             File "prog2.gpr":
             
             with "common";
             project Prog2 is
             
                 for Source_Dirs use ("prog2");
                 for Object_Dir  use "prog2/.build/" & Common.Build;
             
                 package Compiler renames Common.Compiler;
             end Prog2;
             

3. We create a wrapping project Main:

             File "main.gpr":
             
             with "common";
             with "prog1";
             with "prog2";
             project Main is
             
                package Compiler renames Common.Compiler;
             
             end Main;
             

4. Finally we need to create a dummy procedure that withs (either explicitly or implicitly) all the sources of our two programs.

Now we can build the programs using the command

        gnatmake -Pmain dummy
     

for the Debug mode, or

        gnatmake -Pmain -XBUILD=release
     

for the Release mode.

10.15 Project File Complete Syntax

     project ::=
       context_clause project_declaration
     
     context_clause ::=
       {with_clause}
     
     with_clause ::=
       with literal_string { , literal_string } ;
     
     project_declaration ::=
       project <project_>simple_name [ extends literal_string ] is
         {declarative_item}
       end <project_>simple_name;
     
     declarative_item ::=
       package_declaration |
       typed_string_declaration |
       other_declarative_item
     
     package_declaration ::=
       package <package_>simple_name package_completion
     
     package_completion ::=
       package_body | package_renaming
     
     package body ::=
       is
         {other_declarative_item}
       end <package_>simple_name ;
     
     package_renaming ::==
       renames <project_>simple_name.<package_>simple_name ;
     
     typed_string_declaration ::=
       type <typed_string_>_simple_name is
        ( literal_string {, literal_string} );
     
     other_declarative_item ::=
       attribute_declaration |
       typed_variable_declaration |
       variable_declaration |
       case_construction
     
     attribute_declaration ::=
       for attribute use expression ;
     
     attribute ::=
       <simple_attribute_>simple_name |
       <associative_array_attribute_>simple_name ( literal_string )
     
     typed_variable_declaration ::=
       <typed_variable_>simple_name : <typed_string_>name :=  string_expression ;
     
     variable_declaration ::=
       <variable_>simple_name := expression;
     
     expression ::=
       term {& term}
     
     term ::=
       literal_string |
       string_list |
       <variable_>name |
       external_value |
       attribute_reference
     
     literal_string ::=
       (same as Ada)
     
     string_list ::=
       ( <string_>expression { , <string_>expression } )
     
     external_value ::=
       external ( literal_string [, literal_string] )
     
     attribute_reference ::=
       attribute_parent ' <simple_attribute_>simple_name [ ( literal_string ) ]
     
     attribute_parent ::=
       project |
       <project_or_package>simple_name |
       <project_>simple_name . <package_>simple_name
     
     case_construction ::=
       case <typed_variable_>name is
         {case_item}
       end case ;
     
     case_item ::=
       when discrete_choice_list => {case_construction | attribute_declaration}
     
     discrete_choice_list ::=
       literal_string {| literal_string}
     
     name ::=
       simple_name {. simple_name}
     
     simple_name ::=
       identifier (same as Ada)
     
     

11 Elaboration Order Handling in GNAT

• Elaboration Code in Ada 95:
• Checking the Elaboration Order in Ada 95:
• Controlling the Elaboration Order in Ada 95:
• Controlling Elaboration in GNAT - Internal Calls:
• Controlling Elaboration in GNAT - External Calls:
• Default Behavior in GNAT - Ensuring Safety:
• Elaboration Issues for Library Tasks:
• Mixing Elaboration Models:
• What to Do If the Default Elaboration Behavior Fails:
• Elaboration for Access-to-Subprogram Values:
• Summary of Procedures for Elaboration Control:
• Other Elaboration Order Considerations:

This chapter describes the handling of elaboration code in Ada 95 and in GNAT, and discusses how the order of elaboration of program units can be controlled in GNAT, either automatically or with explicit programming features.

11.1 Elaboration Code in Ada 95

Ada 95 provides rather general mechanisms for executing code at elaboration time, that is to say before the main program starts executing. Such code arises in three contexts:

Initializers for variables.

Variables declared at the library level, in package specs or bodies, can require initialization that is performed at elaboration time, as in:

          

          Sqrt_Half : Float := Sqrt (0.5);
          
          

Package initialization code

Code in a BEGIN-END section at the outer level of a package body is executed as part of the package body elaboration code.

Library level task allocators

Tasks that are declared using task allocators at the library level start executing immediately and hence can execute at elaboration time.

Subprogram calls are possible in any of these contexts, which means that any arbitrary part of the program may be executed as part of the elaboration code. It is even possible to write a program which does all its work at elaboration time, with a null main program, although stylistically this would usually be considered an inappropriate way to structure a program.

An important concern arises in the context of elaboration code: we have to be sure that it is executed in an appropriate order. What we have is a series of elaboration code sections, potentially one section for each unit in the program. It is important that these execute in the correct order. Correctness here means that, taking the above example of the declaration of Sqrt_Half, if some other piece of elaboration code references Sqrt_Half, then it must run after the section of elaboration code that contains the declaration of Sqrt_Half.

There would never be any order of elaboration problem if we made a rule that whenever you with a unit, you must elaborate both the spec and body of that unit before elaborating the unit doing the with'ing:

     

     with Unit_1;
     package Unit_2 is ...
     
     

would require that both the body and spec of Unit_1 be elaborated before the spec of Unit_2. However, a rule like that would be far too restrictive. In particular, it would make it impossible to have routines in separate packages that were mutually recursive.

You might think that a clever enough compiler could look at the actual elaboration code and determine an appropriate correct order of elaboration, but in the general case, this is not possible. Consider the following example.

In the body of Unit_1, we have a procedure Func_1 that references the variable Sqrt_1, which is declared in the elaboration code of the body of Unit_1:

     

     Sqrt_1 : Float := Sqrt (0.1);
     
     

The elaboration code of the body of Unit_1 also contains:

     

     if expression_1 = 1 then
        Q := Unit_2.Func_2;
     end if;
     
     

Unit_2 is exactly parallel, it has a procedure Func_2 that references the variable Sqrt_2, which is declared in the elaboration code of the body Unit_2:

     

     Sqrt_2 : Float := Sqrt (0.1);
     
     

The elaboration code of the body of Unit_2 also contains:

     

     if expression_2 = 2 then
        Q := Unit_1.Func_1;
     end if;
     
     

Now the question is, which of the following orders of elaboration is acceptable:

     Spec of Unit_1
     Spec of Unit_2
     Body of Unit_1
     Body of Unit_2
     

or

     Spec of Unit_2
     Spec of Unit_1
     Body of Unit_2
     Body of Unit_1
     

If you carefully analyze the flow here, you will see that you cannot tell at compile time the answer to this question. If expression_1 is not equal to 1, and expression_2 is not equal to 2, then either order is acceptable, because neither of the function calls is executed. If both tests evaluate to true, then neither order is acceptable and in fact there is no correct order.

If one of the two expressions is true, and the other is false, then one of the above orders is correct, and the other is incorrect. For example, if expression_1 = 1 and expression_2 /= 2, then the call to Func_2 will occur, but not the call to Func_1. This means that it is essential to elaborate the body of Unit_1 before the body of Unit_2, so the first order of elaboration is correct and the second is wrong.

By making expression_1 and expression_2 depend on input data, or perhaps the time of day, we can make it impossible for the compiler or binder to figure out which of these expressions will be true, and hence it is impossible to guarantee a safe order of elaboration at run time.

11.2 Checking the Elaboration Order in Ada 95

In some languages that involve the same kind of elaboration problems, e.g. Java and C++, the programmer is expected to worry about these ordering problems himself, and it is common to write a program in which an incorrect elaboration order gives surprising results, because it references variables before they are initialized. Ada 95 is designed to be a safe language, and a programmer-beware approach is clearly not sufficient. Consequently, the language provides three lines of defense:

Standard rules

Some standard rules restrict the possible choice of elaboration order. In particular, if you with a unit, then its spec is always elaborated before the unit doing the with. Similarly, a parent spec is always elaborated before the child spec, and finally a spec is always elaborated before its corresponding body.

Dynamic elaboration checks

Dynamic checks are made at run time, so that if some entity is accessed before it is elaborated (typically by means of a subprogram call) then the exception (Program_Error) is raised.

Elaboration control

Facilities are provided for the programmer to specify the desired order of elaboration.

Let's look at these facilities in more detail. First, the rules for dynamic checking. One possible rule would be simply to say that the exception is raised if you access a variable which has not yet been elaborated. The trouble with this approach is that it could require expensive checks on every variable reference. Instead Ada 95 has two rules which are a little more restrictive, but easier to check, and easier to state:

Restrictions on calls

A subprogram can only be called at elaboration time if its body has been elaborated. The rules for elaboration given above guarantee that the spec of the subprogram has been elaborated before the call, but not the body. If this rule is violated, then the exception Program_Error is raised.

Restrictions on instantiations

A generic unit can only be instantiated if the body of the generic unit has been elaborated. Again, the rules for elaboration given above guarantee that the spec of the generic unit has been elaborated before the instantiation, but not the body. If this rule is violated, then the exception Program_Error is raised.

The idea is that if the body has been elaborated, then any variables it references must have been elaborated; by checking for the body being elaborated we guarantee that none of its references causes any trouble. As we noted above, this is a little too restrictive, because a subprogram that has no non-local references in its body may in fact be safe to call. However, it really would be unsafe to rely on this, because it would mean that the caller was aware of details of the implementation in the body. This goes against the basic tenets of Ada.

A plausible implementation can be described as follows. A Boolean variable is associated with each subprogram and each generic unit. This variable is initialized to False, and is set to True at the point body is elaborated. Every call or instantiation checks the variable, and raises Program_Error if the variable is False.

Note that one might think that it would be good enough to have one Boolean variable for each package, but that would not deal with cases of trying to call a body in the same package as the call that has not been elaborated yet. Of course a compiler may be able to do enough analysis to optimize away some of the Boolean variables as unnecessary, and GNAT indeed does such optimizations, but still the easiest conceptual model is to think of there being one variable per subprogram.

11.3 Controlling the Elaboration Order in Ada 95

In the previous section we discussed the rules in Ada 95 which ensure that Program_Error is raised if an incorrect elaboration order is chosen. This prevents erroneous executions, but we need mechanisms to specify a correct execution and avoid the exception altogether. To achieve this, Ada 95 provides a number of features for controlling the order of elaboration. We discuss these features in this section.

First, there are several ways of indicating to the compiler that a given unit has no elaboration problems:

packages that do not require a body

In Ada 95, a library package that does not require a body does not permit a body. This means that if we have a such a package, as in:

          

          package Definitions is
             generic
                type m is new integer;
             package Subp is
                type a is array (1 .. 10) of m;
                type b is array (1 .. 20) of m;
             end Subp;
          end Definitions;
          
          

A package that with's Definitions may safely instantiate Definitions.Subp because the compiler can determine that there definitely is no package body to worry about in this case

pragma Pure

Places sufficient restrictions on a unit to guarantee that no call to any subprogram in the unit can result in an elaboration problem. This means that the compiler does not need to worry about the point of elaboration of such units, and in particular, does not need to check any calls to any subprograms in this unit.

pragma Preelaborate

This pragma places slightly less stringent restrictions on a unit than does pragma Pure, but these restrictions are still sufficient to ensure that there are no elaboration problems with any calls to the unit.

pragma Elaborate_Body

This pragma requires that the body of a unit be elaborated immediately after its spec. Suppose a unit A has such a pragma, and unit B does a with of unit A. Recall that the standard rules require the spec of unit A to be elaborated before the with'ing unit; given the pragma in A, we also know that the body of A will be elaborated before B, so that calls to A are safe and do not need a check.

Note that, unlike pragma Pure and pragma Preelaborate, the use of Elaborate_Body does not guarantee that the program is free of elaboration problems, because it may not be possible to satisfy the requested elaboration order. Let's go back to the example with Unit_1 and Unit_2. If a programmer marks Unit_1 as Elaborate_Body, and not Unit_2, then the order of elaboration will be:

     Spec of Unit_2
     Spec of Unit_1
     Body of Unit_1
     Body of Unit_2
     

Now that means that the call to Func_1 in Unit_2 need not be checked, it must be safe. But the call to Func_2 in Unit_1 may still fail if Expression_1 is equal to 1, and the programmer must still take responsibility for this not being the case.

If all units carry a pragma Elaborate_Body, then all problems are eliminated, except for calls entirely within a body, which are in any case fully under programmer control. However, using the pragma everywhere is not always possible. In particular, for our Unit_1/Unit_2 example, if we marked both of them as having pragma Elaborate_Body, then clearly there would be no possible elaboration order.

The above pragmas allow a server to guarantee safe use by clients, and clearly this is the preferable approach. Consequently a good rule in Ada 95 is to mark units as Pure or Preelaborate if possible, and if this is not possible, mark them as Elaborate_Body if possible. As we have seen, there are situations where neither of these three pragmas can be used. So we also provide methods for clients to control the order of elaboration of the servers on which they depend:

pragma Elaborate (unit)

This pragma is placed in the context clause, after a with clause, and it requires that the body of the named unit be elaborated before the unit in which the pragma occurs. The idea is to use this pragma if the current unit calls at elaboration time, directly or indirectly, some subprogram in the named unit.

pragma Elaborate_All (unit)

This is a stronger version of the Elaborate pragma. Consider the following example:

          Unit A with's unit B and calls B.Func in elab code
          Unit B with's unit C, and B.Func calls C.Func
          

Now if we put a pragma Elaborate (B) in unit A, this ensures that the body of B is elaborated before the call, but not the body of C, so the call to C.Func could still cause Program_Error to be raised.

The effect of a pragma Elaborate_All is stronger, it requires not only that the body of the named unit be elaborated before the unit doing the with, but also the bodies of all units that the named unit uses, following with links transitively. For example, if we put a pragma Elaborate_All (B) in unit A, then it requires not only that the body of B be elaborated before A, but also the body of C, because B with's C.

We are now in a position to give a usage rule in Ada 95 for avoiding elaboration problems, at least if dynamic dispatching and access to subprogram values are not used. We will handle these cases separately later.

The rule is simple. If a unit has elaboration code that can directly or indirectly make a call to a subprogram in a with'ed unit, or instantiate a generic unit in a with'ed unit, then if the with'ed unit does not have pragma Pure or Preelaborate, then the client should have a pragma Elaborate_All for the with'ed unit. By following this rule a client is assured that calls can be made without risk of an exception. If this rule is not followed, then a program may be in one of four states:

No order exists

No order of elaboration exists which follows the rules, taking into account any Elaborate, Elaborate_All, or Elaborate_Body pragmas. In this case, an Ada 95 compiler must diagnose the situation at bind time, and refuse to build an executable program.

One or more orders exist, all incorrect

One or more acceptable elaboration orders exists, and all of them generate an elaboration order problem. In this case, the binder can build an executable program, but Program_Error will be raised when the program is run.

Several orders exist, some right, some incorrect

One or more acceptable elaboration orders exists, and some of them work, and some do not. The programmer has not controlled the order of elaboration, so the binder may or may not pick one of the correct orders, and the program may or may not raise an exception when it is run. This is the worst case, because it means that the program may fail when moved to another compiler, or even another version of the same compiler.

One or more orders exists, all correct

One ore more acceptable elaboration orders exist, and all of them work. In this case the program runs successfully. This state of affairs can be guaranteed by following the rule we gave above, but may be true even if the rule is not followed.

Note that one additional advantage of following our Elaborate_All rule is that the program continues to stay in the ideal (all orders OK) state even if maintenance changes some bodies of some subprograms. Conversely, if a program that does not follow this rule happens to be safe at some point, this state of affairs may deteriorate silently as a result of maintenance changes.

You may have noticed that the above discussion did not mention the use of Elaborate_Body. This was a deliberate omission. If you with an Elaborate_Body unit, it still may be the case that code in the body makes calls to some other unit, so it is still necessary to use Elaborate_All on such units.

11.4 Controlling Elaboration in GNAT - Internal Calls

In the case of internal calls, i.e. calls within a single package, the programmer has full control over the order of elaboration, and it is up to the programmer to elaborate declarations in an appropriate order. For example writing:

     

     function One return Float;
     
     Q : Float := One;
     
     function One return Float is
     begin
          return 1.0;
     end One;
     
     

will obviously raise Program_Error at run time, because function One will be called before its body is elaborated. In this case GNAT will generate a warning that the call will raise Program_Error:

     

      1. procedure y is
      2.    function One return Float;
      3.
      4.    Q : Float := One;
                         |
         >>> warning: cannot call "One" before body is elaborated
         >>> warning: Program_Error will be raised at run time
     
      5.
      6.    function One return Float is
      7.    begin
      8.         return 1.0;
      9.    end One;
     10.
     11. begin
     12.    null;
     13. end;
     
     

Note that in this particular case, it is likely that the call is safe, because the function One does not access any global variables. Nevertheless in Ada 95, we do not want the validity of the check to depend on the contents of the body (think about the separate compilation case), so this is still wrong, as we discussed in the previous sections.

The error is easily corrected by rearranging the declarations so that the body of One appears before the declaration containing the call (note that in Ada 95, declarations can appear in any order, so there is no restriction that would prevent this reordering, and if we write:

     

     function One return Float;
     
     function One return Float is
     begin
          return 1.0;
     end One;
     
     Q : Float := One;
     
     

then all is well, no warning is generated, and no Program_Error exception will be raised. Things are more complicated when a chain of subprograms is executed:

     

     function A return Integer;
     function B return Integer;
     function C return Integer;
     
     function B return Integer is begin return A; end;
     function C return Integer is begin return B; end;
     
     X : Integer := C;
     
     function A return Integer is begin return 1; end;
     
     

Now the call to C at elaboration time in the declaration of X is correct, because the body of C is already elaborated, and the call to B within the body of C is correct, but the call to A within the body of B is incorrect, because the body of A has not been elaborated, so Program_Error will be raised on the call to A. In this case GNAT will generate a warning that Program_Error may be raised at the point of the call. Let's look at the warning:

     

      1. procedure x is
      2.    function A return Integer;
      3.    function B return Integer;
      4.    function C return Integer;
      5.
      6.    function B return Integer is begin return A; end;
                                                         |
         >>> warning: call to "A" before body is elaborated may
                      raise Program_Error
         >>> warning: "B" called at line 7
         >>> warning: "C" called at line 9
     
      7.    function C return Integer is begin return B; end;
      8.
      9.    X : Integer := C;
     10.
     11.    function A return Integer is begin return 1; end;
     12.
     13. begin
     14.    null;
     15. end;
     
     

Note that the message here says "may raise", instead of the direct case, where the message says "will be raised". That's because whether A is actually called depends in general on run-time flow of control. For example, if the body of B said

     

     function B return Integer is
     begin
        if some-condition-depending-on-input-data then
           return A;
        else
           return 1;
        end if;
     end B;
     
     

then we could not know until run time whether the incorrect call to A would actually occur, so Program_Error might or might not be raised. It is possible for a compiler to do a better job of analyzing bodies, to determine whether or not Program_Error might be raised, but it certainly couldn't do a perfect job (that would require solving the halting problem and is provably impossible), and because this is a warning anyway, it does not seem worth the effort to do the analysis. Cases in which it would be relevant are rare.

In practice, warnings of either of the forms given above will usually correspond to real errors, and should be examined carefully and eliminated. In the rare case where a warning is bogus, it can be suppressed by any of the following methods:

• Compile with the -gnatws switch set
• Suppress Elaboration_Checks for the called subprogram
• Use pragma Warnings_Off to turn warnings off for the call

For the internal elaboration check case, GNAT by default generates the necessary run-time checks to ensure that Program_Error is raised if any call fails an elaboration check. Of course this can only happen if a warning has been issued as described above. The use of pragma Suppress (Elaboration_Checks) may (but is not guaranteed to) suppress some of these checks, meaning that it may be possible (but is not guaranteed) for a program to be able to call a subprogram whose body is not yet elaborated, without raising a Program_Error exception.

11.5 Controlling Elaboration in GNAT - External Calls

The previous section discussed the case in which the execution of a particular thread of elaboration code occurred entirely within a single unit. This is the easy case to handle, because a programmer has direct and total control over the order of elaboration, and furthermore, checks need only be generated in cases which are rare and which the compiler can easily detect. The situation is more complex when separate compilation is taken into account. Consider the following:

     

     
package Math is
        function Sqrt (Arg : Float) return Float;
     end Math;
     
     package body Math is
        function Sqrt (Arg : Float) return Float is
        begin
              ...
        end Sqrt;
     end Math;

     
with Math;
     package Stuff is
        X : Float := Math.Sqrt (0.5);
     end Stuff;
     
     with Stuff;
     procedure Main is
     begin
        ...
     end Main;
     
     

where Main is the main program. When this program is executed, the elaboration code must first be executed, and one of the jobs of the binder is to determine the order in which the units of a program are to be elaborated. In this case we have four units: the spec and body of Math, the spec of Stuff and the body of Main). In what order should the four separate sections of elaboration code be executed?

There are some restrictions in the order of elaboration that the binder can choose. In particular, if unit U has a with for a package X, then you are assured that the spec of X is elaborated before U , but you are not assured that the body of X is elaborated before U. This means that in the above case, the binder is allowed to choose the order:

     spec of Math
     spec of Stuff
     body of Math
     body of Main
     

but that's not good, because now the call to Math.Sqrt that happens during the elaboration of the Stuff spec happens before the body of Math.Sqrt is elaborated, and hence causes Program_Error exception to be raised. At first glance, one might say that the binder is misbehaving, because obviously you want to elaborate the body of something you with first, but that is not a general rule that can be followed in all cases. Consider

     

     package X is ...
     
     package Y is ...
     
     with X;
     package body Y is ...
     
     with Y;
     package body X is ...
     
     

This is a common arrangement, and, apart from the order of elaboration problems that might arise in connection with elaboration code, this works fine. A rule that says that you must first elaborate the body of anything you with cannot work in this case: the body of X with's Y, which means you would have to elaborate the body of Y first, but that with's X, which means you have to elaborate the body of X first, but ... and we have a loop that cannot be broken.

It is true that the binder can in many cases guess an order of elaboration that is unlikely to cause a Program_Error exception to be raised, and it tries to do so (in the above example of Math/Stuff/Spec, the GNAT binder will by default elaborate the body of Math right after its spec, so all will be well).

However, a program that blindly relies on the binder to be helpful can get into trouble, as we discussed in the previous sections, so GNAT provides a number of facilities for assisting the programmer in developing programs that are robust with respect to elaboration order.

11.6 Default Behavior in GNAT - Ensuring Safety

The default behavior in GNAT ensures elaboration safety. In its default mode GNAT implements the rule we previously described as the right approach. Let's restate it:

• If a unit has elaboration code that can directly or indirectly make a call to a subprogram in a with'ed unit, or instantiate a generic unit in a with'ed unit, then if the with'ed unit does not have pragma Pure or Preelaborate, then the client should have an Elaborate_All for the with'ed unit.

By following this rule a client is assured that calls and instantiations can be made without risk of an exception.

In this mode GNAT traces all calls that are potentially made from elaboration code, and puts in any missing implicit Elaborate_All pragmas. The advantage of this approach is that no elaboration problems are possible if the binder can find an elaboration order that is consistent with these implicit Elaborate_All pragmas. The disadvantage of this approach is that no such order may exist.

If the binder does not generate any diagnostics, then it means that it has found an elaboration order that is guaranteed to be safe. However, the binder may still be relying on implicitly generated Elaborate_All pragmas so portability to other compilers than GNAT is not guaranteed.

If it is important to guarantee portability, then the compilations should use the -gnatwl (warn on elaboration problems) switch. This will cause warning messages to be generated indicating the missing Elaborate_All pragmas. Consider the following source program:

     

     with k;
     package j is
       m : integer := k.r;
     end;
     
     

where it is clear that there should be a pragma Elaborate_All for unit k. An implicit pragma will be generated, and it is likely that the binder will be able to honor it. However, it is safer to include the pragma explicitly in the source. If this unit is compiled with the -gnatwl switch, then the compiler outputs a warning:

     

     1. with k;
     2. package j is
     3.   m : integer := k.r;
                          |
        >>> warning: call to "r" may raise Program_Error
        >>> warning: missing pragma Elaborate_All for "k"
     
     4. end;
     
     

and these warnings can be used as a guide for supplying manually the missing pragmas.

This default mode is more restrictive than the Ada Reference Manual, and it is possible to construct programs which will compile using the dynamic model described there, but will run into a circularity using the safer static model we have described.

Of course any Ada compiler must be able to operate in a mode consistent with the requirements of the Ada Reference Manual, and in particular must have the capability of implementing the standard dynamic model of elaboration with run-time checks.

In GNAT, this standard mode can be achieved either by the use of the -gnatE switch on the compiler (gcc or gnatmake) command, or by the use of the configuration pragma:

     pragma Elaboration_Checks (RM);
     

Either approach will cause the unit affected to be compiled using the standard dynamic run-time elaboration checks described in the Ada Reference Manual. The static model is generally preferable, since it is clearly safer to rely on compile and link time checks rather than run-time checks. However, in the case of legacy code, it may be difficult to meet the requirements of the static model. This issue is further discussed in What to Do If the Default Elaboration Behavior Fails.

Note that the static model provides a strict subset of the allowed behavior and programs of the Ada Reference Manual, so if you do adhere to the static model and no circularities exist, then you are assured that your program will work using the dynamic model.

11.7 Elaboration Issues for Library Tasks

In this section we examine special elaboration issues that arise for programs that declare library level tasks.

Generally the model of execution of an Ada program is that all units are elaborated, and then execution of the program starts. However, the declaration of library tasks definitely does not fit this model. The reason for this is that library tasks start as soon as they are declared (more precisely, as soon as the statement part of the enclosing package body is reached), that is to say before elaboration of the program is complete. This means that if such a task calls a subprogram, or an entry in another task, the callee may or may not be elaborated yet, and in the standard Reference Manual model of dynamic elaboration checks, you can even get timing dependent Program_Error exceptions, since there can be a race between the elaboration code and the task code.

The static model of elaboration in GNAT seeks to avoid all such dynamic behavior, by being conservative, and the conservative approach in this particular case is to assume that all the code in a task body is potentially executed at elaboration time if a task is declared at the library level.

This can definitely result in unexpected circularities. Consider the following example

     package Decls is
       task Lib_Task is
          entry Start;
       end Lib_Task;
     
       type My_Int is new Integer;
     
       function Ident (M : My_Int) return My_Int;
     end Decls;
     
     with Utils;
     package body Decls is
       task body Lib_Task is
       begin
          accept Start;
          Utils.Put_Val (2);
       end Lib_Task;
     
       function Ident (M : My_Int) return My_Int is
       begin
          return M;
       end Ident;
     end Decls;
     
     with Decls;
     package Utils is
       procedure Put_Val (Arg : Decls.My_Int);
     end Utils;
     
     with Text_IO;
     package body Utils is
       procedure Put_Val (Arg : Decls.My_Int) is
       begin
          Text_IO.Put_Line (Decls.My_Int'Image (Decls.Ident (Arg)));
       end Put_Val;
     end Utils;
     
     with Decls;
     procedure Main is
     begin
        Decls.Lib_Task.Start;
     end;
     

If the above example is compiled in the default static elaboration mode, then a circularity occurs. The circularity comes from the call Utils.Put_Val in the task body of Decls.Lib_Task. Since this call occurs in elaboration code, we need an implicit pragma Elaborate_All for Utils. This means that not only must the spec and body of Utils be elaborated before the body of Decls, but also the spec and body of any unit that is with'ed by the body of Utils must also be elaborated before the body of Decls. This is the transitive implication of pragma Elaborate_All and it makes sense, because in general the body of Put_Val might have a call to something in a with'ed unit.

In this case, the body of Utils (actually its spec) with's Decls. Unfortunately this means that the body of Decls must be elaborated before itself, in case there is a call from the body of Utils.

Here is the exact chain of events we are worrying about:

1. In the body of Decls a call is made from within the body of a library task to a subprogram in the package Utils. Since this call may occur at elaboration time (given that the task is activated at elaboration time), we have to assume the worst, i.e. that the call does happen at elaboration time.
2. This means that the body and spec of Util must be elaborated before the body of Decls so that this call does not cause an access before elaboration.
3. Within the body of Util, specifically within the body of Util.Put_Val there may be calls to any unit with'ed by this package.
4. One such with'ed package is package Decls, so there might be a call to a subprogram in Decls in Put_Val. In fact there is such a call in this example, but we would have to assume that there was such a call even if it were not there, since we are not supposed to write the body of Decls knowing what is in the body of Utils; certainly in the case of the static elaboration model, the compiler does not know what is in other bodies and must assume the worst.
5. This means that the spec and body of Decls must also be elaborated before we elaborate the unit containing the call, but that unit is Decls! This means that the body of Decls must be elaborated before itself, and that's a circularity.

Indeed, if you add an explicit pragma Elaborate_All for Utils in the body of Decls you will get a true Ada Reference Manual circularity that makes the program illegal.

In practice, we have found that problems with the static model of elaboration in existing code often arise from library tasks, so we must address this particular situation.

Note that if we compile and run the program above, using the dynamic model of elaboration (that is to say use the -gnatE switch), then it compiles, binds, links, and runs, printing the expected result of 2. Therefore in some sense the circularity here is only apparent, and we need to capture the properties of this program that distinguish it from other library-level tasks that have real elaboration problems.

We have four possible answers to this question:

• Use the dynamic model of elaboration.

  If we use the -gnatE switch, then as noted above, the program works. Why is this? If we examine the task body, it is apparent that the task cannot proceed past the accept statement until after elaboration has been completed, because the corresponding entry call comes from the main program, not earlier. This is why the dynamic model works here. But that's really giving up on a precise analysis, and we prefer to take this approach only if we cannot solve the problem in any other manner. So let us examine two ways to reorganize the program to avoid the potential elaboration problem.

• Split library tasks into separate packages.

  Write separate packages, so that library tasks are isolated from other declarations as much as possible. Let us look at a variation on the above program.

            package Decls1 is
              task Lib_Task is
                 entry Start;
              end Lib_Task;
            end Decls1;
            
            with Utils;
            package body Decls1 is
              task body Lib_Task is
              begin
                 accept Start;
                 Utils.Put_Val (2);
              end Lib_Task;
            end Decls1;
            
            package Decls2 is
              type My_Int is new Integer;
              function Ident (M : My_Int) return My_Int;
            end Decls2;
            
            with Utils;
            package body Decls2 is
              function Ident (M : My_Int) return My_Int is
              begin
                 return M;
              end Ident;
            end Decls2;
            
            with Decls2;
            package Utils is
              procedure Put_Val (Arg : Decls2.My_Int);
            end Utils;
            
            with Text_IO;
            package body Utils is
              procedure Put_Val (Arg : Decls2.My_Int) is
              begin
                 Text_IO.Put_Line (Decls2.My_Int'Image (Decls2.Ident (Arg)));
              end Put_Val;
            end Utils;
            
            with Decls1;
            procedure Main is
            begin
               Decls1.Lib_Task.Start;
            end;
            

  All we have done is to split Decls into two packages, one containing the library task, and one containing everything else. Now there is no cycle, and the program compiles, binds, links and executes using the default static model of elaboration.

• Declare separate task types.

  A significant part of the problem arises because of the use of the single task declaration form. This means that the elaboration of the task type, and the elaboration of the task itself (i.e. the creation of the task) happen at the same time. A good rule of style in Ada 95 is to always create explicit task types. By following the additional step of placing task objects in separate packages from the task type declaration, many elaboration problems are avoided. Here is another modified example of the example program:

            package Decls is
              task type Lib_Task_Type is
                 entry Start;
              end Lib_Task_Type;
            
              type My_Int is new Integer;
            
              function Ident (M : My_Int) return My_Int;
            end Decls;
            
            with Utils;
            package body Decls is
              task body Lib_Task_Type is
              begin
                 accept Start;
                 Utils.Put_Val (2);
              end Lib_Task_Type;
            
              function Ident (M : My_Int) return My_Int is
              begin
                 return M;
              end Ident;
            end Decls;
            
            with Decls;
            package Utils is
              procedure Put_Val (Arg : Decls.My_Int);
            end Utils;
            
            with Text_IO;
            package body Utils is
              procedure Put_Val (Arg : Decls.My_Int) is
              begin
                 Text_IO.Put_Line (Decls.My_Int'Image (Decls.Ident (Arg)));
              end Put_Val;
            end Utils;
            
            with Decls;
            package Declst is
               Lib_Task : Decls.Lib_Task_Type;
            end Declst;
            
            with Declst;
            procedure Main is
            begin
               Declst.Lib_Task.Start;
            end;
            

  What we have done here is to replace the task declaration in package Decls with a task type declaration. Then we introduce a separate package Declst to contain the actual task object. This separates the elaboration issues for the task type declaration, which causes no trouble, from the elaboration issues of the task object, which is also unproblematic, since it is now independent of the elaboration of Utils. This separation of concerns also corresponds to a generally sound engineering principle of separating declarations from instances. This version of the program also compiles, binds, links, and executes, generating the expected output.

• Use No_Entry_Calls_In_Elaboration_Code restriction.

  The previous two approaches described how a program can be restructured to avoid the special problems caused by library task bodies. in practice, however, such restructuring may be difficult to apply to existing legacy code, so we must consider solutions that do not require massive rewriting.

  Let us consider more carefully why our original sample program works under the dynamic model of elaboration. The reason is that the code in the task body blocks immediately on the accept statement. Now of course there is nothing to prohibit elaboration code from making entry calls (for example from another library level task), so we cannot tell in isolation that the task will not execute the accept statement during elaboration.

  However, in practice it is very unusual to see elaboration code make any entry calls, and the pattern of tasks starting at elaboration time and then immediately blocking on accept or select statements is very common. What this means is that the compiler is being too pessimistic when it analyzes the whole package body as though it might be executed at elaboration time.

  If we know that the elaboration code contains no entry calls, (a very safe assumption most of the time, that could almost be made the default behavior), then we can compile all units of the program under control of the following configuration pragma:

            pragma Restrictions (No_Entry_Calls_In_Elaboration_Code);
            

  This pragma can be placed in the gnat.adc file in the usual manner. If we take our original unmodified program and compile it in the presence of a gnat.adc containing the above pragma, then once again, we can compile, bind, link, and execute, obtaining the expected result. In the presence of this pragma, the compiler does not trace calls in a task body, that appear after the first accept or select statement, and therefore does not report a potential circularity in the original program.

  The compiler will check to the extent it can that the above restriction is not violated, but it is not always possible to do a complete check at compile time, so it is important to use this pragma only if the stated restriction is in fact met, that is to say no task receives an entry call before elaboration of all units is completed.

11.8 Mixing Elaboration Models

So far, we have assumed that the entire program is either compiled using the dynamic model or static model, ensuring consistency. It is possible to mix the two models, but rules have to be followed if this mixing is done to ensure that elaboration checks are not omitted.

The basic rule is that a unit compiled with the static model cannot be with'ed by a unit compiled with the dynamic model. The reason for this is that in the static model, a unit assumes that its clients guarantee to use (the equivalent of) pragma Elaborate_All so that no elaboration checks are required in inner subprograms, and this assumption is violated if the client is compiled with dynamic checks.

The precise rule is as follows. A unit that is compiled with dynamic checks can only with a unit that meets at least one of the following criteria:

• The with'ed unit is itself compiled with dynamic elaboration checks (that is with the -gnatE switch.
• The with'ed unit is an internal GNAT implementation unit from the System, Interfaces, Ada, or GNAT hierarchies.
• The with'ed unit has pragma Preelaborate or pragma Pure.
• The with'ing unit (that is the client) has an explicit pragma Elaborate_All for the with'ed unit.

If this rule is violated, that is if a unit with dynamic elaboration checks with's a unit that does not meet one of the above four criteria, then the binder (gnatbind) will issue a warning similar to that in the following example:

     warning: "x.ads" has dynamic elaboration checks and with's
     warning:   "y.ads" which has static elaboration checks
     

These warnings indicate that the rule has been violated, and that as a result elaboration checks may be missed in the resulting executable file. This warning may be suppressed using the -ws binder switch in the usual manner.

One useful application of this mixing rule is in the case of a subsystem which does not itself with units from the remainder of the application. In this case, the entire subsystem can be compiled with dynamic checks to resolve a circularity in the subsystem, while allowing the main application that uses this subsystem to be compiled using the more reliable default static model.

11.9 What to Do If the Default Elaboration Behavior Fails

If the binder cannot find an acceptable order, it outputs detailed diagnostics. For example:

     error: elaboration circularity detected
     info:   "proc (body)" must be elaborated before "pack (body)"
     info:     reason: Elaborate_All probably needed in unit "pack (body)"
     info:     recompile "pack (body)" with -gnatwl
     info:                             for full details
     info:       "proc (body)"
     info:         is needed by its spec:
     info:       "proc (spec)"
     info:         which is withed by:
     info:       "pack (body)"
     info:  "pack (body)" must be elaborated before "proc (body)"
     info:     reason: pragma Elaborate in unit "proc (body)"
     
     

In this case we have a cycle that the binder cannot break. On the one hand, there is an explicit pragma Elaborate in proc for pack. This means that the body of pack must be elaborated before the body of proc. On the other hand, there is elaboration code in pack that calls a subprogram in proc. This means that for maximum safety, there should really be a pragma Elaborate_All in pack for proc which would require that the body of proc be elaborated before the body of pack. Clearly both requirements cannot be satisfied. Faced with a circularity of this kind, you have three different options.

Fix the program

The most desirable option from the point of view of long-term maintenance is to rearrange the program so that the elaboration problems are avoided. One useful technique is to place the elaboration code into separate child packages. Another is to move some of the initialization code to explicitly called subprograms, where the program controls the order of initialization explicitly. Although this is the most desirable option, it may be impractical and involve too much modification, especially in the case of complex legacy code.

Perform dynamic checks

If the compilations are done using the -gnatE (dynamic elaboration check) switch, then GNAT behaves in a quite different manner. Dynamic checks are generated for all calls that could possibly result in raising an exception. With this switch, the compiler does not generate implicit Elaborate_All pragmas. The behavior then is exactly as specified in the Ada 95 Reference Manual. The binder will generate an executable program that may or may not raise Program_Error, and then it is the programmer's job to ensure that it does not raise an exception. Note that it is important to compile all units with the switch, it cannot be used selectively.

Suppress checks

The drawback of dynamic checks is that they generate a significant overhead at run time, both in space and time. If you are absolutely sure that your program cannot raise any elaboration exceptions, and you still want to use the dynamic elaboration model, then you can use the configuration pragma Suppress (Elaboration_Checks) to suppress all such checks. For example this pragma could be placed in the gnat.adc file.

Suppress checks selectively

When you know that certain calls in elaboration code cannot possibly lead to an elaboration error, and the binder nevertheless generates warnings on those calls and inserts Elaborate_All pragmas that lead to elaboration circularities, it is possible to remove those warnings locally and obtain a program that will bind. Clearly this can be unsafe, and it is the responsibility of the programmer to make sure that the resulting program has no elaboration anomalies. The pragma Suppress (Elaboration_Check) can be used with different granularity to suppress warnings and break elaboration circularities:

• Place the pragma that names the called subprogram in the declarative part that contains the call.
• Place the pragma in the declarative part, without naming an entity. This disables warnings on all calls in the corresponding declarative region.
• Place the pragma in the package spec that declares the called subprogram, and name the subprogram. This disables warnings on all elaboration calls to that subprogram.
• Place the pragma in the package spec that declares the called subprogram, without naming any entity. This disables warnings on all elaboration calls to all subprograms declared in this spec.

These four cases are listed in order of decreasing safety, and therefore require increasing programmer care in their application. Consider the following program:

          
          package Pack1 is
            function F1 return Integer;
            X1 : Integer;
          end Pack1;
          
          package Pack2 is
            function F2 return Integer;
            function Pure (x : integer) return integer;
            --  pragma Suppress (Elaboration_Check, On => Pure);  -- (3)
            --  pragma Suppress (Elaboration_Check);              -- (4)
          end Pack2;
          
          with Pack2;
          package body Pack1 is
            function F1 return Integer is
            begin
              return 100;
            end F1;
            Val : integer := Pack2.Pure (11);    --  Elab. call (1)
          begin
            declare
              --  pragma Suppress(Elaboration_Check, Pack2.F2);   -- (1)
              --  pragma Suppress(Elaboration_Check);             -- (2)
            begin
              X1 := Pack2.F2 + 1;                --  Elab. call (2)
            end;
          end Pack1;
          
          with Pack1;
          package body Pack2 is
            function F2 return Integer is
            begin
               return Pack1.F1;
            end F2;
            function Pure (x : integer) return integer is
            begin
               return x ** 3 - 3 * x;
            end;
          end Pack2;
          
          with Pack1, Ada.Text_IO;
          procedure Proc3 is
          begin
            Ada.Text_IO.Put_Line(Pack1.X1'Img); -- 101
          end Proc3;
          

In the absence of any pragmas, an attempt to bind this program produces the following diagnostics:

          error: elaboration circularity detected
          info:    "pack1 (body)" must be elaborated before "pack1 (body)"
          info:       reason: Elaborate_All probably needed in unit "pack1 (body)"
          info:       recompile "pack1 (body)" with -gnatwl for full details
          info:          "pack1 (body)"
          info:             must be elaborated along with its spec:
          info:          "pack1 (spec)"
          info:             which is withed by:
          info:          "pack2 (body)"
          info:             which must be elaborated along with its spec:
          info:          "pack2 (spec)"
          info:             which is withed by:
          info:          "pack1 (body)"
          

The sources of the circularity are the two calls to Pack2.Pure and Pack2.F2 in the body of Pack1. We can see that the call to F2 is safe, even though F2 calls F1, because the call appears after the elaboration of the body of F1. Therefore the pragma (1) is safe, and will remove the warning on the call. It is also possible to use pragma (2) because there are no other potentially unsafe calls in the block.

The call to Pure is safe because this function does not depend on the state of Pack2. Therefore any call to this function is safe, and it is correct to place pragma (3) in the corresponding package spec.

Finally, we could place pragma (4) in the spec of Pack2 to disable warnings on all calls to functions declared therein. Note that this is not necessarily safe, and requires more detailed examination of the subprogram bodies involved. In particular, a call to F2 requires that F1 be already elaborated.

It is hard to generalize on which of these four approaches should be taken. Obviously if it is possible to fix the program so that the default treatment works, this is preferable, but this may not always be practical. It is certainly simple enough to use -gnatE but the danger in this case is that, even if the GNAT binder finds a correct elaboration order, it may not always do so, and certainly a binder from another Ada compiler might not. A combination of testing and analysis (for which the warnings generated with the -gnatwl switch can be useful) must be used to ensure that the program is free of errors. One switch that is useful in this testing is the -p (pessimistic elaboration order) switch for gnatbind. Normally the binder tries to find an order that has the best chance of of avoiding elaboration problems. With this switch, the binder plays a devil's advocate role, and tries to choose the order that has the best chance of failing. If your program works even with this switch, then it has a better chance of being error free, but this is still not a guarantee.

For an example of this approach in action, consider the C-tests (executable tests) from the ACVC suite. If these are compiled and run with the default treatment, then all but one of them succeed without generating any error diagnostics from the binder. However, there is one test that fails, and this is not surprising, because the whole point of this test is to ensure that the compiler can handle cases where it is impossible to determine a correct order statically, and it checks that an exception is indeed raised at run time.

This one test must be compiled and run using the -gnatE switch, and then it passes. Alternatively, the entire suite can be run using this switch. It is never wrong to run with the dynamic elaboration switch if your code is correct, and we assume that the C-tests are indeed correct (it is less efficient, but efficiency is not a factor in running the ACVC tests.)

11.10 Elaboration for Access-to-Subprogram Values

The introduction of access-to-subprogram types in Ada 95 complicates the handling of elaboration. The trouble is that it becomes impossible to tell at compile time which procedure is being called. This means that it is not possible for the binder to analyze the elaboration requirements in this case.

If at the point at which the access value is created (i.e., the evaluation of P'Access for a subprogram P), the body of the subprogram is known to have been elaborated, then the access value is safe, and its use does not require a check. This may be achieved by appropriate arrangement of the order of declarations if the subprogram is in the current unit, or, if the subprogram is in another unit, by using pragma Pure, Preelaborate, or Elaborate_Body on the referenced unit.

If the referenced body is not known to have been elaborated at the point the access value is created, then any use of the access value must do a dynamic check, and this dynamic check will fail and raise a Program_Error exception if the body has not been elaborated yet. GNAT will generate the necessary checks, and in addition, if the -gnatwl switch is set, will generate warnings that such checks are required.

The use of dynamic dispatching for tagged types similarly generates a requirement for dynamic checks, and premature calls to any primitive operation of a tagged type before the body of the operation has been elaborated, will result in the raising of Program_Error.

11.11 Summary of Procedures for Elaboration Control

First, compile your program with the default options, using none of the special elaboration control switches. If the binder successfully binds your program, then you can be confident that, apart from issues raised by the use of access-to-subprogram types and dynamic dispatching, the program is free of elaboration errors. If it is important that the program be portable, then use the -gnatwl switch to generate warnings about missing Elaborate_All pragmas, and supply the missing pragmas.

If the program fails to bind using the default static elaboration handling, then you can fix the program to eliminate the binder message, or recompile the entire program with the -gnatE switch to generate dynamic elaboration checks, and, if you are sure there really are no elaboration problems, use a global pragma Suppress (Elaboration_Checks).

11.12 Other Elaboration Order Considerations

This section has been entirely concerned with the issue of finding a valid elaboration order, as defined by the Ada Reference Manual. In a case where several elaboration orders are valid, the task is to find one of the possible valid elaboration orders (and the static model in GNAT will ensure that this is achieved).

The purpose of the elaboration rules in the Ada Reference Manual is to make sure that no entity is accessed before it has been elaborated. For a subprogram, this means that the spec and body must have been elaborated before the subprogram is called. For an object, this means that the object must have been elaborated before its value is read or written. A violation of either of these two requirements is an access before elaboration order, and this section has been all about avoiding such errors.

In the case where more than one order of elaboration is possible, in the sense that access before elaboration errors are avoided, then any one of the orders is "correct" in the sense that it meets the requirements of the Ada Reference Manual, and no such error occurs.

However, it may be the case for a given program, that there are constraints on the order of elaboration that come not from consideration of avoiding elaboration errors, but rather from extra-lingual logic requirements. Consider this example:

     with Init_Constants;
     package Constants is
        X : Integer := 0;
        Y : Integer := 0;
     end Constants;
     
     package Init_Constants is
        procedure Calc;
     end Init_Constants;
     
     with Constants;
     package body Init_Constants is
        procedure Calc is begin null; end;
     begin
        Constants.X := 3;
        Constants.Y := 4;
     end Init_Constants;
     
     with Constants;
     package Calc is
        Z : Integer := Constants.X + Constants.Y;
     end Calc;
     
     with Calc;
     with Text_IO; use Text_IO;
     procedure Main is
     begin
        Put_Line (Calc.Z'Img);
     end Main;