4. Building Executable Programs with GNAT

This chapter describes first the gnatmake tool (Building with gnatmake), which automatically determines the set of sources needed by an Ada compilation unit and executes the necessary (re)compilations, binding and linking. It also explains how to use each tool individually: the compiler (gcc, see Compiling with gcc), binder (gnatbind, see Binding with gnatbind), and linker (gnatlink, see Linking with gnatlink) to build executable programs. Finally, this chapter provides examples of how to make use of the general GNU make mechanism in a GNAT context (see Using the GNU make Utility).

For building large systems with components possibly written in different languages (such as Ada, C, C++ and Fortran) and organized into subsystems and libraries, the GPRbuild tool can be used. This tool, and the Project Manager facility that it is based upon, is described in GPRbuild and GPR Companion Tools User’s Guide.

4.1. Building with gnatmake

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; and
  5. Test.

The third step in particular 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.

Note that for advanced forms of project structure, we recommend creating a project file as explained in the GNAT_Project_Manager chapter in the GPRbuild User’s Guide, and using the gprbuild tool which supports building with project files and works similarly to gnatmake.

4.1.1. 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).

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

4.1.2. Switches for gnatmake

You may specify any of the following switches to gnatmake:

--version
Display Copyright and version, then exit disregarding all other options.
--help
If --version was not used, display usage, then exit disregarding all other options.
--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. A limitation of this syntax is that the name and path name of the executable itself must not include any embedded spaces. 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. A limitation of this syntax is that the name and path name of the executable itself must not include any embedded spaces.
--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 limitation of this syntax is that the name and path name of the executable itself must not include any embedded spaces.
--create-map-file
When linking an executable, create a map file. The name of the map file has the same name as the executable with extension ”.map”.
--create-map-file=mapfile
When linking an executable, create a map file with the specified name.
--create-missing-dirs
When using project files (-Pproject), automatically create missing object directories, library directories and exec directories.
--single-compile-per-obj-dir
Disallow simultaneous compilations in the same object directory when project files are used.
--subdirs=subdir
Actual object directory of each project file is the subdirectory subdir of the object directory specified or defaulted in the project file.
--unchecked-shared-lib-imports
By default, shared library projects are not allowed to import static library projects. When this switch is used on the command line, this restriction is relaxed.
--source-info=source info file
Specify a source info file. This switch is active only when project files are used. If the source info file is specified as a relative path, then it is relative to the object directory of the main project. If the source info file does not exist, then after the Project Manager has successfully parsed and processed the project files and found the sources, it creates the source info file. If the source info file already exists and can be read successfully, then the Project Manager will get all the needed information about the sources from the source info file and will not look for them. This reduces the time to process the project files, especially when looking for sources that take a long time. If the source info file exists but cannot be parsed successfully, the Project Manager will attempt to recreate it. If the Project Manager fails to create the source info file, a message is issued, but gnatmake does not fail. gnatmake “trusts” the source info file. This means that if the source files have changed (addition, deletion, moving to a different source directory), then the source info file need to be deleted and recreated.
-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. The switch -a is also useful in conjunction with -f if you need to recompile an entire application, including run-time files, using special configuration pragmas, such as a Normalize_Scalars 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 temporary 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). A mapping file can make the compiler’s file searches faster, especially if there are many source directories, or the sources are read over a slow network connection. If -P is used, a mapping file is always used, so -C is unnecessary; in this case the mapping file is initially populated based on the project file. If -C is used without -P, the mapping file is initially empty. Each invocation of the compiler will add any newly accessed sources to the mapping file.
-C=file
Use a specific mapping file. The file, specified as a path name (absolute or relative) by this switch, should already exist, otherwise the switch is ineffective. The specified mapping file will be communicated to the compiler. This switch is not compatible with a project file (-P`file`) or with multiple compiling processes (-jnnn, when nnn is greater than 1).
-d

Display progress for each source, up to date or not, as a single line:

completed x out of y (zz%)

If the file needs to be compiled this is displayed after the invocation of the compiler. These lines are displayed even in quiet output mode.

-D dir

Put all object files and ALI file in directory dir. If the -D switch is not used, all object files and ALI files go in the current working directory.

This switch cannot be used when using a project file.

-eInnn
Indicates that the main source is a multi-unit source and the rank of the unit in the source file is nnn. nnn needs to be a positive number and a valid index in the source. This switch cannot be used when gnatmake is invoked for several mains.
-eL

Follow all symbolic links when processing project files. This should be used if your project uses symbolic links for files or directories, but is not needed in other cases.

This also assumes that no directory matches the naming scheme for files (for instance that you do not have a directory called “sources.ads” when using the default GNAT naming scheme).

When you do not have to use this switch (i.e., by default), gnatmake is able to save a lot of system calls (several per source file and object file), which can result in a significant speed up to load and manipulate a project file, especially when using source files from a remote system.

-eS
Output the commands for the compiler, the binder and the linker on standard output, instead of standard error.
-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.
-F
When using project files, if some errors or warnings are detected during parsing and verbose mode is not in effect (no use of switch -v), then error lines start with the full path name of the project file, rather than its simple file name.
-g
Enable debugging. This switch is simply passed to the compiler and to the linker.
-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 (see 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. If n is 0, then the maximum number of parallel compilations is the number of core processors on the platform. 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 needs 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
Specify 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.

-p
Same as --create-missing-dirs
-Pproject
Use project file project. Only one such switch can be used.
-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.

This switch is recommended when Integrated Preprocessing is used.

-u
Unique. Recompile at most the main files. It implies -c. Combined with -f, it is equivalent to calling the compiler directly. Note that using -u with a project file and no main has a special meaning.
-U
When used without a project file or with one or several mains on the command line, is equivalent to -u. When used with a project file and no main on the command line, all sources of all project files are checked and compiled if not up to date, and libraries are rebuilt, if necessary.
-v
Verbose. Display the reason for all recompilations gnatmake decides are necessary, with the highest verbosity level.
-vl
Verbosity level Low. Display fewer lines than in verbosity Medium.
-vm
Verbosity level Medium. Potentially display fewer lines than in verbosity High.
-vh
Verbosity level High. Equivalent to -v.
-vPx
Indicate the verbosity of the parsing of GNAT project files. See Switches Related to Project Files.
-x
Indicate that sources that are not part of any Project File may be compiled. Normally, when using Project Files, only sources that are part of a Project File may be compile. When this switch is used, a source outside of all Project Files may be compiled. The ALI file and the object file will be put in the object directory of the main Project. The compilation switches used will only be those specified on the command line. Even when -x is used, mains specified on the command line need to be sources of a project file.
-Xname=value
Indicate 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. Switches Related to Project Files.
-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

Any uppercase or multi-character switch that is not a gnatmake switch 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. Furthermore, under Windows, the sources pointed to by the libraries path set in the registry are not searched for.
-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. GNAT looks for the runtime in the following directories, and stops 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.

4.1.3. 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 gnatbind. 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 gnatlink. They will be passed on to all link steps performed by gnatmake.
-margs switches
Make switches. The switches are directly interpreted by gnatmake, regardless of any previous occurrence of -cargs, -bargs or -largs.

4.1.4. 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 foo.adb, where foo is a subunit or body of a generic unit, gnatmake recompiles foo.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 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 -aI`include-dir`  -aL`obj-dir`  main
    
  • Using gnatmake along with the -m (minimal recompilation) switch provides a mechanism for avoiding unnecessary recompilations. 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.

4.1.5. 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 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.

Note: when using non-standard naming conventions (Using Other File Names), changing through a configuration pragmas file the version of a source and invoking gnatmake to recompile may have no effect, if the previous version of the source is still accessible by gnatmake. It may be necessary to use the switch -f.

4.1.6. 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.

4.2. Compiling with gcc

This section 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.

4.2.1. 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):

cannot generate code for file ``fff`` (package spec)
to check package spec, use -gnatc

cannot generate code for file ``fff`` (missing subunits)
to check parent unit, use -gnatc

cannot generate code for file ``fff`` (subprogram spec)
to check subprogram spec, use -gnatc

cannot generate code for file ``fff`` (subunit)
to check subunit, use -gnatc

As indicated by the above error messages, if you want to submit one of these files to the compiler to check for correct semantics without generating code, then use the -gnatc switch.

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.

TESTING: the --foobarNN switch

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 two separate files to be compiled:

$ gcc -c x.adb y.adb

calls gnat1 (the Ada compiler) twice to compile x.adb and y.adb. The compiler generates two object files x.o and y.o and the two ALI files x.ali and y.ali.

Any switches apply to all the files listed, see Compiler Switches for a list of available gcc switches.

4.2.2. 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:

  • The directory containing the source file of the main unit being compiled (the file name on the command line).

  • Each directory named by an -I switch given on the gcc command line, in the order given.

  • Each of the directories listed in the text file whose name is given by the ADA_PRJ_INCLUDE_FILE environment variable. ADA_PRJ_INCLUDE_FILE is normally set by gnatmake or by the gnat driver when project files are used. It should not normally be set by other means.

  • 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).

  • 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 a 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 types, subprograms, etc. 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.

4.2.3. 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 (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 withs, 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.

4.2.4. 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.

4.3. Compiler Switches

The gcc command accepts switches that control the compilation process. These switches are fully described in this section: first an alphabetical listing of all switches with a brief description, and then functionally grouped sets of switches with more detailed information.

More switches exist for GCC than those documented here, especially for specific targets. However, their use is not recommended as they may change code generation in ways that are incompatible with the Ada run-time library, or can cause inconsistencies between compilation units.

4.3.1. Alphabetical List of All Switches

-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 “Options for Directory Search” section in the Using the GNU Compiler Collection (GCC) manual 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.

-fcallgraph-info[=su,da]
Makes the compiler output callgraph information for the program, on a per-file basis. The information is generated in the VCG format. It can be decorated with additional, per-node and/or per-edge information, if a list of comma-separated markers is additionally specified. When the su marker is specified, the callgraph is decorated with stack usage information; it is equivalent to -fstack-usage. When the da marker is specified, the callgraph is decorated with information about dynamically allocated objects.
-fdump-scos
Generates SCO (Source Coverage Obligation) information in the ALI file. This information is used by advanced coverage tools. See unit SCOs in the compiler sources for details in files scos.ads and scos.adb.
-flto[=n]
Enables Link Time Optimization. This switch must be used in conjunction with the -Ox switches (but not with the -gnatn switch since it is a full replacement for the latter) and instructs the compiler to defer most optimizations until the link stage. The advantage of this approach is that the compiler can do a whole-program analysis and choose the best interprocedural optimization strategy based on a complete view of the program, instead of a fragmentary view with the usual approach. This can also speed up the compilation of big programs and reduce the size of the executable, compared with a traditional per-unit compilation with inlining across modules enabled by the -gnatn switch. The drawback of this approach is that it may require more memory and that the debugging information generated by -g with it might be hardly usable. The switch, as well as the accompanying -Ox switches, must be specified both for the compilation and the link phases. If the n parameter is specified, the optimization and final code generation at link time are executed using n parallel jobs by means of an installed make program.
-fno-inline
Suppresses all inlining, unless requested with pragma Inline_Always. The effect is enforced regardless of other optimization or inlining switches. Note that inlining can also be suppressed on a finer-grained basis with pragma No_Inline.
-fno-inline-functions
Suppresses automatic inlining of subprograms, which is enabled if -O3 is used.
-fno-inline-small-functions
Suppresses automatic inlining of small subprograms, which is enabled if -O2 is used.
-fno-inline-functions-called-once
Suppresses inlining of subprograms local to the unit and called once from within it, which is enabled if -O1 is used.
-fno-ivopts
Suppresses high-level loop induction variable optimizations, which are enabled if -O1 is used. These optimizations are generally profitable but, for some specific cases of loops with numerous uses of the iteration variable that follow a common pattern, they may end up destroying the regularity that could be exploited at a lower level and thus producing inferior code.
-fno-strict-aliasing
Causes the compiler to avoid assumptions regarding non-aliasing of objects of different types. See Optimization and Strict Aliasing for details.
-fno-strict-overflow
Causes the compiler to avoid assumptions regarding the rules of signed integer overflow. These rules specify that signed integer overflow will result in a Constraint_Error exception at run time and are enforced in default mode by the compiler, so this switch should not be necessary in normal operating mode. It might be useful in conjunction with -gnato0 for very peculiar cases of low-level programming.
-fstack-check
Activates stack checking. See Stack Overflow Checking for details.
-fstack-usage
Makes the compiler output stack usage information for the program, on a per-subprogram basis. See Static Stack Usage Analysis for details.
-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.
-gnat05
Allow full Ada 2005 features.
-gnat12
Allow full Ada 2012 features.
-gnat2005
Allow full Ada 2005 features (same as -gnat05)
-gnat2012
Allow full Ada 2012 features (same as -gnat12)
-gnat83
Enforce Ada 83 restrictions.
-gnat95

Enforce Ada 95 restrictions.

Note: for compatibility with some Ada 95 compilers which support only the overriding keyword of Ada 2005, the -gnatd.D switch can be used along with -gnat95 to achieve a similar effect with GNAT.

-gnatd.D instructs GNAT to consider overriding as a keyword and handle its associated semantic checks, even in Ada 95 mode.

-gnata
Assertions enabled. Pragma Assert and pragma Debug to be activated. Note that these pragmas can also be controlled using the configuration pragmas Assertion_Policy and Debug_Policy. It also activates pragmas Check, Precondition, and Postcondition. Note that these pragmas can also be controlled using the configuration pragma Check_Policy. In Ada 2012, it also activates all assertions defined in the RM as aspects: preconditions, postconditions, type invariants and (sub)type predicates. In all Ada modes, corresponding pragmas for type invariants and (sub)type predicates are also activated. The default is that all these assertions are disabled, and have no effect, other than being checked for syntactic validity, and in the case of subtype predicates, constructions such as membership tests still test predicates even if assertions are turned off.
-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.
-gnatB
Assume no invalid (bad) values except for ‘Valid attribute use (Validity Checking).
-gnatc
Check syntax and semantics only (no code generation attempted). When the compiler is invoked by gnatmake, if the switch -gnatc is only given to the compiler (after -cargs or in package Compiler of the project file, gnatmake will fail because it will not find the object file after compilation. If gnatmake is called with -gnatc as a builder switch (before -cargs or in package Builder of the project file) then gnatmake will not fail because it will not look for the object files after compilation, and it will not try to build and link.
-gnatC
Generate CodePeer intermediate format (no code generation attempted). This switch will generate an intermediate representation suitable for use by CodePeer (.scil files). This switch is not compatible with code generation (it will, among other things, disable some switches such as -gnatn, and enable others such as -gnata).
-gnatd
Specify debug options for the compiler. The string of characters after the -gnatd specify the specific debug options. The possible characters are 0-9, a-z, A-Z, optionally preceded by a dot. See compiler source file debug.adb for details of the implemented debug options. Certain debug options are relevant to applications programmers, and these are documented at appropriate points in this users guide.
-gnatD
Create expanded source files for source level debugging. This switch also suppresses generation of cross-reference information (see -gnatx). Note that this switch is not allowed if a previous -gnatR switch has been given, since these two switches are not compatible.
-gnateA

Check that the actual parameters of a subprogram call are not aliases of one another. To qualify as aliasing, the actuals must denote objects of a composite type, their memory locations must be identical or overlapping, and at least one of the corresponding formal parameters must be of mode OUT or IN OUT.

type Rec_Typ is record
   Data : Integer := 0;
end record;

function Self (Val : Rec_Typ) return Rec_Typ is
begin
   return Val;
end Self;

procedure Detect_Aliasing (Val_1 : in out Rec_Typ; Val_2 : Rec_Typ) is
begin
   null;
end Detect_Aliasing;

Obj : Rec_Typ;

Detect_Aliasing (Obj, Obj);
Detect_Aliasing (Obj, Self (Obj));

In the example above, the first call to Detect_Aliasing fails with a Program_Error at runtime because the actuals for Val_1 and Val_2 denote the same object. The second call executes without raising an exception because Self(Obj) produces an anonymous object which does not share the memory location of Obj.

-gnatec=path
Specify a configuration pragma file (the equal sign is optional) (The Configuration Pragmas Files).
-gnateC
Generate CodePeer messages in a compiler-like format. This switch is only effective if -gnatcC is also specified and requires an installation of CodePeer.
-gnated
Disable atomic synchronization
-gnateDsymbol[=value]
Defines a symbol, associated with value, for preprocessing. (Integrated Preprocessing).
-gnateE
Generate extra information in exception messages. In particular, display extra column information and the value and range associated with index and range check failures, and extra column information for access checks. In cases where the compiler is able to determine at compile time that a check will fail, it gives a warning, and the extra information is not produced at run time.
-gnatef
Display full source path name in brief error messages.
-gnateF
Check for overflow on all floating-point operations, including those for unconstrained predefined types. See description of pragma Check_Float_Overflow in GNAT RM.

-gnateg -gnatceg

The -gnatc switch must always be specified before this switch, e.g. -gnatceg. Generate a C header from the Ada input file. See Generating C Headers for Ada Specifications for more information.
-gnateG
Save result of preprocessing in a text file.
-gnateinnn
Set maximum number of instantiations during compilation of a single unit to nnn. This may be useful in increasing the default maximum of 8000 for the rare case when a single unit legitimately exceeds this limit.
-gnateInnn
Indicates that the source is a multi-unit source and that the index of the unit to compile is nnn. nnn needs to be a positive number and need to be a valid index in the multi-unit source.
-gnatel
This switch can be used with the static elaboration model to issue info messages showing where implicit pragma Elaborate and pragma Elaborate_All are generated. This is useful in diagnosing elaboration circularities caused by these implicit pragmas when using the static elaboration model. See See the section in this guide on elaboration checking for further details. These messages are not generated by default, and are intended only for temporary use when debugging circularity problems.
-gnateL
This switch turns off the info messages about implicit elaboration pragmas.
-gnatem=path
Specify a mapping file (the equal sign is optional) (Units to Sources Mapping Files).
-gnatep=file
Specify a preprocessing data file (the equal sign is optional) (Integrated Preprocessing).
-gnateP
Turn categorization dependency errors into warnings. Ada requires that units that WITH one another have compatible categories, for example a Pure unit cannot WITH a Preelaborate unit. If this switch is used, these errors become warnings (which can be ignored, or suppressed in the usual manner). This can be useful in some specialized circumstances such as the temporary use of special test software.
-gnateS
Synonym of -fdump-scos, kept for backwards compatibility.
-gnatet=path
Generate target dependent information. The format of the output file is described in the section about switch -gnateT.
-gnateT=path

Read target dependent information, such as endianness or sizes and alignments of base type. If this switch is passed, the default target dependent information of the compiler is replaced by the one read from the input file. This is used by tools other than the compiler, e.g. to do semantic analysis of programs that will run on some other target than the machine on which the tool is run.

The following target dependent values should be defined, where Nat denotes a natural integer value, Pos denotes a positive integer value, and fields marked with a question mark are boolean fields, where a value of 0 is False, and a value of 1 is True:

Bits_BE                    : Nat; -- Bits stored big-endian?
Bits_Per_Unit              : Pos; -- Bits in a storage unit
Bits_Per_Word              : Pos; -- Bits in a word
Bytes_BE                   : Nat; -- Bytes stored big-endian?
Char_Size                  : Pos; -- Standard.Character'Size
Double_Float_Alignment     : Nat; -- Alignment of double float
Double_Scalar_Alignment    : Nat; -- Alignment of double length scalar
Double_Size                : Pos; -- Standard.Long_Float'Size
Float_Size                 : Pos; -- Standard.Float'Size
Float_Words_BE             : Nat; -- Float words stored big-endian?
Int_Size                   : Pos; -- Standard.Integer'Size
Long_Double_Size           : Pos; -- Standard.Long_Long_Float'Size
Long_Long_Size             : Pos; -- Standard.Long_Long_Integer'Size
Long_Size                  : Pos; -- Standard.Long_Integer'Size
Maximum_Alignment          : Pos; -- Maximum permitted alignment
Max_Unaligned_Field        : Pos; -- Maximum size for unaligned bit field
Pointer_Size               : Pos; -- System.Address'Size
Short_Enums                : Nat; -- Short foreign convention enums?
Short_Size                 : Pos; -- Standard.Short_Integer'Size
Strict_Alignment           : Nat; -- Strict alignment?
System_Allocator_Alignment : Nat; -- Alignment for malloc calls
Wchar_T_Size               : Pos; -- Interfaces.C.wchar_t'Size
Words_BE                   : Nat; -- Words stored big-endian?

The format of the input file is as follows. First come the values of the variables defined above, with one line per value:

name  value

where name is the name of the parameter, spelled out in full, and cased as in the above list, and value is an unsigned decimal integer. Two or more blanks separates the name from the value.

All the variables must be present, in alphabetical order (i.e. the same order as the list above).

Then there is a blank line to separate the two parts of the file. Then come the lines showing the floating-point types to be registered, with one line per registered mode:

name  digs float_rep size alignment

where name is the string name of the type (which can have single spaces embedded in the name (e.g. long double), digs is the number of digits for the floating-point type, float_rep is the float representation (I/V/A for IEEE-754-Binary, Vax_Native, AAMP), size is the size in bits, alignment is the alignment in bits. The name is followed by at least two blanks, fields are separated by at least one blank, and a LF character immediately follows the alignment field.

Here is an example of a target parameterization file:

Bits_BE                       0
Bits_Per_Unit                 8
Bits_Per_Word                64
Bytes_BE                      0
Char_Size                     8
Double_Float_Alignment        0
Double_Scalar_Alignment       0
Double_Size                  64
Float_Size                   32
Float_Words_BE                0
Int_Size                     64
Long_Double_Size            128
Long_Long_Size               64
Long_Size                    64
Maximum_Alignment            16
Max_Unaligned_Field          64
Pointer_Size                 64
Short_Size                   16
Strict_Alignment              0
System_Allocator_Alignment   16
Wchar_T_Size                 32
Words_BE                      0

float         15  I  64  64
double        15  I  64  64
long double   18  I  80 128
TF            33  I 128 128
-gnateu
Ignore unrecognized validity, warning, and style switches that appear after this switch is given. This may be useful when compiling sources developed on a later version of the compiler with an earlier version. Of course the earlier version must support this switch.
-gnateV
Check that all actual parameters of a subprogram call are valid according to the rules of validity checking (Validity Checking).
-gnateY
Ignore all STYLE_CHECKS pragmas. Full legality checks are still carried out, but the pragmas have no effect on what style checks are active. This allows all style checking options to be controlled from the command line.
-gnatE
Full dynamic elaboration checks.
-gnatf
Full errors. Multiple errors per line, all undefined references, do not attempt to suppress cascaded errors.
-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. Note that -gnatg implies -gnatw.ge and -gnatyg so that all standard warnings and all standard style options are turned on. All warnings and style messages are treated as errors.
-gnatG=nn
List generated expanded code in source form.
-gnath
Output usage information. The output is written to stdout.
-gnatic
Identifier character set (c = 1/2/3/4/8/9/p/f/n/w). For details of the possible selections for c, see Character Set Control.
-gnatI

Ignore representation clauses. When this switch is used, representation clauses are treated as comments. This is useful when initially porting code where you want to ignore rep clause problems, and also for compiling foreign code (particularly for use with ASIS). The representation clauses that are ignored are: enumeration_representation_clause, record_representation_clause, and attribute_definition_clause for the following attributes: Address, Alignment, Bit_Order, Component_Size, Machine_Radix, Object_Size, Scalar_Storage_Order, Size, Small, Stream_Size, and Value_Size. Pragma Default_Scalar_Storage_Order is also ignored. Note that this option should be used only for compiling – the code is likely to malfunction at run time.

Note that when -gnatct is used to generate trees for input into ASIS tools, these representation clauses are removed from the tree and ignored. This means that the tool will not see them.

-gnatjnn
Reformat error messages to fit on nn character lines
-gnatk=n
Limit file names to n (1-999) characters (k = krunch).
-gnatl
Output full source listing with embedded error messages.
-gnatL
Used in conjunction with -gnatG or -gnatD to intersperse original source lines (as comment lines with line numbers) in the expanded source output.
-gnatm=n
Limit number of detected error or warning messages to n where n is in the range 1..999999. The default setting if no switch is given is 9999. If the number of warnings reaches this limit, then a message is output and further warnings are suppressed, but the compilation is continued. If the number of error messages reaches this limit, then a message is output and the compilation is abandoned. The equal sign here is optional. A value of zero means that no limit applies.
-gnatn[12]
Activate inlining across modules for subprograms for which pragma Inline is specified. This inlining is performed by the GCC back-end. An optional digit sets the inlining level: 1 for moderate inlining across modules or 2 for full inlining across modules. If no inlining level is specified, the compiler will pick it based on the optimization level.
-gnatN

Activate front end inlining for subprograms for which pragma Inline is specified. This inlining is performed by the front end and will be visible in the -gnatG output.

When using a gcc-based back end (in practice this means using any version of GNAT other than the JGNAT, .NET or GNAAMP versions), then the use of -gnatN is deprecated, and the use of -gnatn is preferred. Historically front end inlining was more extensive than the gcc back end inlining, but that is no longer the case.

-gnato0
Suppresses overflow checking. This causes the behavior of the compiler to match the default for older versions where overflow checking was suppressed by default. This is equivalent to having pragma Suppress (Overflow_Check) in a configuration pragma file.
-gnato??

Set default mode for handling generation of code to avoid intermediate arithmetic overflow. Here ?? is two digits, a single digit, or nothing. Each digit is one of the digits 1 through 3:

Digit Interpretation
1 All intermediate overflows checked against base type (STRICT)
2 Minimize intermediate overflows (MINIMIZED)
3 Eliminate intermediate overflows (ELIMINATED)

If only one digit appears, then it applies to all cases; if two digits are given, then the first applies outside assertions, pre/postconditions, and type invariants, and the second applies within assertions, pre/postconditions, and type invariants.

If no digits follow the -gnato, then it is equivalent to -gnato11, causing all intermediate overflows to be handled in strict mode.

This switch also causes arithmetic overflow checking to be performed (as though pragma Unsuppress (Overflow_Check) had been specified).

The default if no option -gnato is given is that overflow handling is in STRICT mode (computations done using the base type), and that overflow checking is enabled.

Note that division by zero is a separate check that is not controlled by this switch (divide-by-zero checking is on by default).

See also Specifying the Desired Mode.

-gnatp
Suppress all checks. See Run-Time Checks for details. This switch has no effect if cancelled by a subsequent -gnat-p switch.
-gnat-p
Cancel effect of previous -gnatp switch.
-gnatP
Enable polling. This is required on some systems (notably Windows NT) to obtain asynchronous abort and asynchronous transfer of control capability. See Pragma_Polling in the GNAT_Reference_Manual for full details.
-gnatq
Don’t quit. Try semantics, even if parse errors.
-gnatQ
Don’t quit. Generate ALI and tree files even if illegalities. Note that code generation is still suppressed in the presence of any errors, so even with -gnatQ no object file is generated.
-gnatr
Treat pragma Restrictions as Restriction_Warnings.
-gnatR[0/1/2/3][e][m][s]
Output representation information for declared types, objects and subprograms. Note that this switch is not allowed if a previous -gnatD switch has been given, since these two switches are not compatible.
-gnats
Syntax check only.
-gnatS
Print package Standard.
-gnatt
Generate tree output file.
-gnatTnnn
All compiler tables start at nnn times usual starting size.
-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 (Validity Checking).
-gnatwxxx
Warning mode where xxx is a string of option letters that denotes the exact warnings that are enabled or disabled (Warning Message Control).
-gnatWe
Wide character encoding method (e=n/h/u/s/e/8).
-gnatx
Suppress generation of cross-reference information.
-gnatX
Enable GNAT implementation extensions and latest Ada version.
-gnaty
Enable built-in style checks (Style Checking).
-gnatzm
Distribution stub generation and compilation (m=r/c for receiver/caller stubs).
-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.
-nostdinc
Inhibit the search of the default location for the GNAT Run Time Library (RTL) source files.
-nostdlib
Inhibit the search of the default location for the GNAT Run Time Library (RTL) ALI files.
-O[n]

n controls the optimization level:

n Effect
0 No optimization, the default setting if no -O appears
1 Normal optimization, the default if you specify -O without an operand. A good compromise between code quality and compilation time.
2 Extensive optimization, may improve execution time, possibly at the cost of substantially increased compilation time.
3 Same as -O2, and also includes inline expansion for small subprograms in the same unit.
s Optimize space usage

See also Optimization Levels.

-pass-exit-codes
Catch exit codes from the compiler and use the most meaningful as exit status.
--RTS=rts-path
Specifies the default location of the runtime library. Same meaning as the equivalent gnatmake flag (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.
-fverbose-asm
Used in conjunction with -S to cause the generated assembly code file to be annotated with variable names, making it significantly easier to follow.
-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.
-w
Turn off warnings generated by the back end of the compiler. Use of this switch also causes the default for front end warnings to be set to suppress (as though -gnatws had appeared at the start of the options).

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.
  • The switches -gnatzc and -gnatzr may not be combined with any other switches, and only one of them may appear in the command line.
  • The switch -gnat-p may not be combined with any other switch.
  • 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 (Validity Checking).
  • Option ‘em’, ‘ec’, ‘ep’, ‘l=’ and ‘R’ must be the last options in a combined list of options.

4.3.2. 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. GPS 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. In the case where a body is compiled, the corresponding spec is also listed, along with any subunits. Typical output from compiling a package body p.adb might look like:

Compiling: p.adb

     1. package body p is
     2.    procedure a;
     3.    procedure a is separate;
     4. begin
     5.    null
               |
        >>> missing ";"

     6. end;

Compiling: p.ads

     1. package p is
     2.    pragma Elaborate_Body
                                |
        >>> missing ";"

     3. end p;

Compiling: p-a.adb

     1. separate p
                |
        >>> missing "("

     2. procedure a is
     3. begin
     4.    null
               |
        >>> missing ";"

     5. end;

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.

-gnatl=fname
This has the same effect as -gnatl except that the output is written to a file instead of to standard output. If the given name fname does not start with a period, then it is the full name of the file to be written. If fname is an extension, it is appended to the name of the file being compiled. For example, if file xyz.adb is compiled with -gnatl=.lst, then the output is written to file xyz.adb.lst.
-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).
-gnatm=n

The m stands for maximum. n is a decimal integer in the range of 1 to 999999 and limits the number of error or warning 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 number of errors detected
compilation abandoned

The default setting if no switch is given is 9999. If the number of warnings reaches this limit, then a message is output and further warnings are suppressed, but the compilation is continued. If the number of error messages reaches this limit, then a message is output and the compilation is abandoned. A value of zero means that no limit applies.

Note that the equal sign is optional, so the switches -gnatm2 and -gnatm=2 are equivalent.

-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

The -gnatf switch also generates additional information for some error messages. Some examples are:

  • Details on possibly non-portable unchecked conversion
  • List possible interpretations for ambiguous calls
  • Additional details on incorrect parameters
-gnatjnn

In normal operation mode (or if -gnatj0 is used), then error messages with continuation lines are treated as though the continuation lines were separate messages (and so a warning with two continuation lines counts as three warnings, and is listed as three separate messages).

If the -gnatjnn switch is used with a positive value for nn, then messages are output in a different manner. A message and all its continuation lines are treated as a unit, and count as only one warning or message in the statistics totals. Furthermore, the message is reformatted so that no line is longer than nn characters.

-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. This switch implies -gnatq, since the semantic phase must be run to get a meaningful 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.

When -gnatQ is used and the generated ALI file is marked as being in error, gnatmake will attempt to recompile the source when it finds such an ALI file, including with switch -gnatc.

Note that -gnatQ has no effect if -gnats is specified, since ALI files are never generated if -gnats is set.

4.3.3. Warning Message Control

In addition to error messages, which correspond to illegalities as defined in the Ada 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. This list is not complete, and new warnings are often added to subsequent versions of GNAT. The list is intended to give a general idea of the kinds of warnings that are generated.

  • Possible infinitely recursive calls
  • Out-of-range values being assigned
  • Possible order of elaboration problems
  • Size not a multiple of alignment for a record type
  • Assertions (pragma Assert) that are sure to fail
  • Unreachable code
  • Address clauses with possibly unaligned values, or where an attempt is made to overlay a smaller variable with a larger one.
  • Fixed-point type declarations with a null range
  • Direct_IO or Sequential_IO instantiated with a type that has access values
  • 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 statement along some execution path in a function
  • Incorrect (unrecognized) pragmas
  • Incorrect external names
  • Allocation from empty storage pool
  • Potentially blocking operation in protected type
  • Suspicious parenthesization of expressions
  • Mismatching bounds in an aggregate
  • Attempt to return local value by reference
  • 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
  • Standard.Duration used to resolve universal fixed expression
  • Dereference of possibly null value
  • Declaration that is likely to cause storage error
  • Internal GNAT unit withed by application unit
  • Values known to be out of range at compile time
  • Unreferenced or unmodified variables. Note that a special exemption applies to variables which contain any of the substrings DISCARD, DUMMY, IGNORE, JUNK, UNUSED, in any casing. Such variables are considered likely to be intentionally used in a situation where otherwise a warning would be given, so warnings of this kind are always suppressed for such variables.
  • Address overlays that could clobber memory
  • Unexpected initialization when address clause present
  • Bad alignment for address clause
  • Useless type conversions
  • Redundant assignment statements and other redundant constructs
  • Useless exception handlers
  • Accidental hiding of name by child unit
  • 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 section lists compiler switches that are available to control the handling of warning messages. It is also possible to exercise much finer control over what warnings are issued and suppressed using the GNAT pragma Warnings (see the description of the pragma in the GNAT_Reference_manual).

-gnatwa

Activate most optional warnings.

This switch activates most optional warning messages. See the 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:

  • -gnatwd (implicit dereferencing)
  • -gnatw.d (tag warnings with -gnatw switch)
  • -gnatwh (hiding)
  • -gnatw.h (holes in record layouts)
  • -gnatw.j (late primitives of tagged types)
  • -gnatw.k (redefinition of names in standard)
  • -gnatwl (elaboration warnings)
  • -gnatw.l (inherited aspects)
  • -gnatw.n (atomic synchronization)
  • -gnatwo (address clause overlay)
  • -gnatw.o (values set by out parameters ignored)
  • -gnatw.q (questionable layout of record types)
  • -gnatw.s (overridden size clause)
  • -gnatwt (tracking of deleted conditional code)
  • -gnatw.u (unordered enumeration)
  • -gnatw.w (use of Warnings Off)
  • -gnatw.y (reasons for package needing body)

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. Note that unlike switch -gnatws, the use of switch -gnatwA does not suppress warnings that are normally given unconditionally and cannot be individually controlled (for example, the warning about a missing exit path in a function). Also, again unlike switch -gnatws, warnings suppressed by the use of switch -gnatwA can be individually turned back on. For example the use of switch -gnatwA followed by switch -gnatwd will suppress all optional warnings except the warnings for implicit dereferencing.

-gnatw.a

Activate warnings on failing assertions.

This switch activates warnings for assertions where the compiler can tell at compile time that the assertion will fail. Note that this warning is given even if assertions are disabled. The default is that such warnings are generated.

-gnatw.A

Suppress warnings on failing assertions.

This switch suppresses warnings for assertions where the compiler can tell at compile time that the assertion will fail.

-gnatwb

Activate warnings on bad fixed values.

This switch activates warnings for static fixed-point expressions whose value is not an exact multiple of Small. Such values are implementation dependent, since an implementation is free to choose either of the multiples that surround the value. GNAT always chooses the closer one, but this is not required behavior, and it is better to specify a value that is an exact multiple, ensuring predictable execution. The default is that such warnings are not generated.

-gnatwB

Suppress warnings on bad fixed values.

This switch suppresses warnings for static fixed-point expressions whose value is not an exact multiple of Small.

-gnatw.b

Activate warnings on biased representation.

This switch activates warnings when a size clause, value size clause, component clause, or component size clause forces the use of biased representation for an integer type (e.g. representing a range of 10..11 in a single bit by using 0/1 to represent 10/11). The default is that such warnings are generated.

-gnatw.B

Suppress warnings on biased representation.

This switch suppresses warnings for representation clauses that force the use of biased representation.

-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. Note that this warning does not get issued for the use of boolean variables or constants whose values are known at compile time, since this is a standard technique for conditional compilation in Ada, and this would generate too many false positive warnings.

This warning option also activates a special test for comparisons using the operators ‘>=’ and’ <=’. If the compiler can tell that only the equality condition is possible, then it will warn that the ‘>’ or ‘<’ part of the test is useless and that the operator could be replaced by ‘=’. An example would be comparing a Natural variable <= 0.

This warning option also generates warnings if one or both tests is optimized away in a membership test for integer values if the result can be determined at compile time. Range tests on enumeration types are not included, since it is common for such tests to include an end point.

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.

-gnatw.c

Activate warnings on missing component clauses.

This switch activates warnings for record components where a record representation clause is present and has component clauses for the majority, but not all, of the components. A warning is given for each component for which no component clause is present.

-gnatw.C

Suppress warnings on missing component clauses.

This switch suppresses warnings for record components that are missing a component clause in the situation described above.

-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.

-gnatwD

Suppress warnings on implicit dereferencing.

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

-gnatw.d

Activate tagging of warning and info messages.

If this switch is set, then warning messages are tagged, with one of the following strings:

  • [-gnatw?] Used to tag warnings controlled by the switch -gnatwx where x is a letter a-z.
  • [-gnatw.?] Used to tag warnings controlled by the switch -gnatw.x where x is a letter a-z.
  • [-gnatel] Used to tag elaboration information (info) messages generated when the static model of elaboration is used and the -gnatel switch is set.
  • [restriction warning] Used to tag warning messages for restriction violations, activated by use of the pragma Restriction_Warnings.
  • [warning-as-error] Used to tag warning messages that have been converted to error messages by use of the pragma Warning_As_Error. Note that such warnings are prefixed by the string “error: ” rather than “warning: ”.
  • [enabled by default] Used to tag all other warnings that are always given by default, unless warnings are completely suppressed using pragma Warnings(Off) or the switch -gnatws.
-gnatw.D

Deactivate tagging of warning and info messages messages.

If this switch is set, then warning messages return to the default mode in which warnings and info messages are not tagged as described above for -gnatw.d.

-gnatwe

Treat warnings and style checks as errors.

This switch causes warning messages and style check 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. Note that this is the only -gnatw switch that affects the handling of style check messages. Note also that this switch has no effect on info (information) messages, which are not treated as errors if this switch is present.

-gnatw.e

Activate every optional warning.

This switch activates all optional warnings, including those which are not activated by -gnatwa. The use of this switch is not recommended for normal use. If you turn this switch on, it is almost certain that you will get large numbers of useless warnings. The warnings that are excluded from -gnatwa are typically highly specialized warnings that are suitable for use only in code that has been specifically designed according to specialized coding rules.

-gnatwE

Treat all run-time exception warnings as errors.

This switch causes warning messages regarding errors that will be raised during run-time execution to be treated as errors.

-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 -gnatwu. The default is that these warnings are not generated.

-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.

-gnatwg

Activate warnings on unrecognized pragmas.

This switch causes a warning to be generated if an unrecognized pragma is encountered. Apart from issuing this warning, the pragma is ignored and has no effect. The default is that such warnings are issued (satisfying the Ada Reference Manual requirement that such warnings appear).

-gnatwG

Suppress warnings on unrecognized pragmas.

This switch suppresses warnings for unrecognized pragmas.

-gnatw.g

Warnings used for GNAT sources.

This switch sets the warning categories that are used by the standard GNAT style. Currently this is equivalent to -gnatwAao.q.s.CI.V.X.Z but more warnings may be added in the future without advanced notice.

-gnatwh

Activate warnings on hiding.

This switch activates warnings on hiding declarations that are considered potentially confusing. Not all cases of hiding cause warnings; for example an overriding declaration hides an implicit declaration, which is just normal code. The default is that warnings on hiding are not generated.

-gnatwH

Suppress warnings on hiding.

This switch suppresses warnings on hiding declarations.

-gnatw.h

Activate warnings on holes/gaps in records.

This switch activates warnings on component clauses in record representation clauses that leave holes (gaps) in the record layout. If this warning option is active, then record representation clauses should specify a contiguous layout, adding unused fill fields if needed.

-gnatw.H

Suppress warnings on holes/gaps in records.

This switch suppresses warnings on component clauses in record representation clauses that leave holes (haps) in the record layout.

-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 withed by user programs. The default is that such warnings are generated

-gnatwI

Disable warnings on implementation units.

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

-gnatw.i

Activate warnings on overlapping actuals.

This switch enables a warning on statically detectable overlapping actuals in a subprogram call, when one of the actuals is an in-out parameter, and the types of the actuals are not by-copy types. This warning is off by default.

-gnatw.I

Disable warnings on overlapping actuals.

This switch disables warnings on overlapping actuals in a call..

-gnatwj

Activate warnings on obsolescent features (Annex J).

If this warning option is activated, then warnings are generated for calls to subprograms marked with pragma Obsolescent and for use of features in Annex J of the Ada Reference Manual. In the case of Annex J, not all features are flagged. In particular use of the renamed packages (like Text_IO) and use of package ASCII are not flagged, since these are very common and would generate many annoying positive warnings. The default is that such warnings are not generated.

In addition to the above cases, warnings are also generated for GNAT features that have been provided in past versions but which have been superseded (typically by features in the new Ada standard). For example, pragma Ravenscar will be flagged since its function is replaced by pragma Profile(Ravenscar), and pragma Interface_Name will be flagged since its function is replaced by pragma Import.

Note that this warning option functions differently from the restriction No_Obsolescent_Features in two respects. First, the restriction applies only to annex J features. Second, the restriction does flag uses of package ASCII.

-gnatwJ

Suppress warnings on obsolescent features (Annex J).

This switch disables warnings on use of obsolescent features.

-gnatw.j

Activate warnings on late declarations of tagged type primitives.

This switch activates warnings on visible primitives added to a tagged type after deriving a private extension from it.

-gnatw.J

Suppress warnings on late declarations of tagged type primitives.

This switch suppresses warnings on visible primitives added to a tagged type after deriving a private extension from it.

-gnatwk

Activate warnings on variables that could be constants.

This switch activates warnings for variables that are initialized but never modified, and then could be declared constants. The default is that such warnings are not given.

-gnatwK

Suppress warnings on variables that could be constants.

This switch disables warnings on variables that could be declared constants.

-gnatw.k

Activate warnings on redefinition of names in standard.

This switch activates warnings for declarations that declare a name that is defined in package Standard. Such declarations can be confusing, especially since the names in package Standard continue to be directly visible, meaning that use visibiliy on such redeclared names does not work as expected. Names of discriminants and components in records are not included in this check.

-gnatw.K

Suppress warnings on redefinition of names in standard.

This switch activates warnings for declarations that declare a name that is defined in package Standard.

-gnatwl

Activate warnings for elaboration pragmas.

This switch activates warnings for possible elaboration problems, including suspicious use of Elaborate pragmas, when using the static elaboration model, and possible situations that may raise Program_Error when using the dynamic elaboration model. See the section in this guide on elaboration checking for further details. The default is that such warnings are not generated.

-gnatwL

Suppress warnings for elaboration pragmas.

This switch suppresses warnings for possible elaboration problems.

-gnatw.l

List inherited aspects.

This switch causes the compiler to list inherited invariants, preconditions, and postconditions from Type_Invariant’Class, Invariant’Class, Pre’Class, and Post’Class aspects. Also list inherited subtype predicates.

-gnatw.L

Suppress listing of inherited aspects.

This switch suppresses listing of inherited aspects.

-gnatwm

Activate warnings on modified but unreferenced variables.

This switch activates warnings for variables that are assigned (using an initialization value or with one or more assignment statements) but whose value is never read. The warning is suppressed for volatile variables and also for variables that are renamings of other variables or for which an address clause is given. The default is that these warnings are not given.

-gnatwM

Disable warnings on modified but unreferenced variables.

This switch disables warnings for variables that are assigned or initialized, but never read.

-gnatw.m

Activate warnings on suspicious modulus values.

This switch activates warnings for modulus values that seem suspicious. The cases caught are where the size is the same as the modulus (e.g. a modulus of 7 with a size of 7 bits), and modulus values of 32 or 64 with no size clause. The guess in both cases is that 2**x was intended rather than x. In addition expressions of the form 2*x for small x generate a warning (the almost certainly accurate guess being that 2**x was intended). The default is that these warnings are given.

-gnatw.M

Disable warnings on suspicious modulus values.

This switch disables warnings for suspicious modulus values.

-gnatwn

Set normal warnings mode.

This switch sets normal warning mode, in which enabled warnings are issued and treated as warnings rather than errors. This is the default mode. the switch -gnatwn can be used to cancel the effect of an explicit -gnatws or -gnatwe. It also cancels the effect of the implicit -gnatwe that is activated by the use of -gnatg.

-gnatw.n

Activate warnings on atomic synchronization.

This switch actives warnings when an access to an atomic variable requires the generation of atomic synchronization code. These warnings are off by default.

-gnatw.N

Suppress warnings on atomic synchronization.

This switch suppresses warnings when an access to an atomic variable requires the generation of atomic synchronization code.

-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.

-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.

-gnatw.o

Activate warnings on modified but unreferenced out parameters.

This switch activates warnings for variables that are modified by using them as actuals for a call to a procedure with an out mode formal, where the resulting assigned value is never read. It is applicable in the case where there is more than one out mode formal. If there is only one out mode formal, the warning is issued by default (controlled by -gnatwu). The warning is suppressed for volatile variables and also for variables that are renamings of other variables or for which an address clause is given. The default is that these warnings are not given.

-gnatw.O

Disable warnings on modified but unreferenced out parameters.

This switch suppresses warnings for variables that are modified by using them as actuals for a call to a procedure with an out mode formal, where the resulting assigned value is never read.

-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. The default is that such warnings are not given. Warnings on ineffective inlining by the gcc back-end can be activated separately, using the gcc switch -Winline.

-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.

-gnatw.p

Activate warnings on parameter ordering.

This switch activates warnings for cases of suspicious parameter ordering when the list of arguments are all simple identifiers that match the names of the formals, but are in a different order. The warning is suppressed if any use of named parameter notation is used, so this is the appropriate way to suppress a false positive (and serves to emphasize that the “misordering” is deliberate). The default is that such warnings are not given.

-gnatw.P

Suppress warnings on parameter ordering.

This switch suppresses warnings on cases of suspicious parameter ordering.

-gnatwq

Activate warnings on questionable missing parentheses.

This switch activates warnings for cases where parentheses are not used and the result is potential ambiguity from a readers point of view. For example (not a > b) when a and b are modular means ((not a) > b) and very likely the programmer intended (not (a > b)). Similarly (-x mod 5) means (-(x mod 5)) and quite likely ((-x) mod 5) was intended. In such situations it seems best to follow the rule of always parenthesizing to make the association clear, and this warning switch warns if such parentheses are not present. The default is that these warnings are given.

-gnatwQ

Suppress warnings on questionable missing parentheses.

This switch suppresses warnings for cases where the association is not clear and the use of parentheses is preferred.

-gnatw.q

Activate warnings on questionable layout of record types.

This switch activates warnings for cases where the default layout of a record type, that is to say the layout of its components in textual order of the source code, would very likely cause inefficiencies in the code generated by the compiler, both in terms of space and speed during execution. One warning is issued for each problematic component without representation clause in the nonvariant part and then in each variant recursively, if any.

The purpose of these warnings is neither to prescribe an optimal layout nor to force the use of representation clauses, but rather to get rid of the most blatant inefficiencies in the layout. Therefore, the default layout is matched against the following synthetic ordered layout and the deviations are flagged on a component-by-component basis:

  • first all components or groups of components whose length is fixed and a multiple of the storage unit,
  • then the remaining components whose length is fixed and not a multiple of the storage unit,
  • then the remaining components whose length doesn’t depend on discriminants (that is to say, with variable but uniform length for all objects),
  • then all components whose length depends on discriminants,
  • finally the variant part (if any),

for the nonvariant part and for each variant recursively, if any.

The exact wording of the warning depends on whether the compiler is allowed to reorder the components in the record type or precluded from doing it by means of pragma No_Component_Reordering.

The default is that these warnings are not given.

-gnatw.Q

Suppress warnings on questionable layout of record types.

This switch suppresses warnings for cases where the default layout of a record type would very likely cause inefficiencies.

-gnatwr

Activate warnings on redundant constructs.

This switch activates warnings for redundant constructs. The following is the current list of constructs regarded as redundant:

  • 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.
  • Exception handler containing only a reraise statement (raise with no operand) which has no effect.
  • Use of the operator abs on an operand that is known at compile time to be non-negative
  • Comparison of an object or (unary or binary) operation of boolean type to an explicit True value.

The default is that warnings for redundant constructs are not given.

-gnatwR

Suppress warnings on redundant constructs.

This switch suppresses warnings for redundant constructs.

-gnatw.r

Activate warnings for object renaming function.

This switch activates warnings for an object renaming that renames a function call, which is equivalent to a constant declaration (as opposed to renaming the function itself). The default is that these warnings are given.

-gnatw.R

Suppress warnings for object renaming function.

This switch suppresses warnings for object renaming function.

-gnatws

Suppress all warnings.

This switch completely suppresses the output of all warning messages from the GNAT front end, including both warnings that can be controlled by switches described in this section, and those that are normally given unconditionally. The effect of this suppress action can only be cancelled by a subsequent use of the switch -gnatwn.

Note that switch -gnatws 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. Also this switch has no effect on the handling of style check messages.

-gnatw.s

Activate warnings on overridden size clauses.

This switch activates warnings on component clauses in record representation clauses where the length given overrides that specified by an explicit size clause for the component type. A warning is similarly given in the array case if a specified component size overrides an explicit size clause for the array component type.

-gnatw.S

Suppress warnings on overridden size clauses.

This switch suppresses warnings on component clauses in record representation clauses that override size clauses, and similar warnings when an array component size overrides a size clause.

-gnatwt

Activate warnings for tracking of deleted conditional code.

This switch activates warnings for tracking of code in conditionals (IF and CASE statements) that is detected to be dead code which cannot be executed, and which is removed by the front end. This warning is off by default. This may be useful for detecting deactivated code in certified applications.

-gnatwT

Suppress warnings for tracking of deleted conditional code.

This switch suppresses warnings for tracking of deleted conditional code.

-gnatw.t

Activate warnings on suspicious contracts.

This switch activates warnings on suspicious contracts. This includes warnings on suspicious postconditions (whether a pragma Postcondition or a Post aspect in Ada 2012) and suspicious contract cases (pragma or aspect Contract_Cases). A function postcondition or contract case is suspicious when no postcondition or contract case for this function mentions the result of the function. A procedure postcondition or contract case is suspicious when it only refers to the pre-state of the procedure, because in that case it should rather be expressed as a precondition. This switch also controls warnings on suspicious cases of expressions typically found in contracts like quantified expressions and uses of Update attribute. The default is that such warnings are generated.

-gnatw.T

Suppress warnings on suspicious contracts.

This switch suppresses warnings on suspicious contracts.

-gnatwu

Activate warnings on unused entities.

This switch activates warnings to be generated for entities that are declared but not referenced, and for units that are withed 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 a with’ed 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 withed 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 includes the effect of -gnatwf).

-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).

-gnatw.u

Activate warnings on unordered enumeration types.

This switch causes enumeration types to be considered as conceptually unordered, unless an explicit pragma Ordered is given for the type. The effect is to generate warnings in clients that use explicit comparisons or subranges, since these constructs both treat objects of the type as ordered. (A client is defined as a unit that is other than the unit in which the type is declared, or its body or subunits.) Please refer to the description of pragma Ordered in the GNAT Reference Manual for further details. The default is that such warnings are not generated.

-gnatw.U

Deactivate warnings on unordered enumeration types.

This switch causes all enumeration types to be considered as ordered, so that no warnings are given for comparisons or subranges for any type.

-gnatwv

Activate warnings on unassigned variables.

This switch activates warnings for access to variables which may not be properly initialized. The default is that such warnings are generated.

-gnatwV

Suppress warnings on unassigned variables.

This switch suppresses warnings for access to variables which may not be properly initialized. For variables of a composite type, the warning can also be suppressed in Ada 2005 by using a default initialization with a box. For example, if Table is an array of records whose components are only partially uninitialized, then the following code:

Tab : Table := (others => <>);

will suppress warnings on subsequent statements that access components of variable Tab.

-gnatw.v

Activate info messages for non-default bit order.

This switch activates messages (labeled “info”, they are not warnings, just informational messages) about the effects of non-default bit-order on records to which a component clause is applied. The effect of specifying non-default bit ordering is a bit subtle (and changed with Ada 2005), so these messages, which are given by default, are useful in understanding the exact consequences of using this feature.

-gnatw.V

Suppress info messages for non-default bit order.

This switch suppresses information messages for the effects of specifying non-default bit order on record components with component clauses.

-gnatww

Activate warnings on wrong low bound assumption.

This switch activates warnings for indexing an unconstrained string parameter with a literal or S’Length. This is a case where the code is assuming that the low bound is one, which is in general not true (for example when a slice is passed). The default is that such warnings are generated.

-gnatwW

Suppress warnings on wrong low bound assumption.

This switch suppresses warnings for indexing an unconstrained string parameter with a literal or S’Length. Note that this warning can also be suppressed in a particular case by adding an assertion that the lower bound is 1, as shown in the following example:

procedure K (S : String) is
   pragma Assert (S'First = 1);
   ...
-gnatw.w

Activate warnings on Warnings Off pragmas.

This switch activates warnings for use of pragma Warnings (Off, entity) where either the pragma is entirely useless (because it suppresses no warnings), or it could be replaced by pragma Unreferenced or pragma Unmodified. Also activates warnings for the case of Warnings (Off, String), where either there is no matching Warnings (On, String), or the Warnings (Off) did not suppress any warning. The default is that these warnings are not given.

-gnatw.W

Suppress warnings on unnecessary Warnings Off pragmas.

This switch suppresses warnings for use of pragma Warnings (Off, ...).

-gnatwx

Activate warnings on Export/Import pragmas.

This switch activates warnings on Export/Import pragmas when the compiler detects a possible conflict between the Ada and foreign language calling sequences. For example, the use of default parameters in a convention C procedure is dubious because the C compiler cannot supply the proper default, so a warning is issued. The default is that such warnings are generated.

-gnatwX

Suppress warnings on Export/Import pragmas.

This switch suppresses warnings on Export/Import pragmas. The sense of this is that you are telling the compiler that you know what you are doing in writing the pragma, and it should not complain at you.

-gnatw.x

Activate warnings for No_Exception_Propagation mode.

This switch activates warnings for exception usage when pragma Restrictions (No_Exception_Propagation) is in effect. Warnings are given for implicit or explicit exception raises which are not covered by a local handler, and for exception handlers which do not cover a local raise. The default is that these warnings are given for units that contain exception handlers.

-gnatw.X

Disable warnings for No_Exception_Propagation mode.

This switch disables warnings for exception usage when pragma Restrictions (No_Exception_Propagation) is in effect.

-gnatwy

Activate warnings for Ada compatibility issues.

For the most part, newer versions of Ada are upwards compatible with older versions. For example, Ada 2005 programs will almost always work when compiled as Ada 2012. However there are some exceptions (for example the fact that some is now a reserved word in Ada 2012). This switch activates several warnings to help in identifying and correcting such incompatibilities. The default is that these warnings are generated. Note that at one point Ada 2005 was called Ada 0Y, hence the choice of character.

-gnatwY

Disable warnings for Ada compatibility issues.

This switch suppresses the warnings intended to help in identifying incompatibilities between Ada language versions.

-gnatw.y

Activate information messages for why package spec needs body.

There are a number of cases in which a package spec needs a body. For example, the use of pragma Elaborate_Body, or the declaration of a procedure specification requiring a completion. This switch causes information messages to be output showing why a package specification requires a body. This can be useful in the case of a large package specification which is unexpectedly requiring a body. The default is that such information messages are not output.

-gnatw.Y

Disable information messages for why package spec needs body.

This switch suppresses the output of information messages showing why a package specification needs a body.

-gnatwz

Activate warnings on unchecked conversions.

This switch activates warnings for unchecked conversions where the types are known at compile time to have different sizes. The default is that such warnings are generated. Warnings are also generated for subprogram pointers with different conventions.

-gnatwZ

Suppress warnings on unchecked conversions.

This switch suppresses warnings for unchecked conversions where the types are known at compile time to have different sizes or conventions.

-gnatw.z

Activate warnings for size not a multiple of alignment.

This switch activates warnings for cases of record types with specified Size and Alignment attributes where the size is not a multiple of the alignment, resulting in an object size that is greater than the specified size. The default is that such warnings are generated.

-gnatw.Z

Suppress warnings for size not a multiple of alignment.

This switch suppresses warnings for cases of record types with specified Size and Alignment attributes where the size is not a multiple of the alignment, resulting in an object size that is greater than the specified size. The warning can also be suppressed by giving an explicit Object_Size value.

-Wunused
The warnings controlled by the -gnatw switch are generated by the front end of the compiler. The GCC back end can provide additional warnings and they are controlled by the -W switch. For example, -Wunused activates back end warnings for entities that are declared but not referenced.
-Wuninitialized
Similarly, -Wuninitialized activates the back end warning for uninitialized variables. This switch must be used in conjunction with an optimization level greater than zero.
-Wstack-usage=len
Warn if the stack usage of a subprogram might be larger than len bytes. See Static Stack Usage Analysis for details.
-Wall
This switch enables most warnings from the GCC back end. The code generator detects a number of warning situations that are missed by the GNAT front end, and this switch can be used to activate them. The use of this switch also sets the default front end warning mode to -gnatwa, that is, most front end warnings activated as well.
-w
Conversely, this switch suppresses warnings from the GCC back end. The use of this switch also sets the default front end warning mode to -gnatws, that is, front end warnings suppressed as well.
-Werror
This switch causes warnings from the GCC back end 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.

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

-gnatwaGe

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

When no switch -gnatw is used, this is equivalent to:

  • -gnatw.a
  • -gnatwB
  • -gnatw.b
  • -gnatwC
  • -gnatw.C
  • -gnatwD
  • -gnatw.D
  • -gnatwF
  • -gnatw.F
  • -gnatwg
  • -gnatwH
  • -gnatw.H
  • -gnatwi
  • -gnatwJ
  • -gnatw.J
  • -gnatwK
  • -gnatw.K
  • -gnatwL
  • -gnatw.L
  • -gnatwM
  • -gnatw.m
  • -gnatwn
  • -gnatw.N
  • -gnatwo
  • -gnatw.O
  • -gnatwP
  • -gnatw.P
  • -gnatwq
  • -gnatw.Q
  • -gnatwR
  • -gnatw.R
  • -gnatw.S
  • -gnatwT
  • -gnatw.t
  • -gnatwU
  • -gnatw.U
  • -gnatwv
  • -gnatw.v
  • -gnatww
  • -gnatw.W
  • -gnatwx
  • -gnatw.X
  • -gnatwy
  • -gnatw.Y
  • -gnatwz
  • -gnatw.z

4.3.4. Debugging and Assertion Control

-gnata

The -gnata option is equivalent to the following Assertion_Policy pragma:

pragma Assertion_Policy (Check);

Which is a shorthand for:

pragma Assertion_Policy
  (Assert               => Check,
   Static_Predicate     => Check,
   Dynamic_Predicate    => Check,
   Pre                  => Check,
   Pre'Class            => Check,
   Post                 => Check,
   Post'Class           => Check,
   Type_Invariant       => Check,
   Type_Invariant'Class => Check);

The pragmas Assert and Debug normally have no effect and are ignored. This switch, where a stands for ‘assert’, causes pragmas Assert and Debug to be activated. This switch also causes preconditions, postconditions, subtype predicates, and type invariants to be activated.

The pragmas have the form:

pragma Assert (<Boolean-expression> [, <static-string-expression>])
pragma Debug (<procedure call>)
pragma Type_Invariant (<type-local-name>, <Boolean-expression>)
pragma Predicate (<type-local-name>, <Boolean-expression>)
pragma Precondition (<Boolean-expression>, <string-expression>)
pragma Postcondition (<Boolean-expression>, <string-expression>)

The aspects have the form:

with [Pre|Post|Type_Invariant|Dynamic_Predicate|Static_Predicate]
  => <Boolean-expression>;

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 containing 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.

For the aspect specification, the Boolean-expression is evaluated. If the result is True, the aspect has no effect. If the result is False, the exception Assert_Failure is raised.

4.3.5. Validity Checking

The Ada Reference Manual defines the concept of invalid values (see RM 13.9.1). The primary source of invalid values is uninitialized variables. A scalar variable that is left uninitialized may contain an invalid value; the concept of invalid does not apply to access or composite types.

It is an error to read an invalid value, but the RM does not require run-time checks to detect such errors, except for some minimal checking to prevent erroneous execution (i.e. unpredictable behavior). This corresponds to the -gnatVd switch below, which is the default. For example, by default, if the expression of a case statement is invalid, it will raise Constraint_Error rather than causing a wild jump, and if an array index on the left-hand side of an assignment is invalid, it will raise Constraint_Error rather than overwriting an arbitrary memory location.

The -gnatVa may be used to enable additional validity checks, which are not required by the RM. These checks are often very expensive (which is why the RM does not require them). These checks are useful in tracking down uninitialized variables, but they are not usually recommended for production builds, and in particular we do not recommend using these extra validity checking options in combination with optimization, since this can confuse the optimizer. If performance is a consideration, leading to the need to optimize, then the validity checking options should not be used.

The other -gnatVx switches below allow finer-grained control; you can enable whichever validity checks you desire. However, for most debugging purposes, -gnatVa is sufficient, and the default -gnatVd (i.e. standard Ada behavior) is usually sufficient for non-debugging use.

The -gnatB switch tells the compiler to assume that all values are valid (that is, within their declared subtype range) except in the context of a use of the Valid attribute. This means the compiler can generate more efficient code, since the range of values is better known at compile time. However, an uninitialized variable can cause wild jumps and memory corruption in this mode.

The -gnatVx switch allows control over the validity checking mode as described below. The x argument is a string of letters that indicate validity checks that are performed or not performed in addition to the default checks required by Ada as described above.

-gnatVa

All validity checks.

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

-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.

-gnatVe

Validity checks for elementary components.

In the absence of this switch, assignments to record or array components are not validity checked, even if validity checks for assignments generally (-gnatVc) are turned on. In Ada, assignment of composite values do not require valid data, but assignment of individual components does. So for example, there is a difference between copying the elements of an array with a slice assignment, compared to assigning element by element in a loop. This switch allows you to turn off validity checking for components, even when they are assigned component by component.

-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 NaNs 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.

-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.

-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 suppresses all run-time checks, including validity checks, and thus implies -gnatVn. When this switch is used, it cancels any other -gnatV previously issued.

-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. Checks are also made on individual component values for composite comparisons, and on the expressions in type conversions and qualified expressions. Checks are also made on explicit ranges using .. (e.g., slices, loops etc).

-gnatVp

Validity checks for parameters.

This controls the treatment of parameters within a subprogram (as opposed to -gnatVi and -gnatVm which control validity testing of parameters on a call. If either of these call options is used, then normally an assumption is made within a subprogram that the input arguments have been validity checking at the point of call, and do not need checking again within a subprogram). If -gnatVp is set, then this assumption is not made, and parameters are not assumed to be valid, so their validity will be checked (or rechecked) within the subprogram.

-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.

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 are to be validity checked. In order to make it easier to specify the desired combination of effects, the upper case letters CDFIMORST may be used to turn off the corresponding lower case option. Thus -gnatVaM turns on all validity checking options except for checking of in out parameters.

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 detecting 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.

4.3.6. 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 message is given, preceded by the character sequence ‘(style)’. This message does not prevent successful compilation (unless the -gnatwe switch is used).

Note that this is by no means intended to be a general facility for checking arbitrary coding standards. It is simply an embedding of the style rules we have chosen for the GNAT sources. If you are starting a project which does not have established style standards, you may find it useful to adopt the entire set of GNAT coding standards, or some subset of them.

If you already have an established set of coding standards, then the selected style checking options may indeed correspond to choices you have made, but for general checking of an existing set of coding rules, you should look to the gnatcheck tool, which is designed for that purpose.

The string x is a sequence of letters or digits indicating the particular style checks to be performed. The following checks are defined:

-gnaty0

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. A value of zero turns off this style check. 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, or they may be aligned the same way as the following non-blank line (this is useful when full line comments appear in the middle of a statement, or they may be aligned with the source line on the previous non-blank line.

-gnatya

Check attribute casing.

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.

-gnatyA

Use of array index numbers in array attributes.

When using the array attributes First, Last, Range, or Length, the index number must be omitted for one-dimensional arrays and is required for multi-dimensional arrays.

-gnatyb

Blanks not allowed at statement end.

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.

-gnatyB

Check Boolean operators.

The use of AND/OR operators is not permitted except in the cases of modular operands, array operands, and simple stand-alone boolean variables or boolean constants. In all other cases and then/or else are required.

-gnatyc

Check comments, double space.

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 at least 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 allows proper processing of the output from specialized tools such as gnatprep (where --! is used) and in earlier versions of the SPARK annotation 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#. Note that this usage is not permitted in GNAT implementation units (i.e., when -gnatg is used).

  • 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.

  • A comment that 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.  --
    ---------------------------
    
-gnatyC

Check comments, single space.

This is identical to c except that only one space is required following the -- of a comment instead of two.

-gnatyd

Check no DOS line terminators present.

All lines must be terminated by a single ASCII.LF character (in particular the DOS line terminator sequence CR/LF is not allowed).

-gnatye

Check end/exit labels.

Optional labels on end statements ending subprograms and on exit statements exiting named loops, are required to be present.

-gnatyf

No form feeds or vertical tabs.

Neither form feeds nor vertical tab characters are permitted in the source text.

-gnatyg

GNAT style mode.

The set of style check switches is set to match that used by the GNAT sources. This may be useful when developing code that is eventually intended to be incorporated into GNAT. Currently this is equivalent to -gnatwydISux) but additional style switches may be added to this set in the future without advance notice.

-gnatyh

No horizontal tabs.

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.

-gnatyi

Check if-then layout.

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.

-gnatyI

check mode IN keywords.

Mode in (the default mode) is not allowed to be given explicitly. in out is fine, but not in on its own.

-gnatyk

Check keyword casing.

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).

-gnatyl

Check layout.

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. This is also permitted when the type declaration is split on two lines. For example, any of the following three layouts is acceptable:

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

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;
-gnatyL

Set maximum nesting level.

The maximum level of nesting of constructs (including subprograms, loops, blocks, packages, and conditionals) may not exceed the given value nnn. A value of zero disconnects this style check.

-gnatym

Check maximum line length.

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. Note that this count is of characters in the source text. This means that a tab character counts as one character in this count and a wide character sequence counts as a single character (however many bytes are needed in the encoding).

-gnatyM

Set maximum line length.

The length of lines must not exceed the given value nnn. The maximum value that can be specified is 32767. If neither style option for setting the line length is used, then the default is 255. This also controls the maximum length of lexical elements, where the only restriction is that they must fit on a single line.

-gnatyn

Check casing of entities in Standard.

Any identifier from Standard must be cased to match the presentation in the Ada Reference Manual (for example, Integer and ASCII.NUL).

-gnatyN

Turn off all style checks.

All style check options are turned off.

-gnatyo

Check order of subprogram bodies.

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).

-gnatyO

Check that overriding subprograms are explicitly marked as such.

This applies to all subprograms of a derived type that override a primitive operation of the type, for both tagged and untagged types. In particular, the declaration of a primitive operation of a type extension that overrides an inherited operation must carry an overriding indicator. Another case is the declaration of a function that overrides a predefined operator (such as an equality operator).

-gnatyp

Check pragma casing.

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. An exception is that SPARK_Mode is allowed as an alternative for Spark_Mode.

-gnatyr

Check references.

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.

-gnatys

Check separate specs.

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.

-gnatyS

Check no statements after then/else.

No statements are allowed on the same line as a then or else keyword following the keyword in an if statement. or else and and then are not affected, and a special exception allows a pragma to appear after else.

-gnatyt

Check token spacing.

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, initialization) 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 parenthesis ends with a letter or digit, then a space must separate the two tokens.
  • If the token following a right parenthesis starts 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.

Exactly one blank (and no other white space) must appear between a not token and a following in token.

-gnatyu

Check unnecessary blank lines.

Unnecessary blank lines are not allowed. A blank line is considered unnecessary if it appears at the end of the file, or if more than one blank line occurs in sequence.

-gnatyx

Check extra parentheses.

Unnecessary extra level of parentheses (C-style) are not allowed around conditions in if statements, while statements and exit statements.

-gnatyy

Set all standard style check options.

This is equivalent to gnaty3aAbcefhiklmnprst, that is all checking options enabled with the exception of -gnatyB, -gnatyd, -gnatyI, -gnatyLnnn, -gnatyo, -gnatyO, -gnatyS, -gnatyu, and -gnatyx.

-gnaty-

Remove style check options.

This causes any subsequent options in the string to act as canceling the corresponding style check option. To cancel maximum nesting level control, use the L parameter without any integer value after that, because any digit following - in the parameter string of the -gnaty option will be treated as canceling the indentation check. The same is true for the M parameter. y and N parameters are not allowed after -.

-gnaty+

Enable style check options.

This causes any subsequent options in the string to enable the corresponding style check option. That is, it cancels the effect of a previous -, if any.

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 the use of -gnatyy as described above, that is all built-in standard style check options are enabled.

The switch -gnatyN clears any previously set style checks.

4.3.7. Run-Time Checks

By default, the following checks are suppressed: stack overflow checks, and checks for access before elaboration on subprogram calls. All other checks, including overflow checks, range checks and array bounds checks, are turned on by default. The following gcc switches refine this default behavior.

-gnatp

This switch causes the unit to be compiled as though pragma Suppress (All_checks) had been present in the source. Validity checks are also eliminated (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.

Note that when checks are suppressed, the compiler is allowed, but not required, to omit the checking code. If the run-time cost of the checking code is zero or near-zero, the compiler will generate it even if checks are suppressed. In particular, if the compiler can prove that a certain check will necessarily fail, it will generate code to do an unconditional ‘raise’, even if checks are suppressed. The compiler warns in this case. Another case in which checks may not be eliminated is when they are embedded in certain run time routines such as math library routines.

Of course, run-time checks are omitted whenever the compiler can prove that they will not fail, whether or not checks are suppressed.

Note that if you suppress a check that would have failed, program execution is erroneous, which means the behavior is totally unpredictable. The program might crash, or print wrong answers, or do anything else. It might even do exactly what you wanted it to do (and then it might start failing mysteriously next week or next year). The compiler will generate code based on the assumption that the condition being checked is true, which can result in erroneous execution if that assumption is wrong.

The checks subject to suppression include all the checks defined by the Ada standard, the additional implementation defined checks Alignment_Check, Duplicated_Tag_Check, Predicate_Check, Container_Checks, Tampering_Check, and Validity_Check, as well as any checks introduced using pragma Check_Name. Note that Atomic_Synchronization is not automatically suppressed by use of this option.

If the code depends on certain checks being active, you can use pragma Unsuppress either as a configuration pragma or as a local pragma to make sure that a specified check is performed even if gnatp is specified.

The -gnatp switch has no effect if a subsequent -gnat-p switch appears.

-gnat-p
This switch cancels the effect of a previous gnatp switch.
-gnato??

This switch controls the mode used for computing intermediate arithmetic integer operations, and also enables overflow checking. For a full description of overflow mode and checking control, see the ‘Overflow Check Handling in GNAT’ appendix in this User’s Guide.

Overflow checks are always enabled by this switch. The argument controls the mode, using the codes

1 = STRICT
In STRICT mode, intermediate operations are always done using the base type, and overflow checking ensures that the result is within the base type range.
2 = MINIMIZED
In MINIMIZED mode, overflows in intermediate operations are avoided where possible by using a larger integer type for the computation (typically Long_Long_Integer). Overflow checking ensures that the result fits in this larger integer type.
3 = ELIMINATED
In ELIMINATED mode, overflows in intermediate operations are avoided by using multi-precision arithmetic. In this case, overflow checking has no effect on intermediate operations (since overflow is impossible).

If two digits are present after -gnato then the first digit sets the mode for expressions outside assertions, and the second digit sets the mode for expressions within assertions. Here assertions is used in the technical sense (which includes for example precondition and postcondition expressions).

If one digit is present, the corresponding mode is applicable to both expressions within and outside assertion expressions.

If no digits are present, the default is to enable overflow checks and set STRICT mode for both kinds of expressions. This is compatible with the use of -gnato in previous versions of GNAT.

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, unlike for example the failure of a range check, can result in an incorrect value, but cannot cause random memory destruction (like an out of range subscript), or a wild jump (from an out of range case value). Overflow checking is also quite expensive in time and space, since in general it requires the use of double length arithmetic.

Note again that the default is -gnato11 (equivalent to -gnato1), so overflow checking is performed in STRICT mode by default.

-gnatE
Enables dynamic checks for access-before-elaboration on subprogram calls and generic instantiations. Note that -gnatE is not necessary for safety, because in the default mode, GNAT ensures statically that the checks would not fail. For full details of the effect and use of this switch, Compiling with gcc.
-fstack-check
Activates stack overflow checking. For full details of the effect and use of this switch see Stack Overflow Checking.

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.

4.3.8. 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.

When the source file is empty or contains only empty lines and/or comments, the output is a warning:

$ gcc -c -gnats -x ada toto.txt
toto.txt:1:01: warning: empty file, contains no compilation units
$

Otherwise, 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 withs 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 (Renaming Files with gnatchop).

4.3.9. 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 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 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 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).

4.3.10. Compiling Different Versions of Ada

The switches described in this section allow you to explicitly specify the version of the Ada language that your programs are written in. The default mode is Ada 2012, but you can also specify Ada 95, Ada 2005 mode, or indicate Ada 83 compatibility mode.

-gnat83 (Ada 83 Compatibility Mode)

Although GNAT is primarily an Ada 95 / Ada 2005 compiler, this switch specifies that the program is to be compiled in Ada 83 mode. With -gnat83, GNAT rejects most post-Ada 83 extensions and applies Ada 83 semantics where this can be done easily. It is not possible to guarantee this switch does a perfect job; some subtle tests, such as are found in earlier ACVC tests (and that have been removed from the ACATS suite for Ada 95), might not compile correctly. Nevertheless, this switch may be useful in some circumstances, for example where, due to contractual reasons, existing code needs to be maintained using only Ada 83 features.

With few exceptions (most notably the need to use <> on unconstrained generic formal parameters, the use of the new Ada 95 / Ada 2005 reserved words, and the use of packages with optional bodies), it is not necessary to specify the -gnat83 switch when compiling Ada 83 programs, because, with rare exceptions, Ada 95 and Ada 2005 are upwardly compatible with Ada 83. Thus a correct Ada 83 program is usually also a correct program in these later versions of the language standard. For further information please refer to the Compatibility and Porting Guide chapter in the GNAT Reference Manual.

-gnat95 (Ada 95 mode)

This switch directs the compiler to implement the Ada 95 version of the language. Since Ada 95 is almost completely upwards compatible with Ada 83, Ada 83 programs may generally be compiled using this switch (see the description of the -gnat83 switch for further information about Ada 83 mode). If an Ada 2005 program is compiled in Ada 95 mode, uses of the new Ada 2005 features will cause error messages or warnings.

This switch also can be used to cancel the effect of a previous -gnat83, -gnat05/2005, or -gnat12/2012 switch earlier in the command line.

-gnat05 or -gnat2005 (Ada 2005 mode)
This switch directs the compiler to implement the Ada 2005 version of the language, as documented in the official Ada standards document. Since Ada 2005 is almost completely upwards compatible with Ada 95 (and thus also with Ada 83), Ada 83 and Ada 95 programs may generally be compiled using this switch (see the description of the -gnat83 and -gnat95 switches for further information).
-gnat12 or -gnat2012 (Ada 2012 mode)
This switch directs the compiler to implement the Ada 2012 version of the language (also the default). Since Ada 2012 is almost completely upwards compatible with Ada 2005 (and thus also with Ada 83, and Ada 95), Ada 83 and Ada 95 programs may generally be compiled using this switch (see the description of the -gnat83, -gnat95, and -gnat05/2005 switches for further information).
-gnatX (Enable GNAT Extensions)
This switch directs the compiler to implement the latest version of the language (currently Ada 2012) and also to enable certain GNAT implementation extensions that are not part of any Ada standard. For a full list of these extensions, see the GNAT reference manual.

4.3.11. Character Set Control

-gnatic

Normally GNAT recognizes the Latin-1 character set in source program identifiers, as described in the Ada 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 ISO 8859-1 (Latin-1) identifiers
2 ISO 8859-2 (Latin-2) letters allowed in identifiers
3 ISO 8859-3 (Latin-3) letters allowed in identifiers
4 ISO 8859-4 (Latin-4) letters allowed in identifiers
5 ISO 8859-5 (Cyrillic) letters allowed in identifiers
9 ISO 8859-15 (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 these encoding methods 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.

Note that brackets encoding only applies to program text. Within comments, brackets are considered to be normal graphic characters, and bracket sequences are never recognized as wide characters.

If no -gnatW? parameter is present, then the default representation is normally Brackets encoding only. However, if the first three characters of the file are 16#EF# 16#BB# 16#BF# (the standard byte order mark or BOM for UTF-8), then these three characters are skipped and the default representation for the file is set to UTF-8.

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.

When no -gnatW? is specified, then characters (other than wide characters represented using brackets notation) are treated as 8-bit Latin-1 codes. The codes recognized are the Latin-1 graphic characters, and ASCII format effectors (CR, LF, HT, VT). Other lower half control characters in the range 16#00#..16#1F# are not accepted in program text or in comments. Upper half control characters (16#80#..16#9F#) are rejected in program text, but allowed and ignored in comments. Note in particular that the Next Line (NEL) character whose encoding is 16#85# is not recognized as an end of line in this default mode. If your source program contains instances of the NEL character used as a line terminator, you must use UTF-8 encoding for the whole source program. In default mode, all lines must be ended by a standard end of line sequence (CR, CR/LF, or LF).

Note that the convention of simply accepting all upper half characters in comments means that programs that use standard ASCII for program text, but UTF-8 encoding for comments are accepted in default mode, providing that the comments are ended by an appropriate (CR, or CR/LF, or LF) line terminator. This is a common mode for many programs with foreign language comments.

4.3.12. 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, File Naming Rules.

4.3.13. Subprogram Inlining Control

-gnatn[12]

The n here is intended to suggest the first syllable of the word ‘inline’. GNAT recognizes and processes Inline pragmas. However, for inlining to actually occur, optimization must be enabled and, by default, inlining of subprograms across modules is not performed. If you want to additionally enable inlining of subprograms specified by pragma Inline across modules, you must also specify this switch.

In the absence of this switch, GNAT does not attempt inlining across modules and does not access the bodies of subprograms for which pragma Inline is specified if they are not in the current unit.

You can optionally specify the inlining level: 1 for moderate inlining across modules, which is a good compromise between compilation times and performances at run time, or 2 for full inlining across modules, which may bring about longer compilation times. If no inlining level is specified, the compiler will pick it based on the optimization level: 1 for -O1, -O2 or -Os and 2 for -O3.

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 Inlining of Subprograms.

-gnatN

This switch activates front-end inlining which also generates additional dependencies.

When using a gcc-based back end (in practice this means using any version of GNAT other than the JGNAT, .NET or GNAAMP versions), then the use of -gnatN is deprecated, and the use of -gnatn is preferred. Historically front end inlining was more extensive than the gcc back end inlining, but that is no longer the case.

4.3.14. 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. Note that the combination of switches -gnatct generates a tree in the form required by ASIS applications.
-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.

4.3.15. 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[=nn]

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 optional parameter nn if present after -gnatG specifies an alternative maximum line length that overrides the normal default of 72. This value is in the range 40-999999, values less than 40 being silently reset to 40. The equal sign is optional.

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 spec of package Sprint in file sprint.ads for a full list.

If the switch -gnatL is used in conjunction with -gnatG, then the original source lines are interspersed in the expanded source (as comment lines with the original line number).

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.
[subtype or type declaration]
Used to list an equivalent declaration for an internally generated type that is referenced elsewhere in the listing.
freeze type-name [actions]
Shows the point at which type-name 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.
label-name : label
Declaration of label labelname.
#$ subprogram-name
An implicit call to a run-time support routine (to meet the requirement of H.3.1(9) in a convenient manner).
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[=nn]

When used in conjunction with -gnatG, this switch causes the expanded source, as described above for -gnatG to be written to files with names xxx.dg, where xxx is the normal file name, instead of to the standard output file. 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 suppresses generation of cross-reference information (see -gnatx) since otherwise the cross-reference information would refer to the .dg file, which would cause confusion since this is not the original source file.

Note that -gnatD actually implies -gnatG automatically, so it is not necessary to give both options. In other words -gnatD is equivalent to -gnatDG).

If the switch -gnatL is used in conjunction with -gnatDG, then the original source lines are interspersed in the expanded source (as comment lines with the original line number).

The optional parameter nn if present after -gnatD specifies an alternative maximum line length that overrides the normal default of 72. This value is in the range 40-999999, values less than 40 being silently reset to 40. The equal sign is optional.

-gnatr
This switch causes pragma Restrictions to be treated as Restriction_Warnings so that violation of restrictions causes warnings rather than illegalities. This is useful during the development process when new restrictions are added or investigated. The switch also causes pragma Profile to be treated as Profile_Warnings, and pragma Restricted_Run_Time and pragma Ravenscar set restriction warnings rather than restrictions.
-gnatR[0|1|2|3][e][m][s]

This switch controls output from the compiler of a listing showing representation information for declared types, objects and subprograms. 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 declared types and objects. The Linker_Section is also listed for any entity for which the Linker_Section is set explicitly or implicitly (the latter case occurs for objects of a type for which a Linker_Section is set).

For -gnatR3, symbolic expressions for values that are computed at run time for records are included. 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 details on the format of -gnatR3 output.

If the switch is followed by an e (e.g. -gnatR2e), then extended representation information for record sub-components of records are included.

If the switch is followed by an m (e.g. -gnatRm), then subprogram conventions and parameter passing mechanisms for all the subprograms are included.

If the switch is followed by an s (e.g., -gnatR3s), then the output is to a file with the name file.rep where file is the name of the corresponding source file.

Note that it is possible for record components to have zero size. In this case, the component clause uses an obvious extension of permitted Ada syntax, for example at 0 range 0 .. -1.

-gnatS
The use of the switch -gnatS for an Ada compilation will cause the compiler to output a representation of package Standard in a form very close to standard Ada. It is not quite possible to do this entirely in standard Ada (since new numeric base types cannot be created in standard Ada), but the output is easily readable to any Ada programmer, and is useful to determine the characteristics of target dependent types in package Standard.
-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.

4.3.16. Exception Handling Control

GNAT uses two methods for handling exceptions at run-time. The setjmp/longjmp method saves the context when entering a frame with an exception handler. Then when an exception is raised, the context can be restored immediately, without the need for tracing stack frames. This method provides very fast exception propagation, but introduces significant overhead for the use of exception handlers, even if no exception is raised.

The other approach is called ‘zero cost’ exception handling. With this method, the compiler builds static tables to describe the exception ranges. No dynamic code is required when entering a frame containing an exception handler. When an exception is raised, the tables are used to control a back trace of the subprogram invocation stack to locate the required exception handler. This method has considerably poorer performance for the propagation of exceptions, but there is no overhead for exception handlers if no exception is raised. Note that in this mode and in the context of mixed Ada and C/C++ programming, to propagate an exception through a C/C++ code, the C/C++ code must be compiled with the -funwind-tables GCC’s option.

The following switches may be used to control which of the two exception handling methods is used.

--RTS=sjlj
This switch causes the setjmp/longjmp run-time (when available) to be used for exception handling. If the default mechanism for the target is zero cost exceptions, then this switch can be used to modify this default, and must be used for all units in the partition. This option is rarely used. One case in which it may be advantageous is if you have an application where exception raising is common and the overall performance of the application is improved by favoring exception propagation.
--RTS=zcx
This switch causes the zero cost approach to be used for exception handling. If this is the default mechanism for the target (see below), then this switch is unneeded. If the default mechanism for the target is setjmp/longjmp exceptions, then this switch can be used to modify this default, and must be used for all units in the partition. This option can only be used if the zero cost approach is available for the target in use, otherwise it will generate an error.

The same option --RTS must be used both for gcc and gnatbind. Passing this option to gnatmake (Switches for gnatmake) will ensure the required consistency through the compilation and binding steps.

4.3.17. Units to Sources Mapping Files

-gnatem=path

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.

The use of mapping files is not required for correct operation of the compiler, but mapping files can improve efficiency, particularly when sources are read over a slow network connection. In normal operation, you need not be concerned with the format or use of mapping files, and the -gnatem switch is not a switch that you would use explicitly. It is intended primarily for use by automatic tools such as gnatmake running under the project file facility. The description here of the format of mapping files is provided for completeness and for possible use by other tools.

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 specs 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 (nonexistent 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 creates a temporary mapping file and communicates it to the compiler using this switch.

4.3.18. Code Generation Control

The GCC technology provides a wide range of target dependent -m switches for controlling details of code generation with respect to different versions of architectures. This includes variations in instruction sets (e.g., different members of the power pc family), and different requirements for optimal arrangement of instructions (e.g., different members of the x86 family). The list of available -m switches may be found in the GCC documentation.

Use of these -m switches may in some cases result in improved code performance.

The GNAT technology is tested and qualified without any -m switches, so generally the most reliable approach is to avoid the use of these switches. However, we generally expect most of these switches to work successfully with GNAT, and many customers have reported successful use of these options.

Our general advice is to avoid the use of -m switches unless special needs lead to requirements in this area. In particular, there is no point in using -m switches to improve performance unless you actually see a performance improvement.

4.4. Linker Switches

Linker switches can be specified after -largs builder switch.

-fuse-ld=name
Linker to be used. The default is bfd for ld.bfd, the alternative being gold for ld.gold. The later is a more recent and faster linker, but only available on GNU/Linux platforms.

4.5. Binding with gnatbind

This chapter describes the GNAT binder, gnatbind, which is used to bind compiled GNAT objects.

The gnatbind program performs four separate functions:

  • Checks that a program is consistent, in accordance with the rules in Chapter 10 of the Ada Reference Manual. In particular, error messages are generated if a program uses inconsistent versions of a given unit.
  • 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 Language Manual.
  • 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.
  • 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.

4.5.1. 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. gnatbind constructs an Ada package in two files whose names are b~mainprog.ads, and b~mainprog.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:

  • Enter gcc -c hello.adb to compile the main program.
  • Enter gcc -c p.ads to compile package P.
  • Edit file p.ads.
  • 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 for reference purposes a sample binder output file is given in Example of Binder Output File.

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.

4.5.2. Switches for gnatbind

The following switches are available with gnatbind; details will be presented in subsequent sections.

--version
Display Copyright and version, then exit disregarding all other options.
--help
If --version was not used, display usage, then exit disregarding all other options.
-a
Indicates that, if supported by the platform, the adainit procedure should be treated as an initialisation routine by the linker (a constructor). This is intended to be used by the Project Manager to automatically initialize shared Stand-Alone Libraries.
-aO
Specify directory to be searched for ALI files.
-aI
Specify directory to be searched for source file.
-A[=filename]
Output ALI list (to standard output or to the named file).
-b
Generate brief messages to stderr even if verbose mode set.
-c
Check only, no generation of binder output file.
-dnn[k|m]

This switch can be used to change the default task stack size value to a specified size nn, which is expressed in bytes by default, or in kilobytes when suffixed with k or in megabytes when suffixed with m. In the absence of a [k|m] suffix, this switch is equivalent, in effect, to completing all task specs with

pragma Storage_Size (nn);

When they do not already have such a pragma.

-Dnn[k|m]

This switch can be used to change the default secondary stack size value to a specified size nn, which is expressed in bytes by default, or in kilobytes when suffixed with k or in megabytes when suffixed with m.

The secondary stack is used to deal with functions that return a variable sized result, for example a function returning an unconstrained String. There are two ways in which this secondary stack is allocated.

For most targets, the secondary stack grows on demand and is allocated as a chain of blocks in the heap. The -D option is not very relevant. It only give some control over the size of the allocated blocks (whose size is the minimum of the default secondary stack size value, and the actual size needed for the current allocation request).

For certain targets, notably VxWorks 653 and bare board targets, the secondary stack is allocated by carving off a chunk of the primary task stack. By default this is a fixed percentage of the primary task stack as defined by System.Parameter.Sec_Stack_Percentage. This can be overridden per task using the Secondary_Stack_Size pragma/aspect. The -D option is used to define the size of the environment task’s secondary stack.

-e
Output complete list of elaboration-order dependencies.
-Ea

Store tracebacks in exception occurrences when the target supports it. The “a” is for “address”; tracebacks will contain hexadecimal addresses, unless symbolic tracebacks are enabled.

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.

-Es
Store tracebacks in exception occurrences when the target supports it. The “s” is for “symbolic”; symbolic tracebacks are enabled.
-E
Currently the same as -Ea.
-felab-order
Force elaboration order.
-F
Force the checks of elaboration flags. gnatbind does not normally generate checks of elaboration flags for the main executable, except when a Stand-Alone Library is used. However, there are cases when this cannot be detected by gnatbind. An example is importing an interface of a Stand-Alone Library through a pragma Import and only specifying through a linker switch this Stand-Alone Library. This switch is used to guarantee that elaboration flag checks are generated.
-h
Output usage (help) information.
-H32
Use 32-bit allocations for __gnat_malloc (and thus for access types). For further details see Dynamic Allocation Control.
-H64
Use 64-bit allocations for __gnat_malloc (and thus for access types). For further details see Dynamic Allocation Control.
-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
Bind the units for library building. In this case the adainit and adafinal procedures (Binding with Non-Ada Main Programs) are renamed to xxxinit and xxxfinal. Implies -n. (GNAT and Libraries, for more details.)
-Mxyz
Rename generated main program from main to xyz. This option is supported on cross environments only.
-mn
Limit number of detected errors or warnings to n, where n is in the range 1..999999. The default value if no switch is given is 9999. If the number of warnings reaches this limit, then a message is output and further warnings are suppressed, the bind continues in this case. If the number of errors reaches this limit, then a message is output and the bind is abandoned. A value of zero means that no limit is enforced. The equal sign is optional.
-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 (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[=filename]
Output object list (to standard output or to the named file).
-p
Pessimistic (worst-case) elaboration order.
-P
Generate binder file suitable for CodePeer.
-R
Output closure source list, which includes all non-run-time units that are included in the bind.
-Ra
Like -R but the list includes run-time units.
-s
Require all source files to be present.
-Sxxx

Specifies the value to be used when detecting uninitialized scalar objects with pragma Initialize_Scalars. The xxx string specified with the switch is one of:

  • in for an invalid value.

    If zero is invalid for the discrete type in question, then the scalar value is set to all zero bits. For signed discrete types, the largest possible negative value of the underlying scalar is set (i.e. a one bit followed by all zero bits). For unsigned discrete types, the underlying scalar value is set to all one bits. For floating-point types, a NaN value is set (see body of package System.Scalar_Values for exact values).

  • lo for low value.

    If zero is invalid for the discrete type in question, then the scalar value is set to all zero bits. For signed discrete types, the largest possible negative value of the underlying scalar is set (i.e. a one bit followed by all zero bits). For unsigned discrete types, the underlying scalar value is set to all zero bits. For floating-point, a small value is set (see body of package System.Scalar_Values for exact values).

  • hi for high value.

    If zero is invalid for the discrete type in question, then the scalar value is set to all one bits. For signed discrete types, the largest possible positive value of the underlying scalar is set (i.e. a zero bit followed by all one bits). For unsigned discrete types, the underlying scalar value is set to all one bits. For floating-point, a large value is set (see body of package System.Scalar_Values for exact values).

  • xx for hex value (two hex digits).

    The underlying scalar is set to a value consisting of repeated bytes, whose value corresponds to the given value. For example if BF is given, then a 32-bit scalar value will be set to the bit patterm 16#BFBFBFBF#.

In addition, you can specify -Sev to indicate that the value is to be set at run time. In this case, the program will look for an environment variable of the form GNAT_INIT_SCALARS=yy, where yy is one of in/lo/hi/xx with the same meanings as above. If no environment variable is found, or if it does not have a valid value, then the default is in (invalid values).

-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 milliseconds. If the system supports the specification of a specific time slice value, then the indicated value is used. If the system does not support specific time slice values, but does support some general notion of round-robin scheduling, then any nonzero value will activate round-robin scheduling.

A value of zero is treated specially. It turns off time slicing, and in addition, indicates to the tasking run time that the semantics should match as closely as possible the Annex D requirements of the Ada RM, and in particular sets the default scheduling policy to FIFO_Within_Priorities.

-un
Enable dynamic stack usage, with n results stored and displayed at program termination. A result is generated when a task terminates. Results that can’t be stored are displayed on the fly, at task termination. This option is currently not supported on Itanium platforms. (See Dynamic Stack Usage Analysis for details.)
-v
Verbose mode. Write error messages, header, summary output to stdout.
-Vkey=value
Store the given association of key to value in the bind environment. Values stored this way can be retrieved at run time using GNAT.Bind_Environment.
-wx
Warning mode; x = s/e for suppress/treat as error.
-Wxe
Override default wide character encoding for standard Text_IO files.
-x
Exclude source files (check object consistency only).
-Xnnn
Set default exit status value, normally 0 for POSIX compliance.
-y
Enable leap seconds support in Ada.Calendar and its children.
-z
No main subprogram.

You may obtain this listing of switches by running gnatbind with no arguments.

4.5.2.1. Consistency-Checking Modes

As described earlier, 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.
-Wxe
Override default wide character encoding for standard Text_IO files. Normally the default wide character encoding method used for standard [Wide_[Wide_]]Text_IO files is taken from the encoding specified for the main source input (see description of switch -gnatWx for the compiler). The use of this switch for the binder (which has the same set of possible arguments) overrides this default as specified.
-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).

4.5.2.2. 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.

Note

This means that -t should be used only in unusual situations, with extreme care.

4.5.2.3. Elaboration Control

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

-felab-order

Force elaboration order.

elab-order should be the name of a “forced elaboration order file”, that is, a text file containing library item names, one per line. A name of the form “some.unit%s” or “some.unit (spec)” denotes the spec of Some.Unit. A name of the form “some.unit%b” or “some.unit (body)” denotes the body of Some.Unit. Each pair of lines is taken to mean that there is an elaboration dependence of the second line on the first. For example, if the file contains:

this (spec)
this (body)
that (spec)
that (body)

then the spec of This will be elaborated before the body of This, and the body of This will be elaborated before the spec of That, and the spec of That will be elaborated before the body of That. The first and last of these three dependences are already required by Ada rules, so this file is really just forcing the body of This to be elaborated before the spec of That.

The given order must be consistent with Ada rules, or else gnatbind will give elaboration cycle errors. For example, if you say x (body) should be elaborated before x (spec), there will be a cycle, because Ada rules require x (spec) to be elaborated before x (body); you can’t have the spec and body both elaborated before each other.

If you later add “with That;” to the body of This, there will be a cycle, in which case you should erase either “this (body)” or “that (spec)” from the above forced elaboration order file.

Blank lines and Ada-style comments are ignored. Unit names that do not exist in the program are ignored. Units in the GNAT predefined library are also ignored.

-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 and 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.

Note that -p is not intended for production use; it is more for debugging/experimental use.

4.5.2.4. Output Control

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

-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.
-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. 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 Restrictions 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.

4.5.2.5. Dynamic Allocation Control

The heap control switches – -H32 and -H64 – determine whether dynamic allocation uses 32-bit or 64-bit memory. They only affect compiler-generated allocations via __gnat_malloc; explicit calls to malloc and related functions from the C run-time library are unaffected.

-H32
Allocate memory on 32-bit heap
-H64
Allocate memory on 64-bit heap. This is the default unless explicitly overridden by a 'Size clause on the access type.

These switches are only effective on VMS platforms.

4.5.2.6. Binding with Non-Ada Main Programs

The description so far has 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 (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. 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.

4.5.2.7. 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. Note that the default wide character encoding method for standard Text_IO files is always set to Brackets if this switch is set (you can use the binder switch -Wx to override this default).

4.5.3. 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.

4.5.4. 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:

  • The directory containing the ALI file named in the command line, unless the switch -I- is specified.

  • All directories specified by -I switches on the gnatbind command line, in the order given.

  • Each of the directories listed in the text file whose name is given by the ADA_PRJ_OBJECTS_FILE environment variable.

    ADA_PRJ_OBJECTS_FILE is normally set by gnatmake or by the gnat driver when project files are used. It should not normally be set by other means.

  • 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).

  • 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. See Installing a 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 -aO`dir. 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.

4.5.5. Examples of gnatbind Usage

Here are some examples of gnatbind invovations:

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. Note that if this option is used, then linking must be done manually, since gnatlink will not be able to find the generated file.

4.7. Using the GNU make Utility

This chapter offers some examples of makefiles that solve specific problems. It does not explain how to write a makefile, nor does it try to replace the gnatmake utility (Building with gnatmake).

All the examples in this section are specific to the GNU version of make. Although make is a standard utility, and the basic language is the same, these examples use some advanced features found only in GNU make.

4.7.1. Using gnatmake in a Makefile

Complex project organizations can be handled in a very powerful way by using GNU make combined with gnatmake. For instance, here is a Makefile which allows you to build each subsystem of a big project into a separate shared library. Such a makefile allows you to significantly reduce the link time of very big applications while maintaining full coherence at each step of the build process.

The list of dependencies are handled automatically by gnatmake. The Makefile is simply used to call gnatmake in each of the appropriate directories.

Note that you should also read the example on how to automatically create the list of directories (Automatically Creating a List of Directories) which might help you in case your project has a lot of subdirectories.

## This Makefile is intended to be used with the following directory
## configuration:
##  - The sources are split into a series of csc (computer software components)
##    Each of these csc is put in its own directory.
##    Their name are referenced by the directory names.
##    They will be compiled into shared library (although this would also work
##    with static libraries
##  - The main program (and possibly other packages that do not belong to any
##    csc is put in the top level directory (where the Makefile is).
##       toplevel_dir __ first_csc  (sources) __ lib (will contain the library)
##                    \\_ second_csc (sources) __ lib (will contain the library)
##                    \\_ ...
## Although this Makefile is build for shared library, it is easy to modify
## to build partial link objects instead (modify the lines with -shared and
## gnatlink below)
##
## With this makefile, you can change any file in the system or add any new
## file, and everything will be recompiled correctly (only the relevant shared
## objects will be recompiled, and the main program will be re-linked).

# The list of computer software component for your project. This might be
# generated automatically.
CSC_LIST=aa bb cc

# Name of the main program (no extension)
MAIN=main

# If we need to build objects with -fPIC, uncomment the following line
#NEED_FPIC=-fPIC

# The following variable should give the directory containing libgnat.so
# You can get this directory through 'gnatls -v'. This is usually the last
# directory in the Object_Path.
GLIB=...

# The directories for the libraries
# (This macro expands the list of CSC to the list of shared libraries, you
# could simply use the expanded form:
# LIB_DIR=aa/lib/libaa.so bb/lib/libbb.so cc/lib/libcc.so
LIB_DIR=${foreach dir,${CSC_LIST},${dir}/lib/lib${dir}.so}

${MAIN}: objects ${LIB_DIR}
    gnatbind ${MAIN} ${CSC_LIST:%=-aO%/lib} -shared
    gnatlink ${MAIN} ${CSC_LIST:%=-l%}

objects::
    # recompile the sources
    gnatmake -c -i ${MAIN}.adb ${NEED_FPIC} ${CSC_LIST:%=-I%}

# Note: In a future version of GNAT, the following commands will be simplified
# by a new tool, gnatmlib
${LIB_DIR}:
    mkdir -p ${dir $@ }
    cd ${dir $@ } && gcc -shared -o ${notdir $@ } ../*.o -L${GLIB} -lgnat
    cd ${dir $@ } && cp -f ../*.ali .

# The dependencies for the modules
# Note that we have to force the expansion of *.o, since in some cases
# make won't be able to do it itself.
aa/lib/libaa.so: ${wildcard aa/*.o}
bb/lib/libbb.so: ${wildcard bb/*.o}
cc/lib/libcc.so: ${wildcard cc/*.o}

# Make sure all of the shared libraries are in the path before starting the
# program
run::
    LD_LIBRARY_PATH=`pwd`/aa/lib:`pwd`/bb/lib:`pwd`/cc/lib ./${MAIN}

clean::
    ${RM} -rf ${CSC_LIST:%=%/lib}
    ${RM} ${CSC_LIST:%=%/*.ali}
    ${RM} ${CSC_LIST:%=%/*.o}
    ${RM} *.o *.ali ${MAIN}

4.7.2. Automatically Creating a List of Directories

In most makefiles, you will have to specify a list of directories, and store it in a variable. For small projects, it is often easier to specify each of them by hand, since you then have full control over what is the proper order for these directories, which ones should be included.

However, in larger projects, which might involve hundreds of subdirectories, it might be more convenient to generate this list automatically.

The example below presents two methods. The first one, although less general, gives you more control over the list. It involves wildcard characters, that are automatically expanded by make. Its shortcoming is that you need to explicitly specify some of the organization of your project, such as for instance the directory tree depth, whether some directories are found in a separate tree, etc.

The second method is the most general one. It requires an external program, called find, which is standard on all Unix systems. All the directories found under a given root directory will be added to the list.

# The examples below are based on the following directory hierarchy:
# All the directories can contain any number of files
# ROOT_DIRECTORY ->  a  ->  aa  ->  aaa
#                       ->  ab
#                       ->  ac
#                ->  b  ->  ba  ->  baa
#                       ->  bb
#                       ->  bc
# This Makefile creates a variable called DIRS, that can be reused any time
# you need this list (see the other examples in this section)

# The root of your project's directory hierarchy
ROOT_DIRECTORY=.

####
# First method: specify explicitly the list of directories
# This allows you to specify any subset of all the directories you need.
####

DIRS := a/aa/ a/ab/ b/ba/

####
# Second method: use wildcards
# Note that the argument(s) to wildcard below should end with a '/'.
# Since wildcards also return file names, we have to filter them out
# to avoid duplicate directory names.
# We thus use make's ``dir`` and ``sort`` functions.
# It sets DIRs to the following value (note that the directories aaa and baa
# are not given, unless you change the arguments to wildcard).
# DIRS= ./a/a/ ./b/ ./a/aa/ ./a/ab/ ./a/ac/ ./b/ba/ ./b/bb/ ./b/bc/
####

DIRS := ${sort ${dir ${wildcard ${ROOT_DIRECTORY}/*/
                    ${ROOT_DIRECTORY}/*/*/}}}

####
# Third method: use an external program
# This command is much faster if run on local disks, avoiding NFS slowdowns.
# This is the most complete command: it sets DIRs to the following value:
# DIRS= ./a ./a/aa ./a/aa/aaa ./a/ab ./a/ac ./b ./b/ba ./b/ba/baa ./b/bb ./b/bc
####

DIRS := ${shell find ${ROOT_DIRECTORY} -type d -print}

4.7.3. Generating the Command Line Switches

Once you have created the list of directories as explained in the previous section (Automatically Creating a List of Directories), you can easily generate the command line arguments to pass to gnatmake.

For the sake of completeness, this example assumes that the source path is not the same as the object path, and that you have two separate lists of directories.

# see "Automatically creating a list of directories" to create
# these variables
SOURCE_DIRS=
OBJECT_DIRS=

GNATMAKE_SWITCHES := ${patsubst %,-aI%,${SOURCE_DIRS}}
GNATMAKE_SWITCHES += ${patsubst %,-aO%,${OBJECT_DIRS}}

all:
        gnatmake ${GNATMAKE_SWITCHES} main_unit

4.7.4. Overcoming Command Line Length Limits

One problem that might be encountered on big projects is that many operating systems limit the length of the command line. It is thus hard to give gnatmake the list of source and object directories.

This example shows how you can set up environment variables, which will make gnatmake behave exactly as if the directories had been specified on the command line, but have a much higher length limit (or even none on most systems).

It assumes that you have created a list of directories in your Makefile, using one of the methods presented in Automatically Creating a List of Directories. For the sake of completeness, we assume that the object path (where the ALI files are found) is different from the sources patch.

Note a small trick in the Makefile below: for efficiency reasons, we create two temporary variables (SOURCE_LIST and OBJECT_LIST), that are expanded immediately by make. This way we overcome the standard make behavior which is to expand the variables only when they are actually used.

On Windows, if you are using the standard Windows command shell, you must replace colons with semicolons in the assignments to these variables.

# In this example, we create both ADA_INCLUDE_PATH and ADA_OBJECTS_PATH.
# This is the same thing as putting the -I arguments on the command line.
# (the equivalent of using -aI on the command line would be to define
#  only ADA_INCLUDE_PATH, the equivalent of -aO is ADA_OBJECTS_PATH).
# You can of course have different values for these variables.
#
# Note also that we need to keep the previous values of these variables, since
# they might have been set before running 'make' to specify where the GNAT
# library is installed.

# see "Automatically creating a list of directories" to create these
# variables
SOURCE_DIRS=
OBJECT_DIRS=

empty:=
space:=${empty} ${empty}
SOURCE_LIST := ${subst ${space},:,${SOURCE_DIRS}}
OBJECT_LIST := ${subst ${space},:,${OBJECT_DIRS}}
ADA_INCLUDE_PATH += ${SOURCE_LIST}
ADA_OBJECTS_PATH += ${OBJECT_LIST}
export ADA_INCLUDE_PATH
export ADA_OBJECTS_PATH

all:
        gnatmake main_unit