Platform-Specific Information

This appendix contains information relating to the implementation of run-time libraries on various platforms. It also covers topics related to the GNAT implementation on specific Operating Systems.

Run-Time Libraries

The GNAT run-time implementation may vary with respect to both the underlying threads library and the exception-handling scheme. For threads support, the default run-time will bind to the thread package of the underlying operating system.

For exception handling, either or both of two models are supplied:

  • Zero-Cost Exceptions (“ZCX”), which uses binder-generated tables that are interrogated at run time to locate a handler.

  • setjmp / longjmp (‘SJLJ’), which uses dynamically-set data to establish the set of handlers

Most programs should experience a substantial speed improvement by being compiled with a ZCX run-time. This is especially true for tasking applications or applications with many exception handlers. Note however that the ZCX run-time does not support asynchronous abort of tasks (abort and select-then-abort constructs) and will instead implement abort by polling points in the runtime. You can also add additional polling points explicitly if needed in your application via pragma Abort_Defer.

This section summarizes which combinations of threads and exception support are supplied on various GNAT platforms.

Summary of Run-Time Configurations

Platform

Run-Time

Tasking

Exceptions

GNU/Linux

rts-native (default)

pthread library

ZCX

rts-sjlj

pthread library

SJLJ

Windows

rts-native (default)

native Win32 threads

ZCX

rts-sjlj

native Win32 threads

SJLJ

Mac OS

rts-native

pthread library

ZCX

Specifying a Run-Time Library

The adainclude subdirectory containing the sources of the GNAT run-time library and the adalib subdirectory containing the ALI files and the static and/or shared GNAT library are located in the gcc target-dependent area:

target=$prefix/lib/gcc/gcc-*dumpmachine*/gcc-*dumpversion*/

As indicated above, on some platforms, several run-time libraries are supplied. These libraries are installed in the target dependent area and contain a complete source and binary subdirectory. The detailed description below explains the differences between the different libraries in terms of their thread support.

The default run-time library (when GNAT is installed) is rts-native. This default run-time is selected by the means of soft links. For example on x86-linux:

../_images/rtlibrary-structure.png

If you want to select the rts-sjlj library on a permanent basis, you can modify these soft links with the following commands:

$ cd $target
$ rm -f adainclude adalib
$ ln -s rts-sjlj/adainclude adainclude
$ ln -s rts-sjlj/adalib adalib

Alternatively, you can specify rts-sjlj/adainclude in the file $target/ada_source_path and rts-sjlj/adalib in $target/ada_object_path.

You can select another run-time library temporarily by using the --RTS switch, e.g., --RTS=sjlj

GNU/Linux Topics

This section describes topics that are specific to GNU/Linux platforms.

Required Packages on GNU/Linux

GNAT requires the C library developer’s package to be installed. The name of of that package depends on your GNU/Linux distribution:

  • RedHat, SUSE: glibc-devel;

  • Debian, Ubuntu: libc6-dev (normally installed by default).

If you’re using the 32-bit version of GNAT on a 64-bit version of GNU/Linux, you’ll need the 32-bit version of the following packages:

  • RedHat, SUSE: glibc.i686, glibc-devel.i686, ncurses-libs.i686

  • SUSE: glibc-locale-base-32bit

  • Debian, Ubuntu: libc6:i386, libc6-dev:i386, lib32ncursesw5

Other GNU/Linux distributions might choose different name for those packages.

Position Independent Executable (PIE) Enabled by Default on Linux

GNAT generates Position Independent Executable (PIE) code by default. PIE binaries are loaded into random memory locations, introducing an additional layer of protection against attacks.

Building PIE binaries requires that all of their dependencies also be built as Position Independent. If the link of your project fails with an error like:

/[...]/ld: /path/to/object/file: relocation R_X86_64_32S against symbol
`symbol name' can not be used when making a PIE object;
recompile with -fPIE

it means the identified object file has not been built as Position Independent.

If you are not interested in building PIE binaries, you can simply turn this feature off by first compiling your code with -fno-pie and then by linking with -no-pie (note the subtle but important difference in the names of the switches – the linker switch does not have an f after the dash!). When using gprbuild, you do this by updating the Required_Switches attribute in package Compiler and, depending on your type of project, either attribute Switches or attribute Library_Options in package Linker.

On the other hand, if you would like to build PIE binaries and you are getting the error above, a quick and easy workaround to allow linking to succeed again is to disable PIE during the link, thus temporarily lifting the requirement that all dependencies also be Position Independent code. To do so, you simply need to add -no-pie to the list of switches passed to the linker. As part of this workaround, there is no need to adjust the compiler switches.

From there, to be able to link your binaries with PIE and therefore drop the -no-pie workaround, you’ll need to get the identified dependencies rebuilt with PIE enabled (compiled with -fPIE and linked with -pie).

Choosing the Scheduling Policy with GNU/Linux

When using a POSIX threads implementation, you have a choice of several scheduling policies: SCHED_FIFO, SCHED_RR and SCHED_OTHER.

Typically, the default is SCHED_OTHER, while using SCHED_FIFO or SCHED_RR requires special (e.g., root) privileges.

By default, GNAT uses the SCHED_OTHER policy. To specify SCHED_FIFO, you can use one of the following:

  • pragma Time_Slice (0.0)

  • the corresponding binder switch -T0

  • pragma Task_Dispatching_Policy (FIFO_Within_Priorities)

To specify SCHED_RR, you should use pragma Time_Slice with a value greater than 0.0, or else use the corresponding -T binder switch.

To make sure a program is running as root, you can put something like this in a library package body in your application:

function geteuid return Integer;
pragma Import (C, geteuid, "geteuid");
Ignore : constant Boolean :=
  (if geteuid = 0 then True else raise Program_Error with "must be root");

This gets the effective user id and if it’s not 0 (i.e. root), it raises Program_Error. Note that if you’re running the code in a container, this may not be sufficient as you may have sufficient privilege on the container, but not on the host machine running the container, so check that you also have sufficient priviledge for running the container image.

A GNU/Linux Debug Quirk

On SuSE 15, some kernels have a defect causing issues when debugging programs using threads or Ada tasks. Due to the lack of documentation found regarding this kernel issue, we can only provide limited information about which kernels are impacted. Kernel version 5.3.18 is known to be impacted and kernels in the 5.14 range or newer are believed to fix this problem.

The bug affects the debugging of 32-bit processes on a 64-bit system. Symptoms can vary: Unexpected SIGABRT signals being received by the program, “The futex facility returned an unexpected error code” error message, and inferior programs hanging indefinitely range among the symptoms most commonly observed.

Microsoft Windows Topics

This section describes topics that are specific to the Microsoft Windows platforms.

Installing from the Command Line

By default the GNAT installers display a GUI that prompts you to enter the installation path and similar information and then guides you through the installation process. You can also perform silent installations using the command-line interface.

To install one of the GNAT installers from the command line, you should pass parameter /S (and, optionally, /D=<directory>) as command-line arguments.

For example, for an unattended installation of GNAT 19.2 into the default directory C:\GNATPRO\19.2 you would run:

gnatpro-19.2-x86-windows-bin /S

To install into a custom directory, say, C:\TOOLS\GNATPRO\19.2:

gnatpro-19.2-x86-windows-bin /S /D=C:\TOOLS\GNATPRO\19.2

You can use the same syntax for all installers.

Note that unattended installations don’t modify system path, nor create file associations, so you need to do such activities by hand.

Using GNAT on Windows

One of the strengths of the GNAT technology is that its tool set (gcc, gnatbind, gnatlink, gnatmake, the gdb debugger, etc.) is used in the same way regardless of the platform.

On Windows, this tool set is complemented by a number of Microsoft-specific tools that have been provided to facilitate interoperability with Windows when this is required. With these tools:

  • You can build applications using the CONSOLE or WINDOWS subsystems.

  • You can use any Dynamically Linked Library (DLL) in your Ada code (both relocatable and non-relocatable DLLs are supported).

  • You can build Ada DLLs for use in other applications. You can write these applications in a language other than Ada (e.g., C, C++, etc). Again, both relocatable and non-relocatable Ada DLLs are supported.

  • You can include Windows resources in your Ada application.

  • You can use or create COM/DCOM objects.

Listed immediately below are all known general GNAT-for-Windows restrictions. We list other restrictions about specific features such as Windows Resources and DLLs in separate sections below.

  • You cannot use GetLastError and SetLastError when tasking, protected records, or exceptions are used. In these cases, in order to implement Ada semantics, the GNAT run-time system calls certain Win32 routines that set the last error variable to 0 upon success. You may be able to use GetLastError and SetLastError when tasking, protected record, and exception features are not used, but it is not guaranteed to work.

  • You cannot link against Microsoft C++ libraries except for import libraries. You must do interfacing by means of DLLs.

  • You can link against Microsoft C libraries. However, the preferred solution is to use C/C++ compiler that comes with GNAT, since it doesn’t require having two different development environments and makes the inter-language debugging experience smoother.

  • When the compilation environment is located on FAT32 drives, you may experience recompilations of source files that have not changed if Daylight Saving Time (DST) state has changed since the last time files were compiled. NTFS drives do not have this problem.

  • No components of the GNAT toolset use any entries in the Windows registry. The only entries installation of GNAT may create are file associations and PATH settings, provided you chose to create them at installation time, as well as some minimal bookkeeping information needed to correctly uninstall or integrate different GNAT products.

Using a network installation of GNAT

Make sure the system on which GNAT is installed is accessible from the current machine, i.e., the install location is shared over the network. Shared resources are accessed on Windows by means of UNC paths, which have the format \\\\server\\sharename\\path

In order to use such a network installation, simply add the UNC path of the bin directory of your GNAT installation in front of your PATH. For example, if GNAT is installed in \GNAT directory of a share location called c-drive on a machine LOKI, the following command will make it available:

$ path \\loki\c-drive\gnat\bin;%path%`

Be aware that every compilation using the network installation results in the transfer of large amounts of data across the network and will likely cause a serious performance penalty.

CONSOLE and WINDOWS subsystems

There are two main subsystems under Windows. The CONSOLE subsystem (which is the default subsystem) always creates a console when launching the application. This is not something desirable when the application has a Windows GUI. To remove this console, your application must use the WINDOWS subsystem. To do so, you must specify the -mwindows linker switch.

$ gnatmake winprog -largs -mwindows

Temporary Files

You can control where temporary files get created by setting the TMP environment variable. The file will be created:

  • Under the directory pointed to by the TMP environment variable if this directory exists.

  • Under c:\temp, if the TMP environment variable is not set (or not pointing to a directory) and if this directory exists.

  • Under the current working directory otherwise.

This allows you to determine exactly where the temporary file will be created. This is particularly useful in networked environments where you may not have write access to some directories.

Disabling Command Line Argument Expansion

By default, an executable compiled for the Windows platform will do the following postprocessing on the arguments passed on the command line:

  • If the argument contains the characters * and/or ?, file expansion will be attempted. For example, if the current directory contains a.txt and b.txt, then when calling:

    $ my_ada_program *.txt

    The following arguments will effectively be passed to the main program (for example when using Ada.Command_Line.Argument):

    Ada.Command_Line.Argument (1) -> "a.txt"
Ada.Command_Line.Argument (2) -> "b.txt"

  • You can disable filename expansion for a given argument by using single quotes. Thus, calling:

    $ my_ada_program '*.txt'

    will result in:

    Ada.Command_Line.Argument (1) -> "*.txt"

Note that if the program is launched from a shell such as Cygwin Bash, quote removal might be performed by that shell.

In some contexts, it might be useful to disable this feature (for example if the program performs its own argument expansion). In order to do this, a C symbol needs to be defined and set to 0. You can do this by adding the following code fragment in one of your Ada units:

Do_Argv_Expansion : Integer := 0;
pragma Export (C, Do_Argv_Expansion, "__gnat_do_argv_expansion");

The results of previous examples will be respectively:

Ada.Command_Line.Argument (1) -> "*.txt"

and:

Ada.Command_Line.Argument (1) -> "'*.txt'"

Choosing the Scheduling Policy with Windows

Under Windows, the standard 31 priorities of the Ada model are mapped onto Window’s seven standard priority levels by default: Idle, Lowest, Below Normal, Normal, Above Normal,

When using the FIFO_Within_Priorities task dispatching policy, GNAT assigns the REALTIME_PRIORITY_CLASS priority class to the application and maps the Ada priority range to the sixteen priorities made available under REALTIME_PRIORITY_CLASS.

For details on the values of the different priority mappings, see declarations in system.ads. For more information about Windows priorities, please refer to Microsoft documentation.

Windows Socket Timeouts

Microsoft Windows desktops older than 8.0 and Microsoft Windows Servers older than 2019 set a socket timeout 500 milliseconds longer than the value set by setsockopt with SO_RCVTIMEO and SO_SNDTIMEO options. The GNAT runtime makes a correction for the difference in the corresponding Windows versions. For Windows Server starting with version 2019, you must provide a manifest file for the GNAT runtime to be able to recognize that the Windows version does not need the timeout correction. The manifest file should be located in the same directory as the executable file and its file name must match the executable name suffixed by .manifest. For example, if the executable name is sock_wto.exe, the manifest file name must be sock_wto.exe.manifest. The manifest file must contain at least the following data:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<compatibility xmlns="urn:schemas-microsoft-com:compatibility.v1">
<application>
   <!-- Windows Vista -->
   <supportedOS Id="{e2011457-1546-43c5-a5fe-008deee3d3f0}"/>
   <!-- Windows 7 -->
   <supportedOS Id="{35138b9a-5d96-4fbd-8e2d-a2440225f93a}"/>
   <!-- Windows 8 -->
   <supportedOS Id="{4a2f28e3-53b9-4441-ba9c-d69d4a4a6e38}"/>
   <!-- Windows 8.1 -->
   <supportedOS Id="{1f676c76-80e1-4239-95bb-83d0f6d0da78}"/>
   <!-- Windows 10 -->
   <supportedOS Id="{8e0f7a12-bfb3-4fe8-b9a5-48fd50a15a9a}"/>
</application>
</compatibility>
</assembly>

Without the manifest file, the socket timeout will be overcorrected on these Windows Server versions and the actual time wil be 500 milliseconds shorter than what was set with GNAT.Sockets.Set_Socket_Option. Note that on Microsoft Windows versions where correction is necessary, there is no way to set a socket timeout shorter than 500 ms. If a socket timeout shorter than 500 ms is needed on these Windows versions, you should add a call to Check_Selector before any socket read or write operations.

Mixed-Language Programming on Windows

Developing pure Ada applications on Windows is no different than on other GNAT-supported platforms. However, when developing or porting an application that contains a mix of Ada and C/C++, the choice of your Windows C/C++ development environment conditions your overall interoperability strategy.

If you use gcc or Microsoft C to compile the non-Ada part of your application, there are no Windows-specific restrictions that affect the overall interoperability with your Ada code. If you do want to use the Microsoft tools for your C++ code, you have two choices:

  • You can encapsulate your C++ code in a DLL to be linked with your Ada application. In this case, use the Microsoft or other environment to build the DLL and use GNAT to build your executable (Using DLLs with GNAT).

  • You can encapsulate your Ada code in a DLL to be linked with the other part of your application. In this case, use GNAT to build the DLL (Building DLLs with GNAT Project files) and use the Microsoft or other environment to build your executable.

In addition to the description about C main in Mixed Language Programming section, if the C main uses a stand-alone library, it is required on x86-windows to setup the SEH context. For this, the C main must looks like this:

/* main.c */
extern void adainit (void);
extern void adafinal (void);
extern void __gnat_initialize(void*);
extern void call_to_ada (void);

int main (int argc, char *argv[])
{
  int SEH [2];

  /* Initialize the SEH context */
  __gnat_initialize (&SEH);

  adainit();

  /* Then call Ada services in the stand-alone library */

  call_to_ada();

  adafinal();
}

Note that you need not do this on x86_64-windows where the Windows native SEH support is used.

Windows Calling Conventions

This section pertain only to Win32. On Win64, there is a single native calling convention. All convention specifiers are ignored on this platform.

When a subprogram F (caller) calls a subprogram G (callee), there are several ways to push G‘s parameters on the stack and there are several possible scenarios to clean up the stack upon G‘s return. A calling convention is an agreed upon software protocol whereby the responsibilities between the caller (F) and the callee (G) are clearly defined. Several calling conventions are available for Windows:

  • C (Microsoft defined)

  • Stdcall (Microsoft defined)

  • Win32 (GNAT specific)

  • DLL (GNAT specific)

C Calling Convention

This is the default calling convention used when interfacing to C/C++ routines compiled with either gcc or Microsoft Visual C++.

In the C calling convention, subprogram parameters are pushed on the stack by the caller from right to left. The caller itself is in charge of cleaning up the stack after the call. In addition, the name of a routine with C calling convention is mangled by adding a leading underscore.

The name to use on the Ada side when importing (or exporting) a routine with C calling convention is the name of the routine. For example you should import the C function:

int get_val (long);

from Ada as follows:

function Get_Val (V : Interfaces.C.long) return Interfaces.C.int;
pragma Import (C, Get_Val, External_Name => "get_val");

Note that in this particular case, you could have omitted the External_Name parameter since, when missing, this parameter is set to the name of the Ada entity in lower case. When the Link_Name parameter is missing, as in the above example, this parameter is set the External_Name with a leading underscore.

When importing a variable defined in C, you should always use the C calling convention unless the object containing the variable is part of a DLL (in which case you should use the Stdcall calling convention, Stdcall Calling Convention).

Stdcall Calling Convention

This convention, which was the calling convention used for Pascal programs, is used by Microsoft for all the routines in the Win32 API for efficiency reasons. You must use it to import any routine for which this convention was specified.

In the Stdcall calling convention, subprogram parameters are also pushed on the stack by the caller from right to left. However, the callee, not the caller, is in charge of cleaning up the stack on routine exit. In addition, the name of a routine with Stdcall calling convention is mangled by adding a leading underscore (as for the C calling convention) and a trailing @nn, where nn is the overall size (in bytes) of the parameters passed to the routine.

The name to use on the Ada side when importing a C routine with a Stdcall calling convention is the name of the C routine. The leading underscore and trailing @nn are added automatically by the compiler. For example, you could import the Win32 function:

APIENTRY int get_val (long);

from Ada as follows:

function Get_Val (V : Interfaces.C.long) return Interfaces.C.int;
pragma Import (Stdcall, Get_Val);
--  On the x86 a long is 4 bytes, so the Link_Name is "_get_val@4"

Like the case for the C calling convention, when the External_Name parameter is missing, it is the name of the Ada entity in lower case. If instead of writing the above import pragma you write:

function Get_Val (V : Interfaces.C.long) return Interfaces.C.int;
pragma Import (Stdcall, Get_Val, External_Name => "retrieve_val");

the imported routine is _retrieve_val@4. However, if instead of specifying the External_Name parameter, you specify the Link_Name as in the following example:

function Get_Val (V : Interfaces.C.long) return Interfaces.C.int;
pragma Import (Stdcall, Get_Val, Link_Name => "retrieve_val");

the imported routine is retrieve_val. There is no decoration at all; no leading underscore and no Stdcall suffix @nn.

This is especially important as in some special cases a DLL’s entry point name lacks a trailing @nn while the exported name generated for a call has it.

You can also import variables defined in a DLL by using an import pragma for a variable. As an example, if a DLL contains a variable defined as:

int my_var;

then, to access this variable from Ada you should write:

My_Var : Interfaces.C.int;
pragma Import (Stdcall, My_Var);

Note that to ease building cross-platform bindings, this convention will be handled as a C calling convention on non-Windows platforms.

Win32 Calling Convention

This convention, which is GNAT-specific, is fully equivalent to the Stdcall calling convention described above.

DLL Calling Convention

This convention, which is GNAT-specific, is fully equivalent to the Stdcall calling convention described above.

Using DLLs with GNAT

To use the services of a DLL, say API.dll, in your Ada application you must have:

  • The Ada spec for the routines and/or variables you want to access in API.dll. If not available, you must build this Ada spec from the C/C++ header files provided with the DLL.

  • The import library (libAPI.dll.a or API.lib). As previously mentioned, an import library is a statically linked library containing the import table, which is filled at load time to point to the actual API.dll routines. Sometimes you don’t have an import library for the DLL you want to use. The following sections will explain how to build one. Note that this is optional.

  • The actual DLL, API.dll.

Once you have all the above, to compile an Ada application that uses the services of API.dll and whose main subprogram is My_Ada_App, you simply issue the command

$ gnatmake my_ada_app -largs -lAPI

The argument -largs -lAPI at the end of the gnatmake command tells the GNAT linker to look for an import library. The linker will look for a library name in this specific order:

  • libAPI.dll.a

  • API.dll.a

  • libAPI.a

  • API.lib

  • libAPI.dll

  • API.dll

The first three are the GNU-style import libraries. The third is the Microsoft-style import libraries. The last two are the actual DLL names.

Note that if the Ada package spec for API.dll contains the following pragma

pragma Linker_Options ("-lAPI");

you do not have to add -largs -lAPI at the end of the gnatmake command.

If any one of the items above is missing, you will have to create it yourself. The following sections explain how to do so using as an example a fictitious DLL called API.dll.

Creating an Ada Spec for the DLL Services

A DLL typically comes with a C/C++ header file which provides the definitions of the routines and variables exported by the DLL. The Ada equivalent of this header file is a package spec that contains definitions for the imported entities. If the DLL you intend to use does not come with an Ada spec, you have to generate such a spec yourself. For example, if the header file of API.dll is a file api.h containing the following two definitions:

int some_var;
int get (char *);

then the equivalent Ada spec could be:

with Interfaces.C.Strings;
package API is
   use Interfaces;

   Some_Var : C.int;
   function Get (Str : C.Strings.Chars_Ptr) return C.int;

private
   pragma Import (C, Get);
   pragma Import (DLL, Some_Var);
end API;
Creating an Import Library

If a Microsoft-style import library API.lib or a GNAT-style import library libAPI.dll.a or libAPI.a is available with API.dll you can skip this section. You can also skip this section if API.dll or libAPI.dll is built with GNU tools as in this case it is possible to link directly against the DLL. Otherwise read on.

The Definition File

As previously mentioned, and unlike Unix systems, the list of symbols that are exported from a DLL must be provided explicitly in Windows. The main goal of a definition file is precisely that: list the symbols exported by a DLL. A definition file (usually a file with a .def suffix) has the following structure:

[LIBRARY ``name``]
[DESCRIPTION ``string``]
EXPORTS
   ``symbol1``
   ``symbol2``
   ...
LIBRARY name

This section, which is optional, gives the name of the DLL.

DESCRIPTION string

This section, which is optional, gives a description string that will be embedded in the import library.

EXPORTS

This section gives the list of exported symbols (procedures, functions or variables). For example, in the case of API.dll the EXPORTS section of API.def looks like:

EXPORTS
   some_var
   get

Note that you must specify the correct suffix (@nn) (see Windows Calling Conventions) for a Stdcall calling convention function in the exported symbols list.

There can actually be other sections in a definition file, but these sections are not relevant to the discussion at hand.

Creating a Definition File Automatically

You can automatically create the definition file API.def (see The Definition File) from a DLL. To do that, use the dlltool program as follows:

$ dlltool API.dll -z API.def --export-all-symbols

Note that if some routines in the DLL have the Stdcall convention (Windows Calling Conventions) with stripped @nn suffix then you’ll have to edit api.def to add it and specify -k to gnatdll when creating the import library.

Here are some hints to find the right @nn suffix.

  • If you have the Microsoft import library (.lib), you may be able to find the right symbols by using the Microsoft dumpbin tool (see the corresponding Microsoft documentation for further details).

    $ dumpbin /exports api.lib

  • If you get a message about a missing symbol at link time, the compiler tells you what symbol is expected. You then can go back to the definition file and add the right suffix.

GNAT-Style Import Library

To create a static import library from API.dll with the GNAT tools, you should create the .def file and use the gnatdll tool (see Using gnatdll) as follows:

$ gnatdll -e API.def -d API.dll

gnatdll takes as input a definition file API.def and the name of the DLL containing the services listed in the definition file API.dll. The name of the static import library generated is computed from the name of the definition file as follows: if the definition file name is xyz.def, the import library name will be libxyz.a. Note that in the previous example, the switch -e could have been removed because the name of the definition file (before the .def suffix) is the same as the name of the DLL (Using gnatdll for more information about gnatdll).

Microsoft-Style Import Library

A Microsoft import library is needed only if you plan to make an Ada DLL available to applications developed with Microsoft tools (Mixed-Language Programming on Windows).

To create a Microsoft-style import library for API.dll you should create the .def file, then build the actual import library using Microsoft’s lib utility:

$ lib -machine:IX86 -def:API.def -out:API.lib

If you use the above command, the definition file API.def must contain a line giving the name of the DLL:

LIBRARY      "API"

See the Microsoft documentation for further details about the usage of lib.

Building DLLs with GNAT Project files

There is nothing specific to Windows in the build process. See the Library Projects section in the GNAT Project Manager chapter of the GPRbuild User’s Guide.

Due to a system limitation, you cannot create threads under Windows when inside the DllMain routine which is used for auto-initialization of shared libraries, so you can’t have library level tasks in SALs.

Building DLLs with GNAT

This section explains how to build DLLs using the GNAT built-in DLL support. With the following procedure, it is straightforward to build and use DLLs with GNAT.

  • Building object files. The first step is to build all objects files that are to be included into the DLL. This is done using the standard gnatmake tool.

  • Building the DLL. To build the DLL, you must use the gcc -shared and -shared-libgcc switches. It’s quite simple to use this method:

    $ gcc -shared -shared-libgcc -o api.dll obj1.o obj2.o ...

    It’s important to note that in this case all symbols found in the object files are automatically exported. You can restrict the set of symbols to export by passing to gcc a definition file (see The Definition File). For example:

    $ gcc -shared -shared-libgcc -o api.dll api.def obj1.o obj2.o ...

    If you use a definition file, you must export the elaboration procedures for every package that requires one. Elaboration procedures are named using the package name followed by “_E”.

  • Preparing DLL to be used. For the DLL to be used by client programs, the bodies must be hidden from it and the .ali set with read-only attribute. This is very important because otherwise GNAT will recompile all packages and will not actually use the code in the DLL. For example:

    $ mkdir apilib
$ copy *.ads *.ali api.dll apilib
$ attrib +R apilib\\*.ali

At this point, you can use the DLL by directly linking against it. Note that you must use the GNAT shared runtime when using GNAT shared libraries. You do this with the -shared binder switch.

$ gnatmake main -Iapilib -bargs -shared -largs -Lapilib -lAPI

Building DLLs with gnatdll

Note that it is preferred to use GNAT Project files (Building DLLs with GNAT Project files) or the built-in GNAT DLL support (Building DLLs with GNAT) to build DLLs.

This section explains how to build DLLs containing Ada code using gnatdll. These DLLs will be referred to as Ada DLLs in the remainder of this section.

The steps required to build an Ada DLL that is to be used by Ada as well as non-Ada applications are as follows:

  • You need to mark each Ada entity exported by the DLL with a C or Stdcall calling convention to avoid any Ada name mangling for the entities exported by the DLL (see Exporting Ada Entities). You can skip this step if you plan to use the Ada DLL only from Ada applications.

  • Your Ada code must export an initialization routine which calls the routine adainit (generated by gnatbind) to perform the elaboration of the Ada code in the DLL (Ada DLLs and Elaboration). The initialization routine exported by the Ada DLL must be invoked by the clients of the DLL to initialize the DLL.

  • When useful, the DLL should also export a finalization routine which calls routine adafinal (also generated by gnatbind) to perform the finalization of the Ada code in the DLL (Ada DLLs and Finalization). The finalization routine exported by the Ada DLL must be invoked by the clients of the DLL when the DLL services are no further needed.

  • You must provide a spec for the services exported by the Ada DLL in each of the programming languages to which you plan to make the DLL available.

  • You must provide a definition file listing the exported entities (The Definition File).

  • Finally, you must use gnatdll to produce the DLL and the import library (Using gnatdll).

Note that a relocatable DLL stripped using the strip binutils tool is no longer relocatable. To build a DLL without debug information, pass -largs -s to gnatdll. This restriction does not apply to a DLL built using a Library Project. See the Library Projects section in the GNAT Project Manager chapter of the GPRbuild User’s Guide.

Limitations When Using Ada DLLs from Ada

When using Ada DLLs from Ada applications there is a limitation you should be aware of. On Windows, the GNAT run-time is not in a DLL of its own, so each Ada DLL includes a part of the GNAT run-time. Specifically, each Ada DLL includes the services of the GNAT run-time that are necessary for the Ada code inside the DLL. As a result, when an Ada program uses an Ada DLL there are two independent GNAT run-times: one in the Ada DLL and one in the main program.

It is therefore not possible to exchange GNAT run-time objects between the Ada DLL and the main Ada program. Example of GNAT run-time objects are file handles (e.g., Text_IO.File_Type), tasks types, protected objects types, etc.

It is completely safe to exchange plain elementary, array or record types, Windows object handles, etc.

Exporting Ada Entities

Building a DLL is a way to encapsulate a set of services usable from any application. As a result, the Ada entities exported by a DLL should be exported with the C or Stdcall calling conventions to avoid any Ada name mangling. As an example here is an Ada package API, spec and body, exporting two procedures, a function, and a variable:

with Interfaces.C; use Interfaces;
package API is
   Count : C.int := 0;
   function Factorial (Val : C.int) return C.int;

   procedure Initialize_API;
   procedure Finalize_API;
   --  Initialization & Finalization routines. More in the next section.
private
   pragma Export (C, Initialize_API);
   pragma Export (C, Finalize_API);
   pragma Export (C, Count);
   pragma Export (C, Factorial);
end API;

package body API is
   function Factorial (Val : C.int) return C.int is
      Fact : C.int := 1;
   begin
      Count := Count + 1;
      for K in 1 .. Val loop
         Fact := Fact * K;
      end loop;
      return Fact;
   end Factorial;

   procedure Initialize_API is
      procedure Adainit;
      pragma Import (C, Adainit);
   begin
      Adainit;
   end Initialize_API;

   procedure Finalize_API is
      procedure Adafinal;
      pragma Import (C, Adafinal);
   begin
      Adafinal;
   end Finalize_API;
end API;

If the Ada DLL you are building will only be used by Ada applications, you do not have to export Ada entities with a C or Stdcall convention. As an example, the previous package could be written as follows:

package API is
   Count : Integer := 0;
   function Factorial (Val : Integer) return Integer;

   procedure Initialize_API;
   procedure Finalize_API;
   --  Initialization and Finalization routines.
end API;

package body API is
   function Factorial (Val : Integer) return Integer is
      Fact : Integer := 1;
   begin
      Count := Count + 1;
      for K in 1 .. Val loop
         Fact := Fact * K;
      end loop;
      return Fact;
   end Factorial;

   ...
   --  The remainder of this package body is unchanged.
end API;

Note that if you do not export the Ada entities with a C or Stdcall convention, you will have to provide the mangled Ada names in the definition file of the Ada DLL (Creating the Definition File).

Ada DLLs and Elaboration

The DLL that you are building contains your Ada code as well as all the routines in the Ada library that are needed by it. The first thing a user of your DLL must do is elaborate the Ada code (Elaboration Order Handling in GNAT).

To allow this, you must export an initialization routine (Initialize_API in the previous example), which must be invoked before using any of the DLL services. This elaboration routine must call the Ada elaboration routine adainit generated by the GNAT binder (Binding with Non-Ada Main Programs). See the body of Initialize_Api for an example. Note that the GNAT binder is automatically invoked during the DLL build process by the gnatdll tool (Using gnatdll).

When a DLL is loaded, Windows systematically invokes a routine called DllMain. It should therefore be possible to call adainit directly from DllMain without having to provide an explicit initialization routine. Unfortunately, you can’t call adainit from the DllMain if your program has library level tasks because access to the DllMain entry point is serialized by the system (that is, only a single thread can execute ‘through’ it at a time), which means that the GNAT run-time will deadlock waiting for a newly created task to complete its initialization.

Ada DLLs and Finalization

When the services of an Ada DLL are no longer needed, the client code should invoke the DLL finalization routine, if available. The DLL finalization routine is in charge of releasing all resources acquired by the DLL. In the case of the Ada code contained in the DLL, this is achieved by calling routine adafinal generated by the GNAT binder (Binding with Non-Ada Main Programs). See the body of Finalize_Api for an example. As already pointed out the GNAT binder is automatically invoked during the DLL build process by the gnatdll tool (Using gnatdll).

Creating a Spec for Ada DLLs

To use the services exported by the Ada DLL from another programming language (e.g., C), you have to translate the specs of the exported Ada entities in that language. For instance in the case of API.dll, the corresponding C header file could look like:

extern int *_imp__count;
#define count (*_imp__count)
int factorial (int);

It is important to understand that when building an Ada DLL to be used by other Ada applications, you need two different specs for the packages contained in the DLL: one for building the DLL and the other for using the DLL. This is because the DLL calling convention is needed to use a variable defined in a DLL, but when building the DLL, the variable must have either the Ada or C calling convention. As an example consider a DLL consisting of the following package API:

package API is
   Count : Integer := 0;
   ...
   --  Remainder of the package omitted.
end API;

After producing a DLL containing package API, the spec that must be used to import API.Count from Ada code outside of the DLL is:

package API is
   Count : Integer;
   pragma Import (DLL, Count);
end API;
Creating the Definition File

The definition file is the last file you need to build the DLL. It lists the exported symbols. As an example, the definition file for a DLL containing only package API above (where all the entities are exported with a C calling convention) is:

EXPORTS
    count
    factorial
    finalize_api
    initialize_api

If the C calling convention is missing from package API, the definition file contains the mangled Ada names of the above entities, which in this case are:

EXPORTS
    api__count
    api__factorial
    api__finalize_api
    api__initialize_api
Using gnatdll

gnatdll is a tool to automate the DLL build process once all the Ada and non-Ada sources that make up your DLL have been compiled. gnatdll is actually in charge of two distinct tasks: building both the static import library for the DLL and the actual DLL. You invoke the gnatdll command as

$ gnatdll [ switches ] list-of-files [ -largs opts ]

where list-of-files is a list of ALI and object files. The object file list must be the exact list of objects corresponding to the non-Ada sources whose services are to be included in the DLL. The ALI file list must be the exact list of ALI files for the corresponding Ada sources whose services are to be included in the DLL. If list-of-files is missing, only the static import library is generated.

You may specify any of the following switches to gnatdll:

-a[address]

Build a non-relocatable DLL at address. If you don’t specify address, gnatdll uses the default address of 0x11000000. By default, when this switch is missing, gnatdll builds a relocatable DLL. We advise you to build relocatable DLL.

-bargs opts

Binder switches. Pass opts to the binder.

-d dllfile

dllfile is the name of the DLL. You must specify this switch for gnatdll to do anything. gnatdll names the generated import library algorithmically from dllfile as shown in the following example: if dllfile is xyz.dll, the import library name is libxyz.dll.a. gnatdll obtains the name of the definition file (if not specified by switch -e) algorithmically from dllfile as shown in the following example: if dllfile is xyz.dll, the definition file used is xyz.def.

-e deffile

deffile is the name of the definition file.

-g

Generate debugging information. This information is stored in the object file and copied from there to the final DLL 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 or the symbolic stack traceback.

-h

Help mode. Displays gnatdll switch usage information.

-Idir

Direct gnatdll to search the dir directory for source and object files needed to build the DLL. (Search Paths and the Run-Time Library (RTL)).

-k

Removes the @nn suffix from the import library’s exported names, but keeps them for the link names. You must specify this switch if you want to use a Stdcall function in a DLL for which the @nn suffix has been removed. This is the case for most of the Windows NT DLL for example. This switch has no effect if you specify the -n switch.

-l file

The list of ALI and object files used to build the DLL are listed in file, instead of being given in the command line. Each line in file contains the name of an ALI or object file.

-n

No Import. Do not create the import library.

-q

Quiet mode. Do not display unnecessary messages.

-v

Verbose mode. Display extra information.

-largs opts

Linker switches. Pass opts to the linker.

gnatdll Example

As an example, the command to build a relocatable DLL from api.adb once api.adb has been compiled and api.def created is

$ gnatdll -d api.dll api.ali

The above command creates two files: libapi.dll.a (the import library) and api.dll (the actual DLL). If you want to create only the DLL, just type:

$ gnatdll -d api.dll -n api.ali

Alternatively, if you want to create just the import library, type:

$ gnatdll -d api.dll

gnatdll behind the Scenes

This section details the steps involved in creating a DLL. gnatdll does these steps for you. Unless you are interested in understanding what goes on behind the scenes, you should skip this section.

We use the previous example of a DLL containing the Ada package API, to illustrate the steps necessary to build a DLL. The starting point is a set of objects that make up the DLL and the corresponding ALI files. In the case of this example, this means api.o and api.ali. To build a relocatable DLL, gnatdll does the following:

  • builds the base file (api.base). A base file gives the information necessary to generate relocation information for the DLL.

    $ gnatbind -n api
$ gnatlink api -o api.jnk -mdll -Wl,--base-file,api.base

    In addition to the base file, the gnatlink command generates an output file api.jnk, which can be discarded. The -mdll switch asks gnatlink to generate the routines DllMain and DllMainCRTStartup that are called by the Windows loader when the DLL is loaded into memory.

  • uses dlltool (see Using dlltool) to build the export table (api.exp). The export table contains the relocation information in a form which can be used during the final link to ensure that the Windows loader is able to place the DLL anywhere in memory.

    $ dlltool --dllname api.dll --def api.def --base-file api.base \\
          --output-exp api.exp

  • builds the base file using the new export table. Note that gnatbind must be called once again since the binder generated file has been deleted during the previous call to gnatlink.

    $ gnatbind -n api
$ gnatlink api -o api.jnk api.exp -mdll
      -Wl,--base-file,api.base

  • builds the new export table using the new base file and generates the DLL import library libAPI.dll.a.

    $ dlltool --dllname api.dll --def api.def --base-file api.base \\
          --output-exp api.exp --output-lib libAPI.a

  • Finally, builds the relocatable DLL using the final export table.

    $ gnatbind -n api
$ gnatlink api api.exp -o api.dll -mdll

Using dlltool

dlltool is the low-level tool used by gnatdll to build DLLs and static import libraries. This section summarizes the most common dlltool switches. You run dlltool as follows:

$ dlltool [`switches`]

dlltool switches include:

--base-file basefile

Read the base file basefile generated by the linker. You use this switch to create a relocatable DLL.

--def deffile

Read the definition file.

--dllname name

Gives the name of the DLL. You use this switch to embed the name of the DLL in the static import library generated by dlltool with switch --output-lib.

-k

Kill @nn from exported names (Windows Calling Conventions for a discussion about Stdcall-style symbols).

--help

Prints the dlltool switches with a concise description.

--output-exp exportfile

Generate an export file exportfile. The export file contains the export table (list of symbols in the DLL) and is used to create the DLL.

--output-lib libfile

Generate a static import library libfile.

-v

Verbose mode.

--as assembler-name

Use assembler-name as the assembler. The default is as.

GNAT and Windows Resources

Resources are an easy way to add Windows-specific objects to your application. The objects that you can add as resources include:

  • menus

  • accelerators

  • dialog boxes

  • string tables

  • bitmaps

  • cursors

  • icons

  • fonts

  • version information

You can use a version information resource to embed information into an executable or a DLL. This information can be viewed using the file properties from the Windows Explorer. Here’s an example of a version information resource:

1 VERSIONINFO
FILEVERSION     1,0,0,0
PRODUCTVERSION  1,0,0,0
BEGIN
  BLOCK "StringFileInfo"
  BEGIN
    BLOCK "080904E4"
    BEGIN
      VALUE "CompanyName", "My Company Name"
      VALUE "FileDescription", "My application"
      VALUE "FileVersion", "1.0"
      VALUE "InternalName", "my_app"
      VALUE "LegalCopyright", "My Name"
      VALUE "OriginalFilename", "my_app.exe"
      VALUE "ProductName", "My App"
      VALUE "ProductVersion", "1.0"
    END
  END

  BLOCK "VarFileInfo"
  BEGIN
    VALUE "Translation", 0x809, 1252
  END
END

The value 0809 (langID) is for the U.K English language and 04E4 (charsetID), which is equal to 1252 decimal, for multilingual.

This section explains how to build, compile and use resources. Note that this section does not cover all resource objects; for a complete description see the corresponding Microsoft documentation.

Building Resources

A resource file is an ASCII file. By convention, resource files have an .rc extension. The easiest way to build a resource file is to use Microsoft tools such as imagedit.exe to build bitmaps, icons and cursors and dlgedit.exe to build dialogs. You can always build an .rc file yourself by writing a resource script.

It’s not our objective to explain how to write a resource file. A complete description of the resource script language can be found in the Microsoft documentation.

Compiling Resources

This section describes how you can build a GNAT-compatible (COFF) object file containing the resources. You do this using the Resource Compiler windres as follows:

$ windres -i myres.rc -o myres.o

By default windres runs gcc to preprocess the .rc file. You can specify an alternate preprocessor (usually named cpp.exe) using the windres --preprocessor parameter. You can obtain a list of all possible switches by entering the command windres --help.

You can also use the Microsoft resource compiler rc.exe to produce a .res file (binary resource file). See the corresponding Microsoft documentation for further details. In this case, you need to use windres to translate the .res file to a GNAT-compatible object file as follows:

$ windres -i myres.res -o myres.o
Using Resources

To include the resource file in your program just add the GNAT-compatible object file for the resource(s) to the linker arguments. With gnatmake you do this using the -largs switch:

$ gnatmake myprog -largs myres.o

Using GNAT DLLs from Microsoft Visual Studio Applications

This section describes a common case of mixed GNAT/Microsoft Visual Studio application development, where the main program is developed using MSVS and is linked with a DLL developed using GNAT. You should develop such a mixed application following the general guidelines outlined above; below is the cookbook-style sequence of steps to follow:

  1. First develop and build the GNAT shared library using a library project (let’s assume the project is mylib.gpr, producing the library libmylib.dll):

$ gprbuild -p mylib.gpr

  1. Produce a .def file for the symbols you need to interface with, either by hand or automatically with possibly some manual adjustments (see Creating Definition File Automatically):

$ dlltool libmylib.dll -z libmylib.def --export-all-symbols

  1. Make sure that MSVS command-line tools are accessible on the path.

  2. Create the Microsoft-style import library (see MSVS-Style Import Library):

$ lib -machine:IX86 -def:libmylib.def -out:libmylib.lib

If you are using a 64-bit toolchain, the above becomes…

$ lib -machine:X64 -def:libmylib.def -out:libmylib.lib

  1. Build the C main:

$ cl /O2 /MD main.c libmylib.lib

  1. Before running the executable, make sure you have set the PATH to include the DLL or copy the DLL into into the directory containing the .exe.

Debugging a DLL

Debugging a DLL is similar to debugging a standard program, but you have to deal with two different executable parts: the DLL and the program that uses it. There are the following four possibilities:

  • The program and DLL are built with GCC/GNAT.

  • The program is built with foreign tools and the DLL is built with GCC/GNAT.

  • The program is built with GCC/GNAT and the DLL is built with foreign tools.

In this section we address only cases one and two above. Note that there is no point in trying to debug a DLL with GNU/GDB if there is no GDB-compatible debugging information in it. To do so, you must use a debugger compatible with the tools suite used to build the DLL.

Program and DLL Both Built with GCC/GNAT

This is the simplest case. Both the DLL and the program have GDB compatible debugging information. You can then break anywhere in the process. Let’s suppose the main procedure is named ada_main and in the DLL there’s an entry point named ada_dll.

The DLL (Introduction to Dynamic Link Libraries (DLLs)) and program must have been built with the debugging information (see the GNAT -g switch). Here are the step-by-step instructions for debugging it:

  • Launch GDB on the main program.

    $ gdb -nw ada_main

  • Start the program and stop at the beginning of the main procedure

    (gdb) start

    This step is required to be able to set a breakpoint inside the DLL. Until the program is run, the DLL is not loaded. This has the consequence that the DLL debugging information is also not loaded, so it is not possible to set a breakpoint in the DLL.

  • Set a breakpoint inside the DLL

    (gdb) break ada_dll
(gdb) cont

At this stage, a breakpoint is set inside the DLL. From there on you can use standard GDB commands to debug the whole program (Running and Debugging Ada Programs).

Program Built with Foreign Tools and DLL Built with GCC/GNAT

In this case, things are slightly more complex because you can’t start the main program and then break at the beginning to load the DLL and the associated DLL debugging information. It’s not possible to break at the beginning of the program because there’s no GDB debugging information, and therefore there’s no direct way of getting initial control. This section addresses this issue by describing some methods that you can use to break somewhere in the DLL to debug it.

First, suppose that the main procedure is named main (this is the case, for example, for some C code built with Microsoft Visual C) and that there’s a DLL named test.dll containing an Ada entry point named ada_dll.

The DLL (see Introduction to Dynamic Link Libraries (DLLs)) must have been built with debugging information (see the GNAT -g switch).

Debugging the DLL Directly

  • Determine the executable’s starting address

    $ objdump --file-header main.exe

    The starting address is reported on the last line. For example:

    main.exe:     file format pei-i386
architecture: i386, flags 0x0000010a:
EXEC_P, HAS_DEBUG, D_PAGED
start address 0x00401010

  • Launch the debugger on the executable.

    $ gdb main.exe

  • Set a breakpoint at the starting address and launch the program.

    $ (gdb) break *0x00401010
$ (gdb) run

    The program will stop at the specified address.

  • Set a breakpoint on a DLL subroutine.

    (gdb) break ada_dll.adb:45

    Or if you want to break using a symbol on the DLL, you need first to select the Ada language (language used by the DLL).

    (gdb) set language ada
(gdb) break ada_dll

  • Continue the program.

    (gdb) cont

    This runs the program until it reaches the breakpoint that you’ve set. From that point, you can use standard GDB commands to debug a program as described in (Running and Debugging Ada Programs).

You can also debug the DLL by attaching GDB to a running process.

Attaching to a Running Process

With GDB, you can always debug a running process by attaching to it. It’s possible to debug a DLL this way. The limitation of this approach is that the DLL must run long enough to perform the attach operation. To ensure this, you may want, for example, to insert a time-wasting loop in the code of the DLL to allow this to happen.

  • Launch the main program main.exe.

    $ main

  • Use the Windows Task Manager to find the process ID. Let’s say that the process PID for main.exe is 208.

  • Launch gdb.

    $ gdb

  • Attach to the running process to be debugged.

    (gdb) attach 208

  • Load the process debugging information.

    (gdb) symbol-file main.exe

  • Break somewhere in the DLL.

    (gdb) break ada_dll

  • Continue process execution.

    (gdb) cont

This last step will resume the process execution and stop at the breakpoint we have set. From there you can use standard GDB commands to debug a program, as described in Running and Debugging Ada Programs.

Windows Specific Add-Ons

This section describes the Windows specific add-ons.

Win32Ada

Win32Ada is a binding for the Microsoft Win32 API, which you can easily install using the provided installer. To use it, you need to use a project file and add a single with_clause to give you full access to the Win32Ada binding sources and ensure that the proper libraries are passed to the linker.

with "win32ada";
project P is
   for Sources use ...;
end P;

To build the application, you just need to call gprbuild for the application’s project, here p.gpr:

gprbuild p.gpr

wPOSIX

wPOSIX is a minimal POSIX binding whose goal is to help with building cross-platforms applications. This binding is not complete though, as the Win32 API does not provide the necessary support for all POSIX APIs.

To use the wPOSIX binding, you need to use a project file and add a single with clause to give you full access to the wPOSIX binding sources and ensure that the proper libraries are passed to the linker.

with "wposix";
project P is
   for Sources use ...;
end P;

To build the application, you just need to call gprbuild for the application’s project, here p.gpr:

gprbuild p.gpr

Mac OS Topics

This section describes topics that are specific to Apple’s OS X platform.

Codesigning the Debugger

The Darwin Kernel, used by Apple’s OS X, requires the debugger to have special permissions before it’s allowed to control other processes. These permissions are granted by codesigning the GDB executable. Without these permissions, the debugger will report error messages such as:

Starting program: /x/y/foo
Unable to find Mach task port for process-id 28885: (os/kern) failure (0x5).
(please check gdb is codesigned - see taskgated(8))

Codesigning requires a certificate. The following procedure explains how to create one:

  • Start the Keychain Access application (in /Applications/Utilities/Keychain Access.app)

  • Select the Keychain Access -> Certificate Assistant -> Create a Certificate… menu

  • Then:

    • Choose a name for the new certificate (this procedure will use “gdb-cert” as an example)

    • Set “Identity Type” to “Self Signed Root”

    • Set “Certificate Type” to “Code Signing”

    • Activate the “Let me override defaults” option

  • Click several times on “Continue” until the “Specify a Location For The Certificate” screen appears, then set “Keychain” to “System”

  • Click on “Continue” until the certificate is created

  • Finally, in the view, double-click on the new certificate, and set “When using this certificate” to “Always Trust”

  • Exit the Keychain Access application and restart the computer (this is unfortunately required)

Once you’ve created a certificate as above, you can codesign the debugger by running the following command in a Terminal:

$ codesign -f -s  "gdb-cert"  <gnat_install_prefix>/bin/gdb

with gdb-cert replaced by the actual certificate name chosen above, and gnat_install_prefix replaced by the location where you installed GNAT. Also, be sure that users of GDB are in the Unix group _developer.