Platform-Specific Information

This appendix contains information relating to the implementation of run-time libraries on various platforms and also covers topics related to the GNAT implementation on Windows and Mac OS.

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:


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:


If the rts-sjlj library is to be selected on a permanent basis, these soft links can be modified 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.

Selecting another run-time library temporarily can be achieved by using the --RTS switch, e.g., --RTS=sjlj

Choosing the Scheduling Policy

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 option -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 option.

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");

It gets the effective user id, and if it’s not 0 (i.e. root), it raises Program_Error.

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

Other GNU/Linux distributions might be choosing a different name for those packages.

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. It is also possible to perform silent installations using the command-line interface.

In order 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 such activities need to be done 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. These applications can be written 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.

Immediately below are listed all known general GNAT-for-Windows restrictions. Other restrictions about specific features like Windows Resources and DLLs are listed in separate sections below.

  • It is not possible to 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. It should be possible to use GetLastError and SetLastError when tasking, protected record, and exception features are not used, but it is not guaranteed to work.
  • It is not possible to link against Microsoft C++ libraries except for import libraries. Interfacing must be done by the mean of DLLs.
  • It is possible to link against Microsoft C libraries. Yet 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, users may experience recompilations of the 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 that can be created are file associations and PATH settings, provided the user has chosen to create them at installation time, as well as some minimal book-keeping 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 serious performance penalty.

CONSOLE and WINDOWS subsystems

There are two main subsystems under Windows. The CONSOLE subsystem (which is the default subsystem) will always create a console when launching the application. This is not something desirable when the application has a Windows GUI. To get rid of this console the application must be using the WINDOWS subsystem. To do so the -mwindows linker option must be specified.

$ gnatmake winprog -largs -mwindows

Temporary Files

It is possible to control where temporary files gets 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 ?, then 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"
  • Filename expansion can be disabled 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 then quote removal might be performed by the 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"


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

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, the user 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, then the manifest file name has to 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">
   <!-- 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}"/>

Without the manifest file, the socket timeout is going to be overcorrected on these Windows Server versions and the actual time is going to 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, a call to Check_Selector should be added 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:

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


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



Note that this is not needed 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 instance the C function:

int get_val (long);

should be imported from Ada as follows:

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

Note that in this particular case the External_Name parameter could have been omitted since, when missing, this parameter is taken to be 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 to be 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. It must be used to import any routine for which this convention was specified.

In the Stdcall calling convention subprogram parameters are pushed on the stack by the caller from right to left. The callee (and not the caller) is in charge of cleaning 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 instance the Win32 function:

APIENTRY int get_val (long);

should be imported from Ada as follows:

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

As for the C calling convention, when the External_Name parameter is missing, it is taken to be 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;
pragma Import (Stdcall, Get_Val, External_Name => "retrieve_val");

then 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;
pragma Import (Stdcall, Get_Val, Link_Name => "retrieve_val");

then the imported routine is retrieve_val, that is, 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.

It is also possible to 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 :;
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 this Ada spec must be built 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 will be 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 one such 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 :;
   function Get (Str : C.Strings.Chars_Ptr) return;

   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``]
This section, which is optional, gives the name of the DLL.
This section, which is optional, gives a description string that will be embedded in the import library.

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


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. For 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), it is possible to get the right symbols by using Microsoft dumpbin tool (see the corresponding Microsoft documentation for further details).

    $ dumpbin /exports api.lib
  • If you have a message about a missing symbol at link time the compiler tells you what symbol is expected. You just have to 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, then use 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 option -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:


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, it is not possible under Windows to create threads when inside the DllMain routine which is used for auto-initialization of shared libraries, so it is not possible to have library level tasks in SALs.

Building DLLs with GNAT

This section explain how to build DLLs using the GNAT built-in DLL support. With the following procedure it is straight forward 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 by using the standard gnatmake tool.

  • Building the DLL. To build the DLL you must use the gcc -shared and -shared-libgcc options. It is quite simple to use this method:

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

    It is important to note that in this case all symbols found in the object files are automatically exported. It is possible to 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 required 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 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 it is possible to use the DLL by directly linking against it. Note that you must use the GNAT shared runtime when using GNAT shared libraries. This is achieved by using the -shared binder option.

$ 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) or 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 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 will not be relocatable anymore. 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 users should be aware of. Because on Windows the GNAT run-time is not in a DLL of its own, 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 to 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 : := 0;
   function Factorial (Val : return;

   procedure Initialize_API;
   procedure Finalize_API;
   --  Initialization & Finalization routines. More in the next section.
   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 : return is
      Fact : := 1;
      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);
   end Initialize_API;

   procedure Finalize_API is
      procedure Adafinal;
      pragma Import (C, 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;
      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 achieve 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 would therefore be possible to call adainit directly from DllMain without having to provide an explicit initialization routine. Unfortunately, it is not possible to 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 the 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 comprising 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 needed to build the DLL. It lists the exported symbols. As an example, the definition file for a DLL containing only package API (where all the entities are exported with a C calling convention) is:


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

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: build the static import library for the DLL and the actual DLL. The form of the gnatdll command is

$ 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:

Build a non-relocatable DLL at address. If address is not specified the default address 0x11000000 will be used. By default, when this switch is missing, gnatdll builds relocatable DLL. We advise the reader to build relocatable DLL.
-b address
Set the relocatable DLL base address. By default the address is 0x11000000.
-bargs opts
Binder options. Pass opts to the binder.
-d dllfile
dllfile is the name of the DLL. This switch must be present for gnatdll to do anything. The name of the generated import library is obtained algorithmically from dllfile as shown in the following example: if dllfile is xyz.dll, the import library name is libxyz.dll.a. The name of the definition file to use (if not specified by option -e) is obtained 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.
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.
Help mode. Displays gnatdll switch usage information.
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)).
Removes the @nn suffix from the import library’s exported names, but keeps them for the link names. You must specify this option 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 option has no effect when -n option is specified.
-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.
No Import. Do not create the import library.
Quiet mode. Do not display unnecessary messages.
Verbose mode. Display extra information.
-largs opts
Linker options. 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 will make up the DLL and the corresponding ALI files. In the case of this example this means that api.o and api.ali are available. To build a relocatable DLL, gnatdll does the following:

  • gnatdll 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.

  • gnatdll 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
  • gnatdll 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
  • gnatdll 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 gnatdll 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. The form of the dlltool command is

$ dlltool [`switches`]

dlltool switches include:

--base-file basefile
Read the base file basefile generated by the linker. This switch is used to create a relocatable DLL.
--def deffile
Read the definition file.
--dllname name
Gives the name of the DLL. This switch is used to embed the name of the DLL in the static import library generated by dlltool with switch --output-lib.
Kill @nn from exported names (Windows Calling Conventions for a discussion about Stdcall-style symbols.
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.
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 can be added as resources include:

  • menus
  • accelerators
  • dialog boxes
  • string tables
  • bitmaps
  • cursors
  • icons
  • fonts
  • version information

For example, a version information resource can be defined as follow and embedded into an executable or DLL:

A version information resource can be used to embed information into an executable or a DLL. These information can be viewed using the file properties from the Windows Explorer. Here is an example of a version information resource:

FILEVERSION     1,0,0,0
  BLOCK "StringFileInfo"
    BLOCK "080904E4"
      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"

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

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. It is always possible to build an .rc file yourself by writing a resource script.

It is 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 to build a GNAT-compatible (COFF) object file containing the resources. This is done using the Resource Compiler windres as follows:

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

By default windres will run gcc to preprocess the .rc file. You can specify an alternate preprocessor (usually named cpp.exe) using the windres --preprocessor parameter. A list of all possible options may be obtained by entering the command windres --help.

It is also possible to 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 this is done by using the -largs option:

$ 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. Such a mixed application should be developed 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 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 we have to deal with two different executable parts: the DLL and the program that uses it. We have the following four possibilities:

  • The program and the 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. 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. It is then possible to break anywhere in the process. Let’s suppose here that the main procedure is named ada_main and that in the DLL there is 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 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. As long as the program is not 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 the standard approach 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 it is not possible to start the main program and then break at the beginning to load the DLL and the associated DLL debugging information. It is not possible to break at the beginning of the program because there is no GDB debugging information, and therefore there is no direct way of getting initial control. This section addresses this issue by describing some methods that can be used to break somewhere in the DLL to debug it.

First suppose that the main procedure is named main (this is for example some C code built with Microsoft Visual C) and that there is 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 option).

Debugging the DLL Directly

  • Find out the executable 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:
    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 given 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 will run the program until it reaches the breakpoint that has been set. From that point you can use the standard way to debug a program as described in (Running and Debugging Ada Programs).

It is also possible to debug the DLL by attaching to a running process.

Attaching to a Running Process

With GDB it is always possible to debug a running process by attaching to it. It is 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. It may be useful for instance to insert a time wasting loop in the code of the DLL to meet this criterion.

  • 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 the standard approach 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 is a binding for the Microsoft Win32 API. This binding can be easily installed from the provided installer. To use the Win32Ada binding you need to use a project file, and adding a single with_clause will 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 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 adding a single with_clause will 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 requires the debugger to have special permissions before it is 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
  • 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 a certificate has been created, the debugger can be codesigned as follow. In a Terminal, run the following command:

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

where “gdb-cert” should be replaced by the actual certificate name chosen above, and <gnat_install_prefix> should be replaced by the location where you installed GNAT. Also, be sure that users are in the Unix group _developer.