F. Microsoft Windows Topics

This chapter describes topics that are specific to the Microsoft Windows platforms (NT, 2000, and XP Professional).

F.1 Using GNAT on Windows
F.2 Using a network installation of GNAT
F.3 CONSOLE and WINDOWS subsystems
F.4 Temporary Files
F.5 Mixed-Language Programming on Windows
F.6 Windows Calling Conventions
F.7 Introduction to Dynamic Link Libraries (DLLs)
F.8 Using DLLs with GNAT
F.9 Building DLLs with GNAT
F.10 Building DLLs with GNAT Project files
F.11 Building DLLs with gnatdll
F.12 GNAT and Windows Resources
F.13 Debugging a DLL
F.14 GNAT and COM/DCOM Objects

F.1 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 libraries except for import libraries. The library must be built to be compatible with `MSVCRT.LIB' (/MD Microsoft compiler option), `LIBC.LIB' and `LIBCMT.LIB' (/ML or /MT Microsoft compiler options) are known to not be compatible with the GNAT runtime. Even if the library is compatible with `MSVCRT.LIB' it is not guaranteed to work.

• 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.

F.2 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.

F.3 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

F.4 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.

F.5 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 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 plan to use Microsoft tools (e.g. Microsoft Visual C/C++), you should be aware of the following limitations:

• You cannot link your Ada code with an object or library generated with Microsoft tools if these use the .tls section (Thread Local Storage section) since the GNAT linker does not yet support this section.

• You cannot link your Ada code with an object or library generated with Microsoft tools if these use I/O routines other than those provided in the Microsoft DLL: msvcrt.dll. This is because the GNAT run time uses the services of msvcrt.dll for its I/Os. Use of other I/O libraries can cause a conflict with msvcrt.dll services. For instance Visual C++ I/O stream routines conflict with those in msvcrt.dll.

If you do want to use the Microsoft tools for your non-Ada code and hit one of the above limitations, you have two choices:

1. Encapsulate your non Ada 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 (see section F.8 Using DLLs with GNAT).

2. 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 (see section F.9 Building DLLs with GNAT) and use the Microsoft or whatever environment to build your executable.

F.6 Windows Calling Conventions

F.6.1 C Calling Convention
F.6.2 Stdcall Calling Convention
F.6.3 DLL Calling Convention

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)

• DLL (GNAT specific)

F.6.1 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 Interfaces.C.int; 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 DLL calling convention, see section F.6.3 DLL Calling Convention).

F.6.2 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 Interfaces.C.int; 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 Interfaces.C.int; 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 Interfaces.C.int; pragma Import (Stdcall, Get_Val, Link_Name => "retrieve_val");

then the imported routine is retrieve_val@4, that is, there is no trailing underscore but the appropriate @nn is always added at the end of the Link_Name by the compiler.

Note, that in some special cases a DLL's entry point name lacks a trailing @nn while the exported name generated for a call has it. The gnatdll tool, which creates the import library for the DLL, is able to handle those cases (see section F.11.7 Using gnatdll for the description of the switches).

F.6.3 DLL Calling Convention

This convention, which is GNAT-specific, must be used when you want to import in Ada a variables defined in a DLL. For functions and procedures this convention is equivalent to the Stdcall convention. 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 (DLL, My_Var);

The remarks concerning the External_Name and Link_Name parameters given in the previous sections equally apply to the DLL calling convention.

F.7 Introduction to Dynamic Link Libraries (DLLs)

A Dynamically Linked Library (DLL) is a library that can be shared by several applications running under Windows. A DLL can contain any number of routines and variables.

One advantage of DLLs is that you can change and enhance them without forcing all the applications that depend on them to be relinked or recompiled. However, you should be aware than all calls to DLL routines are slower since, as you will understand below, such calls are indirect.

To illustrate the remainder of this section, suppose that an application wants to use the services of a DLL `API.dll'. To use the services provided by `API.dll' you must statically link against the DLL or an import library which contains a jump table with an entry for each routine and variable exported by the DLL. In the Microsoft world this import library is called `API.lib'. When using GNAT this import library is called either `libAPI.a' or `libapi.a' (names are case insensitive).

After you have linked your application with the DLL or the import library and you run your application, here is what happens:

1. Your application is loaded into memory.

2. The DLL `API.dll' is mapped into the address space of your application. This means that:
   • The DLL will use the stack of the calling thread.

   • The DLL will use the virtual address space of the calling process.

   • The DLL will allocate memory from the virtual address space of the calling process.

   • Handles (pointers) can be safely exchanged between routines in the DLL routines and routines in the application using the DLL.

3. The entries in the jump table (from the import library `libAPI.a' or `API.lib' or automatically created when linking against a DLL) which is part of your application are initialized with the addresses of the routines and variables in `API.dll'.

4. If present in `API.dll', routines DllMain or DllMainCRTStartup are invoked. These routines typically contain the initialization code needed for the well-being of the routines and variables exported by the DLL.

There is an additional point which is worth mentioning. In the Windows world there are two kind of DLLs: relocatable and non-relocatable DLLs. Non-relocatable DLLs can only be loaded at a very specific address in the target application address space. If the addresses of two non-relocatable DLLs overlap and these happen to be used by the same application, a conflict will occur and the application will run incorrectly. Hence, when possible, it is always preferable to use and build relocatable DLLs. Both relocatable and non-relocatable DLLs are supported by GNAT. Note that the `-s' linker option (see GNU Linker User's Guide) removes the debugging symbols from the DLL but the DLL can still be relocated.

As a side note, an interesting difference between Microsoft DLLs and Unix shared libraries, is the fact that on most Unix systems all public routines are exported by default in a Unix shared library, while under Windows it is possible (but not required) to list exported routines in a definition file (see section F.8.2.1 The Definition File).

F.8 Using DLLs with GNAT

F.8.1 Creating an Ada Spec for the DLL Services
F.8.2 Creating an Import Library

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

1. 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.

2. The import library (`libAPI.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.

3. 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 first for a library named `API.lib' (Microsoft-style name) and if not found for a library named `libAPI.a' (GNAT-style name). 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'.

F.8.1 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 : C.int; function Get (Str : C.Strings.Chars_Ptr) return C.int; private pragma Import (C, Get); pragma Import (DLL, Some_Var); end API;

Note that a variable is always imported with a DLL convention. A function can have C, Stdcall or DLL convention. For subprograms, the DLL convention is a synonym of Stdcall (see section F.6 Windows Calling Conventions).

F.8.2 Creating an Import Library

F.8.2.1 The Definition File
F.8.2.2 GNAT-Style Import Library
F.8.2.3 Microsoft-Style Import Library

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

F.8.2.1 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 instance 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 section F.6 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.

F.8.2.2 GNAT-Style Import Library

To create a static import library from `API.dll' with the GNAT tools you should proceed as follows:

1. Create the definition file `API.def' (see section F.8.2.1 The Definition File). For that use the dll2def tool as follows:

   $ dll2def API.dll > API.def

   dll2def is a very simple tool: it takes as input a DLL and prints to standard output the list of entry points in the DLL. Note that if some routines in the DLL have the Stdcall convention (see section F.6 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.

   1. 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

   2. 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.

2. Build the import library libAPI.a, using gnatdll (see section F.11.7 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 (see section F.11.7 Using gnatdll for more information about gnatdll).

F.8.2.3 Microsoft-Style Import Library

With GNAT you can either use a GNAT-style or 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 (see section F.5 Mixed-Language Programming on Windows).

To create a Microsoft-style import library for `API.dll' you should proceed as follows:

1. Create the definition file `API.def' from the DLL. For this use either the dll2def tool as described above or the Microsoft dumpbin tool (see the corresponding Microsoft documentation for further details).

2. 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.

F.9 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.

1. 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.

2. building the DLL

   To build the DLL you must use gcc's -shared option. It is quite simple to use this method:

   $ gcc -shared -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 section F.8.2.1 The Definition File. For example:

   $ gcc -shared -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".

3. 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 -shared binder's option.

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

F.10 Building DLLs with GNAT Project files

There is nothing specific to Windows in this area. see section 11.12 Library Projects.

F.11 Building DLLs with gnatdll

F.11.1 Limitations When Using Ada DLLs from Ada
F.11.2 Exporting Ada Entities
F.11.3 Ada DLLs and Elaboration
F.11.4 Ada DLLs and Finalization
F.11.5 Creating a Spec for Ada DLLs
F.11.6 Creating the Definition File
F.11.7 Using gnatdll

Note that it is prefered to use the built-in GNAT DLL support (see section F.9 Building DLLs with GNAT) or GNAT Project files (see section F.10 Building DLLs with GNAT Project files) 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:

1. 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 section F.11.2 Exporting Ada Entities). You can skip this step if you plan to use the Ada DLL only from Ada applications.

2. 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 (see section F.11.3 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.

3. 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 (see section F.11.4 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.

4. 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.

5. You must provide a definition file listing the exported entities (see section F.8.2.1 The Definition File).

6. Finally you must use gnatdll to produce the DLL and the import library (see section F.11.7 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.

F.11.1 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.

F.11.2 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. Please note that the Stdcall convention should only be used for subprograms, not for variables. 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 (see section F.11.6 Creating the Definition File).

F.11.3 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 (see section C. 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 (see section 4.2.5 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 (see section F.11.7 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.

F.11.4 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 (see section 4.2.5 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 (see section F.11.7 Using gnatdll).

F.11.5 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;

F.11.6 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:

EXPORTS count factorial finalize_api initialize_api

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:

EXPORTS api__count api__factorial api__finalize_api api__initialize_api

F.11.7 Using gnatdll

F.11.7.1 gnatdll Example
F.11.7.2 gnatdll behind the Scenes
F.11.7.3 Using dlltool

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:

-a[address]

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.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.

-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. (see section 3.3 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 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.

-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 options. Pass opts to the linker.

F.11.7.1 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.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

F.11.7.2 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:

1. 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.

2. gnatdll uses dlltool (see section F.11.7.3 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

3. 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 -Wl,--base-file,api.base

4. gnatdll builds the new export table using the new base file and generates the DLL import library `libAPI.a'.

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

5. Finally gnatdll builds the relocatable DLL using the final export table.

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

F.11.7.3 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'.

`-k'

Kill @nn from exported names (see section F.6 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.

F.12 GNAT and Windows Resources

F.12.1 Building Resources
F.12.2 Compiling Resources
F.12.3 Using 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

This section explains how to build, compile and use resources.

F.12.1 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.

F.12.2 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

F.12.3 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

F.13 Debugging a DLL

F.13.1 Program and DLL Both Built with GCC/GNAT
F.13.2 Program Built with Foreign Tools and DLL Built with GCC/GNAT

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:

1. The program and the DLL are built with GCC/GNAT.
2. The program is built with foreign tools and the DLL is built with GCC/GNAT.
3. 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.

F.13.1 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 (see section F.7 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:

1. Launch GDB on the main program.

   $ gdb -nw ada_main

2. Break on the main procedure and run the program.

   (gdb) break ada_main (gdb) run

   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.

3. Set a breakpoint inside the DLL

   (gdb) break ada_dll (gdb) run

At this stage a breakpoint is set inside the DLL. From there on you can use the standard approach to debug the whole program (see section 24. Running and Debugging Ada Programs).

To break on the DllMain routine it is not possible to follow the procedure above. At the time the program stop on ada_main the DllMain routine as already been called. Either you can use the procedure below see section F.13.2.1 Debugging the DLL Directly or this procedure:

1. Launch GDB on the main program.

   $ gdb -nw ada_main

2. Load DLL symbols

   (gdb) add-sym api.dll

3. Set a breakpoint inside the DLL

   (gdb) break ada_dll.adb:45

   Note that at this point it is not possible to break using the routine symbol directly as the program is not yet running. The solution is to break on the proper line (break in `ada_dll.adb' line 45).

4. Start the program

   (gdb) run

F.13.2 Program Built with Foreign Tools and DLL Built with GCC/GNAT

F.13.2.1 Debugging the DLL Directly
F.13.2.2 Attaching to a Running Process

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 section F.7 Introduction to Dynamic Link Libraries (DLLs)) must have been built with debugging information (see GNAT -g option).

F.13.2.1 Debugging the DLL Directly

1. Launch the debugger on the DLL.

   $ gdb -nw test.dll

2. Set a breakpoint on a DLL subroutine.

   (gdb) break ada_dll.adb:45

   Note that at this point it is not possible to break using the routine symbol directly as the program is not yet running. The solution is to break on the proper line (break in `ada_dll.adb' line 45).

3. Specify the executable file to GDB.

   (gdb) exec-file main.exe

4. Run the program.

   (gdb) run

   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 (see section 24. Running and Debugging Ada Programs).

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

F.13.2.2 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.

1. Launch the main program `main.exe'.

   $ main

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

3. Launch gdb.

   $ gdb -nw

4. Attach to the running process to be debugged.

   (gdb) attach 208

5. Load the process debugging information.

   (gdb) symbol-file main.exe

6. Break somewhere in the DLL.

   (gdb) break ada_dll

7. Continue process execution.

   (gdb) continue

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 (see section 24. Running and Debugging Ada Programs).

F.14 GNAT and COM/DCOM Objects

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