F.9.2 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 Creating the Definition File).