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From: Felix Krause <fordprefect86@googlemail.com>
Newsgroups: comp.lang.ada
Subject: StdCall and pragma Import_Function
Date: Fri, 19 Aug 2011 09:24:33 -0700 (PDT)
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Date: 2011-08-19T09:24:33-07:00
List-Id: <comp.lang.ada>

I'm writing a cross-platform Ada binding for a C library. It currently work=
s on OSX and Linux, and I'm trying to get it to work on Windows. I have som=
e code like this:

package P is
   type ULong is mod 2 ** 64;
   for ULong'Size use 64;

   type Bit_Vector is record
      First_Bit  : Boolean;
      Second_Bit : Boolean;
   end record;
   for Bit_Vector use record
      First_Bit  at 0 range 0 .. 0;
      Second_Bit at 0 range 1 .. 1;
   end record;
   for Bit_Vector'Size use ULong'Size;

   function C_Function (Param : Bit_Vector) return ULong;
   pragma Import (StdCall, C_Function, "cFunc");
   pragma Import_Function (Internal =3D> C_Function, External =3D> "cFunc",=
 Mechanism =3D> (Param =3D> Value));
end P;

On Linux and OSX, this works without problems. On Windows however, I get th=
is error: "undefined reference to 'cFunc@4'". The GNAT user manual tells me=
 that for StdCall, the number behind the '@' is the total number of bytes u=
sed by the function parameters. Now I told the compiler quite explicitly th=
at Param should be passed by value and therefore needs 8 bytes, not 4 bytes=
.

I'm not sure what goes wrong here. Is Import_Function ignored on Windows? A=
s I compile for 32bit, the number of bytes used by Param would be 4 if it w=
as passed by reference.

Of course, this example is simplified and the actual C function has more pa=
rameters that could be the cause of the wrong byte number, but if I use "Pa=
ram : ULong" for the function definition, it works, so I'm pretty sure the =
problem lies in the usage of Bit_Vector for the parameter. I also used dump=
bin.exe to extract the correct function names in the C library .lib file, a=
nd indeed, the symbol name is "_cFunc@8".

So, I do have a possibility to work around this problem; namely defining C_=
Function having a ULong parameter and then converting all Bit_Vector variab=
les to ULong when calling C_Function. But I'm still curious what the proble=
m is here, and if there is an easier workaround. Can anybody tell me?

Thanks,
Felix