Problem with a C string to Ada conversion

Problem with a C string to Ada conversion

[Gautier]

Hi.
I'm trying to convert a C string to an Ada one and I don't see what's 
wrong with my code (also tried the variant commented out)...

with GL;
with Interfaces.C.Strings;
with Ada.Text_IO; use Ada.Text_IO;
with Ada.Unchecked_Conversion;

procedure Get_GL_Info is

   use GL;

   function GetString (name: StringEnm) return String is
     function Cvt is new 
Ada.Unchecked_Conversion(ubytePtr,Interfaces.C.Strings.chars_ptr);
     ps: constant ubytePtr:= GL.GetString(name);
     --ca: Interfaces.C.char_array:= Interfaces.C.Strings.Value(Cvt(ps));
   begin
     return Interfaces.C.Strings.Value(Cvt(ps));
     --return Interfaces.C.To_Ada(ca);
   end GetString;

begin
   for i in GL.StringEnm loop
     Put_Line(GL.StringEnm'Image(i) & ": [" & GetString(i) & ']');
   end loop;
end Get_GL_Info;

Here is what happens with this code:
The .exe built by GNAT 3.15p / Windows enters seemingly a memory 
munching race ending by a crash;
the one built by ObjectAda 7.2.2 says immediately:

Program terminated by an exception propagated out of the main subprogram.
Exception raised : INTERFACES.C.STRINGS.DEREFERENCE_ERROR
Executable name: GET_GL_INFO.EXE

Line      Subprogram name                 File
--------  ------------------------------  ------------
      181  interfaces.c.strings.strlen     interfaces.c.strings.bdy
      253  interfaces.c.strings.value      interfaces.c.strings.bdy
      297  interfaces.c.strings.value__3   interfaces.c.strings.bdy
       15  get_gl_info.getstring           get_gl_info.adb
       21  get_gl_info                     get_gl_info.adb
      320  _rtsadamain                     init.c

End of propagation.
Program aborted.

Any clue ? TIA! Gautier
_______________________________________________________________
Ada programming -- http://www.mysunrise.ch/users/gdm/gsoft.htm

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Re: Problem with a C string to Ada conversion

[Gautier]

OK, found it (beginning with reading the Manual about 
Dereference_Error...) - it was a null pointer on the C side.
By initializing GL before calling GetString
   GLUT.Init;
   if GLUT.CreateWindow( "Test" ) = 0 then return; end if;
it works now...
G.

Re: Problem with a C string to Ada conversion

[Gautier]

NB: GNAT 3.15p also outputs the Dereference_Error on the wrong version 
of the program, but only on a WinXP machine; on Win98 only ObjectAda 
does (topic discussed long time ago...). G.