binding to C: popen

[Poul-Erik Andreasen <poulerik@pea.dk>]

Hi

i am trying to make a biding to the C function popen, it goes like this:

with System.Address_To_Access_Conversions;
with Interfaces.C_Streams;

package Pipe_Pkg is

    subtype Command_Pipe is Interfaces.C_Streams.FILEs;

    procedure Pipe_Open (Pipe    : in out Command_Pipe;
                         Command : in  String);
private

   function To_C (Input : in String) return Interfaces.C_Streams.Chars;

end Pipe_Pkg;


package body Pipe_Pkg is


    procedure Pipe_Open
      (Pipe    : in out Command_Pipe;
       Command : in  String)
    is
       use Interfaces.C_Streams;
       function C_Popen (C_Command : Chars;
                         C_Type    : Chars) return Command_Pipe;

       pragma Import (C, C_Popen, "popen");

    begin
       Pipe := C_Popen ((To_C (Command)), To_C ("w"));
    end Pipe_Open;

    function To_C (Input : in String) return Interfaces.C_Streams.Chars 


    is
      package String_Conversion is
         new System.Address_To_Access_Conversions (String);

      String_Pointer : String_Conversion.Object_Pointer :=
                          new String'(Input & ascii.nul);
    begin
       return String_Conversion.To_Address (String_Pointer);
    end;


end Pipe_Pkg;



This ictually going as it is suppose to, but i would like set in an 
exception if the piping dosn't have succes. Acording to the C manual
popen returns null if it fails. But how do i test on that in the FILEs type?