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From: Tilman Gloetzner <tilman@uundz.de>
Newsgroups: comp.lang.ada
Subject: Question: Exporting ADA functions to C
Date: Wed, 21 Mar 2001 17:16:57 +0100
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Date: 2001-03-21T17:16:57+01:00
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Hello,

I am trying to hand over a c-struct to an ada record and vice versa. The
gnat RM says that there are
2 ways to do it: 

Ada RM (page 215):
Interfacing to C
 
   Interfacing to C with GNAT can use one of two approaches:
    1. The types in the package Interfaces.C may be used.
    2. Standard Ada types may be used directly. This may be less
portable
       to other compilers, but will work on all GNAT compilers, which
       guarantee correspondence between the C and Ada types.
                                                                 
...
     * Ada arrays map directly to C arrays.
     * Ada records map directly to C structures.     
...


I choose the 1st way because the RM suggests that the code is better
protable. Unfortunatelly, my test progam crashes with a core dump. Not
that I am surprised, but what did I do wrong ? First, I suspected that I
better hand over the string as pointer, so I commented it(now just
handing over an Integer and a Float). The program still crashed.


Thank you,

Tilman

======================= Test Program
====================================

with INTERFACES.C;use INTERFACES.C;
with INTERFACES.C.STRINGS; use INTERFACES.C.STRINGS;
use INTERFACES;

package Ada_2_C is

   type Rec is
      record
         FLOAT_FIELD: FLOAT;
         STR_FIELD:   STRING(1..16);
         INT_FIELD:   INTEGER;
      end record;

   type Rec_C is
      record
         FLOAT_FIELD: C_Float;
         STR_FIELD:   Char_Array(1..16);
         INT_FIELD:   Int;
      end record;

   pragma Convention (C,Rec_C);

   procedure Register_Record(C_Structure: in Rec_C);
   procedure Get_Record(C_Structure: out Rec_C);

   pragma Export(C,Register_Record,"C_Register_Rec");
   pragma Export(C,Get_Record,"C_Get_Rec");

end Ada_2_C;


package body Ada_2_C is
   Temp: Rec ;

   procedure Register_Record(C_Structure: in Rec_C) is
   begin
      Temp.FLOAT_FIELD  := FLOAT(C_Structure.Float_FIELD)+1.0;
      Temp.Str_Field    := To_Ada(C_Structure.Str_Field);
      Temp.Str_Field(2) := 'B';
      Temp.Int_Field    :=INTEGER(C_Structure.Int_Field) + 1;
   end Register_Record;

   procedure Get_Record(C_Structure: out Rec_C) is
   begin
     C_Structure.FLOAT_FIELD  := C_FLOAT(Temp.FLOAT_FIELD);
     C_Structure.STR_FIELD    := To_C(Temp.STR_FIELD);
     C_Structure.Int_Field    := Int(Temp.Int_Field);
   end Get_Record;

end Ada_2_C;

// C program containing "main" 
#include<stdio.h>

struct Ada_rec {
  float  Float_Field;
  char   Str_Field[16];
  int    Int_Field;
};

extern void adainit (void);
extern void adafinal (void);
extern void C_Register_Rec(struct Ada_rec Structure);
extern void C_Get_Rec(struct Ada_rec Structure);

 
main(int argc,char **argv) {
  struct Ada_rec Structure = {0.0,"TEST TEST TEST T",0};

  printf("Start main\n");
  
  adainit();
  
  printf("pushing struct from c to ada...\n"); 
  C_Register_Rec(Structure);

  printf("pulling  struct (record) from ADA \n");
  C_Get_Rec(Structure);

  printf("Float = %f\nStr = \nInt = %d",
	 Structure.Float_Field,
	 Structure.Str_Field, 
	 Structure.Int_Field);

  adafinal();

  printf("End main\n");
}                 

# Compilation
gnatmake -c ada_2_c.adb -o ada_2_c.o
gcc -c ada_2_c.adb
gnatbind -n ada_2_c
gnatlink ada_2_c.ali c_main.o -o ada_n_c