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Date: Fri, 18 Nov 2011 23:32:24 +0100
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Calling Ada Procedure from C - how to handle a char ptr/array
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Date: 2011-11-18T23:32:25+01:00
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On 18.11.11 21:41, awdorrin wrote:
> function Get_Label return Interfaces.C.Char_array is
>    Label : String(1..8);
> begin
>    Label(1..8) := "12345678";
>
>    return Interfaces.C.To_C(Label);
> end;

While only partly following GNAT's advice to not have Get_Label
return an unconstrained array (not following because this requires
defining a constrained subtype somewhere and I was lazy),
this works for me. It works with or without defining a constrained
Result array in Get_Label.

with Interfaces.C;
function Get_Label return Interfaces.C.Char_Array;
pragma Export (C, Get_Label, "get_label");

function Get_Label return Interfaces.C.Char_array is
   Label : String(1..8);
   Result : Interfaces.C.char_array(0..8);
begin
   Label(1..8) := "12345678";

   Result := Interfaces.C.To_C(Label);
   return Result;
end;

#include <stdio.h>

int main()
{
    extern char* get_label();
    extern void adainit(void);
    extern void adafinal(void);
  
    char* label;

    adainit();
    label = get_label();
    adafinal();

    fputs(label, stdout);
    return 0;
}


$ gnatmake -c -gnatwa -fstack-check -gnato -f get_label.adb
gcc -c -gnatwa -fstack-check -gnato get_label.adb
get_label.ads:2:10: warning: foreign convention function "Get_Label" should not return unconstrained array
$ cc -W -c with_label.c
$ gnatbind -n get_label.ali
$ gnatlink get_label.ali with_label.o -o test_get_label
$ ./test_get_label
12345678$