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From: awdorrin <awdorrin@gmail.com>
Newsgroups: comp.lang.ada
Subject: Calling Ada Procedure from C - how to handle a char ptr/array
Date: Fri, 18 Nov 2011 11:40:46 -0800 (PST)
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Date: 2011-11-18T11:40:46-08:00
List-Id: <comp.lang.ada>

What would be the recommended way to pass a C char pointer or char
array into an Ada function or procedure that will be called from a C
program?

After reading through the Interfaces.C.* packages, I'm assuming I
could do something like this:

In my Ada procedure:

procedure Get_Text( s : out Interfaces.C.char_array) is
  Label : String(1..8)
begin
  Label(1..8) == "12345678";
  S := Interfaces.C.To_C(Label);
 end;

with a C program that does the following:

int main() {
  char temp[8+1];
  memset( temp, '\0', 9);

  adainit();
  Get_Text(temp);
  printf("Temp is: %s\n", temp);
  adafinal();
}

however when I compile the code I get the following warning:
  warning: type of argument "Get_Text.s" is unconstrained array
  warning: foreign caller must pass bounds explicitly

Then when I run, I get an error from the following line of code: S :=
Interfaces.C.To_C(Label);
"raised CONSTRAINT_ERROR : length check failed.

What length check?

Is there a better way to be doing this?

Thanks!
-Al