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From: Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Calling Ada Procedure from C - how to handle a char ptr/array
Date: Fri, 18 Nov 2011 12:56:08 -0700
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Date: 2011-11-18T12:56:08-07:00
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On 11/18/2011 12:40 PM, awdorrin wrote:
> 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";

Someone's been writing C.

>    S := Interfaces.C.To_C(Label);
>   end;
 >
 > 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

The problem here is that Ada doesn't know the bounds of S. C has to also pass 
that information.

 > 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?

An assignment to S must have a right-hand side that's the same length as S. I 
suspect you'll have to assign to a slice:

S (S'First .. S'First + Label'Length) := To_C (Label);

> Is there a better way to be doing this?

A function that returns a Char_Array might be better than an out parameter.

-- 
Jeff Carter
"Go and boil your bottoms."
Monty Python & the Holy Grail
01