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From: Jerry <lanceboyle@qwest.net>
Newsgroups: comp.lang.ada
Subject: Re: Trouble writing Ada callback from C
Date: Wed, 16 Sep 2009 16:19:14 -0700 (PDT)
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Date: 2009-09-16T16:19:14-07:00
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On Sep 12, 11:40=A0am, Vadim Godunko <vgodu...@gmail.com> wrote:
> On Sep 11, 11:57=A0am, Jerry <lancebo...@qwest.net> wrote:
>
> > I have a problem with writing an Ada procedure, used as a callback
> > from C, in which one of the arguments is an output char * (in the C
> > version). The actual length of the returned "string" isn't known at
> > calling time, even though the caller apparently allocates 40
> > characters (see below). However, the actual number of characters
> > returned by the C version is less than 40.
>
> > My problem is how to pass probably a char_array back out which is the
> > correct length, but which length is not known at the time the Ada
> > callback is called from C.
>
> subtype constrained_char_array is Interfaces.C.char_array (1 .. 40);
>
> procedure Y (X : out constrained_char_array);
> pragma Convention (C, Y);

Sorry for being slow to respond--I had to step away from this project
for a few days.

This works--thanks! After seeing this suggestion, I felt a little
embarrassed because it's so simple, and in hindsight, obvious.

Also, I realized that part of the problem was that to match the C
results, I had to insert my string into the left of the allocated 41
characters, then place a null character, then fill the rest with
spaces, like this:

-- Declared elsewhere is this, with Label_String_Length set to 40:
subtype Label_String_Type is
    Interfaces.C.char_array (0 .. size_t(Label_String_Length));

-- Then:
function Unbounded_To_Weird_C
   (Item     : Unbounded_String;
    C_Length : size_t)
return Label_String_Type is
begin
   return To_C(To_String(Item & ASCII.nul &
        (Label_String_Length - Length(Item)) * " "), False);
end Unbounded_To_Weird_C;


Thanks, everyone, for the help.
Jerry