Interfaces.C.Strings_Chars_Ptr as an out mode parameter

Interfaces.C.Strings_Chars_Ptr as an out mode parameter

[David C. Hoos, Sr.]

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RM B.3.1 (1) may have something important to say by
what it does _not_ say, viz. :

The package Interfaces.C.Strings declares types and
subprograms allowing an Ada program to allocate,
reference, update, and free C-style strings. In
particular, the private type chars_ptr corresponds
to a common use of �char *� in C programs, and an
object of this type can be passed to a subprogram to
which pragma Import(C,...) has been applied, and for
which �char *� is the type of the argument of the C
function.

What is does _not_ say is that an object of this
type can be passed to a  subprogram to which pragma
Export(C,...) has been applied, and for which
�char *� is the type of the argument of the C
function.

So... the question is.. if one wishes to have a
procedure with a profile of, say:

procedure My_Proc (Result : out Interfaces.C.Strings.Chars_Ptr);

is it legal to export it like so:

pragma Export (C, My_Proc, "My_Proc");

where the C prototype is void My_Proc (char * result);

The results I'm having are that if the exported procedure
is called from Ada, everything works as expected.

If called from C, a Segmentation fault occurs at the
point where the out mode parameter is assigned its
value in the Ada procedure My_Proc.

What say ye?

Re: Interfaces.C.Strings_Chars_Ptr as an out mode parameter

[dennison]

In article <7jpe2e$m7i@hobbes.crc.com>,
  "David C. Hoos, Sr." <david.c.hoos.sr@ada95.com> wrote:
> procedure My_Proc (Result : out Interfaces.C.Strings.Chars_Ptr);
>
> is it legal to export it like so:
>
> pragma Export (C, My_Proc, "My_Proc");
>
> where the C prototype is void My_Proc (char * result);
>
> The results I'm having are that if the exported procedure
> is called from Ada, everything works as expected.
>
> If called from C, a Segmentation fault occurs at the
> point where the out mode parameter is assigned its
> value in the Ada procedure My_Proc.

If the implementation advice in the RM is followed, then your Ada
"myproc" is equivalent to a C "My_Proc (char ** result)". To get the
equivalent of a C "My_Proc (char * result)" you should either change
your mode to "in" or change your type to "Interfaces.C.Signed_Char"


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Re: Interfaces.C.Strings_Chars_Ptr as an out mode parameter

[Tucker Taft]

David C. Hoos, Sr. wrote:
> ...
> So... the question is.. if one wishes to have a
> procedure with a profile of, say:
> 
> procedure My_Proc (Result : out Interfaces.C.Strings.Chars_Ptr);
> 
> is it legal to export it like so:
> 
> pragma Export (C, My_Proc, "My_Proc");
> 
> where the C prototype is void My_Proc (char * result);

The C prototype should be:  void My_Proc (char ** result);
because "out" parameters are given an extra level of indirection.
The caller must also use "&" on the actual parameter (in C).

> The results I'm having are that if the exported procedure
> is called from Ada, everything works as expected.
> 
> If called from C, a Segmentation fault occurs at the
> point where the out mode parameter is assigned its
> value in the Ada procedure My_Proc.

Doesn't surprise me.

> What say ye?

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-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
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