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From: Keith Thompson <kst@cts.com>
Subject: Re: Newbie question about interface to C
Date: 2000/05/30
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David Gressett <gressett@iglobal.net> writes:
> I need to call C routines which  are prototyped like these examples:
> 
> shapelessblob *somefunction(char *s);  /* creates a shaplessblob */
> 
> int otherfunction(shapelessblob *b);  /* uses a shaplessblob */
> 
> where shaplessblob is a pointer to a structure  that is created by
> somefunction. The calling Ada code will never look at the insides of a
> shapelessblob; It will only pass it to a C routine.
> 
> How do I import such a pointer as an Ada access type?
> 
> The  Ada LRM that comes with gnat3.12p is not very helpful. (it is,
> after all, a RM, not a tutorial.) The Cohen book has only a single
> trivial example of a C interface.

I'd probably do something like this:

    with Interfaces.C;
    with Interfaces.C.Strings;
    package Foo is
        type Shapeless_Blob is private;
        type Shapeless_Blob_Pointer is access Shapeless_Blob;

        function Some_Function
            (S: Interfaces.C.Strings.chars_ptr)
            return Shapeless_Blob_Pointer
        pragma Import(C, Some_Function, "somefunction");

        function Other_Function
            (B: Shapeless_Blob_Pointer)
            return Interfaces.C.Int;
        pragma Import(C, Other_Function, "otherfunction");

    private
        type Shapeless_Blob is
            record
                Dummy: Interfaces.C.int; -- C doesn't allow empty structs
            end record;
        pragma Convention(C, Shapeless_Blob);
        pragma Convention(C, Shapeless_Blob_Pointer);
    end Foo;

This takes advantage of the fact that all struct pointers in C are
represented the same way (something that's subtly implied by the C
standard).

You could probably get away with declaring
    type Shapeless_Blob_Pointer is new System.Address;
and omitting the declaration of Shapeless_Blob, but it's potentially
less portable; some C implementations might represent, say, struct
pointers and char pointers differently.

-- 
Keith Thompson (The_Other_Keith) kst@cts.com  <http://www.ghoti.net/~kst>
San Diego Supercomputer Center           <*>  <http://www.sdsc.edu/~kst>
Welcome to the last year of the 20th century.