Re: Newbie question about interface to C

[Jeff Carter <jrcarter@acm.org>]

David Gressett wrote:
> 
> 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?

This cannot be answered completely without knowing how the functions
deal with their parameters. Do they use the initial value? Do they
modify them? For example, if the char* parameter s to somefunction is
used for input only (Ada mode in), then you could declare the function
as taking an Interfaces.C.char_array; pragma Import will perform the
necessary gymnastics to pass a char* to the C.

As for type shapelessblob*, you could probably get by with
System.Address since you don't care what it points to. To be safe,
though, you should do something like

type C_Struct is record
   I : Interfaces.C.Int;
end record;
pragma Convention (C, C_Struct);

type Shapeless_Blob is access all C_Struct;
pragma Convention (C, Shapeless_Blob);

and use Shapeless_Blob for shapelessblob*.

-- 
Jeff Carter
"Hello! Smelly English K...niggets."
Monty Python & the Holy Grail