Re: Importing C structs?

["Matthew Heaney" <matthewjheaney@earthlink.net>]

"Ching Bon Lam" <c.lam_REMOVE_THIS@student.utwente.nl> wrote in message
news:Xns93C9C9EB630Fcblamstudentutwenten@194.109.133.20...
>
> that would be:
>
> type DB is interfaces.c.extensions.opaque_structure_def;

This is probably overkill.  For an opaque type all you need to do is:

type DB_Type is limited null record;
type DB_Access is access all DB_Type;
for DB_Access'Storage_Size use 0;
pragma Convention (C, DB_Access);


> -- i have too less information about DBTYPE, so i assume
> -- it's just a struct
> type DBTYPE is interfaces.c.extensions.opaque_structure_def;
>
> function dbopen
>    (file     : interfaces.c.char_array;
>     flags    : interfaces.c.int;
>     mode     : interfaces.c.int;
>     type     : DBTYPE;
>     openinfo : interfaces.c.extensions.void_ptr)
>    return DB;
>
> pragma Import(C, dbopen, "dbopen');

The type of parameter file is incorrect.  The char_array type is an
unconstrained Ada array type, which does not correspond directly to any C
type.  Specifically, your problem is that on the Ada side a dope vector
describing the array object is passed -- this is definitely *not* what you
want.

You need to use the type Interfaces.C.Strings.chars_ptr instead.

You can probably use System.Address for void*. The C type void* was
accidently omitted from Interfaces.C in the Ada95 standard, but I assume
that oversite will be corrected in a future version of the language.

It's not clear whether your return type should be DB or an explicit access
type (as in DB_Access), because I'm not familiar with the package
Interfaces.C.extensions.  I would use DB_Access, which is completely
portable.

The type DBTYPE is a bit of a mystery at this point, since structs aren't
typically passed by value in C.  Perhaps DBTYPE is really a pointer?