Re: Importing C structs?

[Ching Bon Lam <c.lam_REMOVE_THIS@student.utwente.nl>]

"Matthew Heaney" <matthewjheaney@earthlink.net> wrote in
news:1V1Wa.299$jp.55@newsread4.news.pas.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);

overkill? i don't think so.. opaque_structure_def is defined in 
Interfaces.C.Extensions (i-cexten.ads in gnat) as:

subtype opaque_structure_def is System.Address;

just a memory address. That's the same as DB *. Since the api (well, most 
api written in C) only use DB * and not DB, you don't need another access 
type. that's why 

subtype DB is interfaces.c.extensions.opaque_structure_def;

Note that it's actually subtype of opaque_structure_def. small error on my 
side.

> 
> 
>> -- 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.

const char * is in this context the same as a string, which is in fact an 
array of char. and interfaces.c.char_array is the same thing. and with 
chars_ptr you need to make an extra temp variable for the conversion.

> 
> 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. 

again defined in interfaces.c.extensions (at least in gnat):

subtype void_ptr is System.Address;

> 
> 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.

i have used interfaces.c.extensions with success, that's why i'm using 
opaque_structure_def because then i don't have to use 'hacks'.

> 
> 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? 
> 

i guess :)

CBL