Re: Ada equivalent for C-pointer?

[Martin Krischik <martin@krischik.com>]

Niklas Holsti wrote:

> Martin Krischik wrote:
>> Pascal Obry wrote:
>> 
>> 
>>>Nick Roberts <nick.roberts@acm.org> writes:
>>>
>>>
>>>>For example, if there is a C function defined thus:
>>>>
>>>>   int wibble(int *n);
>>>>
>>>>then in Ada you can declare:
>>>>
>>>>   function Wibble (Number: in out Interfaces.C.int);
>>>>   pragma Import(C,Wibble,"wibble");
>>>>
>>>>and it should work. (Obviously, you must link the C functions in
>>>>somehow.)
>>>
>>>I don't think so. Here Number will be passed by value, no? And you also
>>>need the returned value ;)
>> 
>> 
>> No all out and in out parameter are passed as pointer when pragma
>> Import(C is used - thats unless you use C_Pass_By_Copy. I have written
>> quite a lot of code interfaceing with C.
> 
> But according to LRM B.3(68), for an "in out" parameter of an
> *elementary* type (as here) the pointer that is passed to the C function
> is a pointer to a *temporary copy* of the parameter (here Number), not a
> pointer to the parameter itself. This may not be what the C function
> expects.

Interesting. I have to think about that. Question is if the optimizer is
allowed to remove the copy.

> For example, it would not make sense for "wibble" to save the 
> pointer for later use.

Well storing a pointer to a stack object is allways wrong. If you want to
store a pointer to the object neither "in out" nor "anomymous access" is
appropiate - a "pool access" should be used instead. And the Ada interface
should reflect that - if only to document propper use.

Martin
-- 
mailto://krischik@users.sourceforge.net
http://www.ada.krischik.com