Re: Help interfacing C

[Keith Thompson <kst@cts.com>]

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"Jes�s M. Mil�n-Franco" <milanjm@inta.es> writes:
> I have to do an Ada interface to a C library and I have found the
> following function declaration:
> 
> int  zzz_VaSend (zzz_mbox_cap mbox_c, int send_type,
>                  int flag, int len, char *buf , char *va_list, ...);
> 
> Where va_list is a list with the receiver names finished by 'null', so
> the function has a variable number of parameters (the last is always
> null)
> 
> Does anybody know if it is possible to have something similar in Ada ??

Not directly, at least not portably.

A C function whose declaration includes a "..." token takes a variable
number of arguments; the body of the function uses the facilities
defined in the standard header <stdarg.h> to retrieve them.  Ada has
no such facility.  (Overloading and default parameters can provide
much of the same functionality, but that doesn't help much when you're
trying to interface directly to a C function like this one.)

You can define a list data structure that you can construct on the Ada
side and pass to an intermediate C function, but even in C building a
variable argument list from a list or array of values is tricky or
even impossible.  It's done with compiler magic.

Your best bet is to find or create a function or set of functions that
does what zzz_VaSend does, but without using "...".

-- 
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.