Help interfacing C

Help interfacing C

[Jes�s M. Mil�n-Franco]

Hi all.

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 ??

----
Jesús M. Milán-Franco (milanjm@sip.ucm.es)
Dpto. Sistemas Informáticos y Programación
Facultad de Informática (U. C. M.)
Ciudad Universitaria s/n - 28040 Madrid (Spain)
phone: (+34) 91 394 43 50 / fax: (+34) 91 394 46 02

Re: Help interfacing C

[Keith Thompson]

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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.

Re: Help interfacing C

[Ken Garlington]

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"Jes�s M. Mil�n-Franco" <milanjm@inta.es> wrote in message
news:8ljlti$7u0$1@polaris.inta.es...
> Hi all.
>
> 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 ??

It's certainly possible to pass a list of items in Ada - what method do you
use to get the elements of va_list?

Re: Help interfacing C

[tmoran]

> int  zzz_VaSend (zzz_mbox_cap mbox_c, int send_type,
>                  int flag, int len, char *buf , char *va_list, ...);

You could use overloading
  function f(some_params:some_type; a:another_type);
  function f(some_params:some_type; a,b:another_type);
  function f(some_params:some_type; a,b,c:another_type);
That works well if you usually only have a few items in the variable
parameter list.

Or, since Ada allows easy creation of temporary arrays, you could do
  function f(some_params:some_type; list : array_type);
and then call it with
  f(x, (1=>a));  -- perhaps replaced with something like the 1st f above.
  f(x, (a,b));
  f(x, (a,b,c));
  etc.

Your function(s) will of course have to transmit their parameters
appropriately to the C function.  You will need separate pragma
Imports for the C function for each number of parameters.

Re: Help interfacing C

[Keith Thompson]

tmoran@bix.com writes:
> > int  zzz_VaSend (zzz_mbox_cap mbox_c, int send_type,
> >                  int flag, int len, char *buf , char *va_list, ...);
> 
> You could use overloading
>   function f(some_params:some_type; a:another_type);
>   function f(some_params:some_type; a,b:another_type);
>   function f(some_params:some_type; a,b,c:another_type);
> That works well if you usually only have a few items in the variable
> parameter list.

If the overloaded function f is interfaced directly to zzz_VaSend(),
this will work only if the C implementation passes ordinary arguments
and variable (stdarg) arguments the same way.  This is probably true
of many, but not all, implementations.

On the other hand, assuming a C function like this:

    int f(some_type arg1, ...);

you could do something like this:

    function F1(Arg1: Some_Type)
        return Interfaces.C.int;
    pragma Import(C, F1);

    function F2(Arg1: Some_Type; Arg2: Another_Type)
        return Interfaces.C.int;
    pragma Import(C, F2);

    function F3(Arg1: Some_Type; Arg2, Arg3: Another_Type)
        return Interfaces.C.int;
    pragma Import(C, F3);

    -- Use renaming declarations to overload F1, F2, F3 as F

and in C:

    int f1(some_type arg1)
    {
        return f(arg1);
    }

    int f2(some_type arg1, another_type arg2)
    {
        return f(arg1, arg2);
    }

    int f3(some_type arg1, another_type arg2, another_type arg3)
    {
        return f(arg1, arg2, arg3);
    }

-- 
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.