Path: utzoo!utgpu!watmath!clyde!att!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c
Subject: Re: varargs strangeness?
Message-ID: <8554@alice.UUCP>
Date: 16 Dec 88 23:26:33 GMT
References: <353@bdrc.UUCP>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 51

In article <353@bdrc.UUCP>, jcl@bdrc.UUCP (John C. Lusth) writes:
> I have a variadic function to which I pass a pointer to a function returning
> void.  Distilled, my function looks like
> 
> error (va_alist)
> 
> va_dcl
> 
> {
>     va_list	ap;
>     void	(*f)();
> 
>     va_start(ap);
> 
>     f = va_arg(ap, void (*)());
>     .
>     .
>     .
> }

The C preprocessor is pretty simple-minded.  This makes it
hard (probably impossible) for varargs to cope with argument
types that cannot be transformed into their related pointer
types by appending a *.

void(*)() is definitely not such a type.

To make it into one, use a typedef:

error (va_alist)

va_dcl

{
    va_list	ap;
    typedef 	void (*voidfuncp)();
    voidfuncp	f;

    va_start(ap);

    f = va_arg(ap, voidfuncp);
    .
    .
    .
}

``C Traps and Pitfalls'' describes varargs.h in some detail
starting on page 134.
-- 
				--Andrew Koenig
				  ark@europa.att.com