Path: utzoo!attcan!uunet!husc6!uwvax!tank!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: Re: passing variable numbers of arguments
Message-ID: <15341@mimsy.UUCP>
Date: 7 Jan 89 21:16:53 GMT
References: <899@thor.stolaf.edu>
Distribution: na
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 53

In article <899@thor.stolaf.edu> mackenzi@agnes.uucp (David MacKenzie) writes:
>The following program produces unexpected results:
>foo (va_alist)
>  va_dcl
>{
>  bar (va_alist);
>}

This program is quite thoroughly illegal.  `va_alist' is allowed to be
defined as a `magic token' that tells the compiler `I accept variable
argument lists'; this magic token may well not be a valid argument.  If
you get lucky, the compiler will object to the call to bar().

>In other words, if I call bar () directly, it works, but if I call it via
>foo (), it breaks, in an implementation-dependant way, no less.  Why?

Because indirect calls to bar() can never pass a variable argument list.

>Is there anyway to get this to work?

You cannot call bar() from foo() (except with a fixed argument list).
What you CAN do is call a common routine from both foo() and bar():

foo(va_alist)
	va_dcl
{
	va_list ap;

	va_start(ap);
	vbar(ap);
	va_end(ap);
}

bar(va_alist)
	va_dcl
{
	va_list ap;

	va_start(ap);
	vbar(ap);
	va_end(ap);
}

vbar(ap)
	va_list ap;
{
	... code to deal with variable arguments ...
}

This is why vprintf, vfprintf, and vsprintf exist at all.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris