Path: utzoo!mnetor!uunet!husc6!cmcl2!rutgers!bellcore!faline!ulysses!allegra!alice!ark
From: ark@alice.UUCP
Newsgroups: comp.unix.wizards
Subject: Re: (simple?) varargs question
Message-ID: <7559@alice.UUCP>
Date: 22 Dec 87 01:55:11 GMT
References: <10937@brl-adm.ARPA>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 48

I hope the following helps.

Every function that can be called with a variable number of
arguments must have the following form:

	#include <varargs.h>

	T F (va_alist) va_dcl
	{
		B
	}

Here T represents the type of the function's return value,
F represents the name of the function, and B represents the
body of the function.  Everything else must appear as written
above, including the lack of a semicolon after `va_dcl'.

Within the body of this function, you may declare one or
more objects of type va_list (not va_alist).  Such an object
represents a "pointer" into the argument list.

If v is an object of type va_list, the following operations
are defined on v:

	va_start (v);
	va_end (v);
	va_arg (v, AT)

Before v is used, va_start(v) must be executed.  A function that
executes va_start(v) should execute va_end(v) before returning.
Both va_start and va_end must be executed in the function that
has va_dcl in its header.

The va_arg macro says the following:  "I know that v is now
pointing at an argument of type AT.  Fetch that argument and
return it as your value.  Also increment v to point to the next
argument."  It is the programmer's responsibility to ensure
that the next argument exists and is indeed of type AT.  Since
char and short arguments are automatically promoted to int,
AT must not be `char' or `short'.

The va_arg macro need not physically appear in the function
with the variable argument list.  Indeed, the first argument
of va_arg need not be the name of a va_list variable at
all -- it can be any expression that evaluates to a va_list
lvalue.  In particular, a function with a variable argument list
may pass the address of an initialized va_arg object to some
other function, which uses va_arg on it with or without copying it.