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Path: utzoo!utgpu!water!watmath!clyde!att-cb!att-ih!pacbell!ames!ncar!boulder!sunybcs!bingvaxu!leah!itsgw!steinmetz!uunet!vsi!friedl
From: friedl@vsi.UUCP (Stephen J. Friedl)
Newsgroups: comp.lang.c,comp.sys.ibm.pc
Subject: Re: cdecl keyword in MSC
Summary: arg-passing direction is not defined by the language
Keywords: cdecl MicroSoft C keywords
Message-ID: <476@vsi.UUCP>
Date: 2 Apr 88 23:29:39 GMT
References: <1238@wjvax.UUCP> <297@ho7cad.ATT.COM> <1242@wjvax.UUCP> <185@premise.ZONE1.COM>
Organization: V-Systems, Inc. -- Santa Ana, CA
Lines: 41

In article <185@premise.ZONE1.COM>, chang@premise.ZONE1.COM writes:
> 
> [ comments about "useless" cdecl and function calling conventions in MSC ]
>
> The calling convention we're talking about here isn't call-by-name or
> call-by-value.  It's simply the order in which parameters are passed
> on the stack.  And in C, that order *must* be right to left in order
> to support variable length functions like printf.  If it were left to
> right, then the printf function wouldn't know where to find its format
> string.

Note that while right-to-left arg passing may be used in the PC,
this is defined *by* the PC and/or MSC and not by the C language
(it is specifically "Unspecified Behavior").  There are indeed
machines such as the WE32100 (AT&T 3B2) and I believe the PDP-11
that are happy to pass args in the other direction, and *my* 3B2
printf() still says "hello, world\n" :-)

My limited cause-and-effect detectors indicate that when the
stack grows up (i.e., "push" increments the stack pointer) then
args go left->right, and if the stack grows down then args go
right->left.  Upward-growing stacks seem to be much less common;
anybody know of others?  Which direction do their args pass?

Also, I imagine that a clever compiler writer could manage to
define a calling convention that would work properly in either
direction, albeit less efficiently in the "wrong" direction.

Try this on your favorite compiler to see what you get.

	main()
	{
	static char	*dirwords[] = { "left", "right" };
	char		**dirptr = dirwords;

		printf("Direction is %s to %s\n", *dirptr++, *dirptr++);
	}

-- 
Steve Friedl   V-Systems, Inc.   "Yes, I'm jeff@unh's brother"
friedl@vsi.com {uunet,ihnp4}!vsi.com!friedl attmail!vsi!friedl