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From: chris@mimsy.UUCP
Newsgroups: comp.sources.d,comp.lang.c
Subject: varargs (was Re: Public Domain _doprnt in C)
Message-ID: <6152@mimsy.UUCP>
Date: Mon, 6-Apr-87 05:00:03 EST
Article-I.D.: mimsy.6152
Posted: Mon Apr  6 05:00:03 1987
Date-Received: Thu, 9-Apr-87 01:45:29 EST
References: <938@copper.TEK.COM> <100@asi.UUCP> <2285@ncoast.UUCP>
Followup-To: comp.lang.c
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
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Xref: utgpu comp.sources.d:547 comp.lang.c:1489

In article <2285@ncoast.UUCP> allbery@ncoast.UUCP (Brandon Allbery) writes:
>Note that varargs is defined such that it will work even on machines such as
>the Pyramid which pass arguments in registers (common on RISC machines).

Or on the MIPS machines.

>You MUST declare only one argument -- va_alist -- and include the type of it
>as "va_dcl" without a semicolon.

`va_alist' and `va_dcl' are often (always?) macros.  It is conceivable
that on a machine that passes the first two arguments in registers and
the third through nth on the stack, these might be

	#define va_alist  _p1, _p2, _p3
	#define va_dcl int _p1, _p2, _p3;

in which case va_start might be

	#define va_start(l) { \
		struct _va_info _va; \
		_va.reg = 2; _va.r[1] = _p1; _va._r[0] = _p2; \
		_va.addr = &_p3; \
		(l) = &_va;

and va_arg then

	#define va_arg(l, t) \
		(sizeof(t) == 8 ? \	/* size always 8 or 4 in this arch. */
		 (ugly code to handle double float with spanning &c) : \
		 (--(l)->reg >= 0 ? *(t *)&(l)->r[(l)->reg] : \
		  ((l)->addr++, ((t *)((l)->addr))[-1])))

(and va_end would simply be defined as `}').

This would indeed preclude usage like

	f(fmt, va_alist)
		char *fmt;
		va_dcl
	{
		...

The reason I mention this is that in fact, on the Pyramid, Sun,
and Vax (all implementations of which I am aware), usages such as
that above in fact work.  I wonder, then, if this has been guaranteed
anywhere.  Has it?  (I know, I should dig out a copy of the X3J11
draft....)

>(C compilers can then recognize this directly to do any special
>processing needed, as for the Pyramid.)

Actually, Pyramid manages it without compiler hooks.  I believe
MIPS uses compiler hooks, though.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris@mimsy.umd.edu