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From: allbery@ncoast.UUCP
Newsgroups: comp.lang.c
Subject: varargs (was Re: Public Domain _doprnt in C)
Message-ID: <2321@ncoast.UUCP>
Date: Tue, 7-Apr-87 20:51:18 EST
Article-I.D.: ncoast.2321
Posted: Tue Apr  7 20:51:18 1987
Date-Received: Sat, 11-Apr-87 05:20:14 EST
References: <938@copper.TEK.COM> <100@asi.UUCP> <2285@ncoast.UUCP> <6152@mimsy.UUCP>
Reply-To: allbery@ncoast.UUCP (Brandon Allbery)
Followup-To: comp.lang.c
Organization: Cleveland Public Access UN*X, Cleveland, Oh
Lines: 55

As quoted from <6152@mimsy.UUCP> by chris@mimsy.UUCP (Chris Torek):
+---------------
| 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.
+---------------

All I know about the MIPS machines is that Plexus supposedly makes one of them
(Plexus calls it the P/95, I think; no idea what MIPS calls it or what archi-
tecture it uses, but it's Plexus's only BSD machine.  Sigh.)

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

Dunno about X3J11, POSIX, /usr/group, etc., but the System V.2 docs state that
varargs must be used with va_alist as the ONLY argument.  Sure, it works on
stack machines where typecasting is mostly size changing or punning; but the
va_alist-only stuff is the only way to be compatible with all machines.

+---------------
| >(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.
+---------------

I've never used a Pyramid, so I was guessing.  However, it makes sense that
varargs should be set up so a compiler can recognize va_alist etc. as tokens
and generate the appropriate code.  I would guess that some architectures
might still not work; the ANSI C standard would fix this by allowing the
function's varargs status to be pre-declared.  (I hope; I know it was under
consideration once, but varargs might not be supported in a pre-declaration
in the current version of P1003.1.)

++Brando
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