Newsgroups: comp.os.minix
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Important new program: cleanit.c
Message-ID: <1989Oct8.025909.13399@utzoo.uucp>
Organization: U of Toronto Zoology
References: <3492@ast.cs.vu.nl> <3498@solo10.cs.vu.nl> <3539@ast.cs.vu.nl> <3554@solo2.cs.vu.nl>
Date: Sun, 8 Oct 89 02:59:09 GMT

In article <3554@solo2.cs.vu.nl> maart@cs.vu.nl (Maarten Litmaath) writes:
>\   ... just leaving out
>\   the last actual parameter.  Given the C calling sequence of pushing the
>\   last parameter first, this is legal, [...]
>
>I think it's always illegal: there might exist machines with `weird' calling
>conventions which would choke over such mis-invocations (while still allowing
>varargs functions).  (Aside: could you comment on that, Henry?)

The order of parameter pushing has never been defined; in fact, order of
evaluation of parameters, and stack layout of same, has always been quite
explicitly undefined.  (Even K&R 1st ed. specifically warns that compilers
differ.)  In pre-ANSI C, there was no officially-blessed way to call a
function with arguments not matching those at the function's definition.
(The <varargs.h> header existed in V7 but was not documented.)  Printf
and friends had to be done *somehow*, with a strong implication that the
method was implementation-specific and collaboration with the compiler
was needed.  ANSI C blesses mismatches specifically and only for varargs
functions.

In practice, the usual approach to printf was to have the compiler use a
push order that put the first argument in a predictable place, which
usually meant last-first so that the first argument was at the top of
the stack.  This had the side effect of making trailing arguments optional
in general.  Exploiting this is probably not a good idea; the portability
problems are substantial and growing.  Varargs functions do not necessarily
use the same calling sequence as ordinary ones.
-- 
A bit of tolerance is worth a  |     Henry Spencer at U of Toronto Zoology
megabyte of flaming.           | uunet!attcan!utzoo!henry henry@zoo.toronto.edu