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Path: utzoo!henry
From: henry@utzoo.UUCP (Henry Spencer)
Newsgroups: comp.lang.c
Subject: Re: another auto-increment problem
Message-ID: <8672@utzoo.UUCP>
Date: Mon, 28-Sep-87 21:59:21 EDT
Article-I.D.: utzoo.8672
Posted: Mon Sep 28 21:59:21 1987
Date-Received: Mon, 28-Sep-87 21:59:21 EDT
References: <3680001@wdl1.UUCP>
Organization: U of Toronto Zoology
Lines: 22

> ... all our UNIX systems have
> implementations that evaluate function arguments right to left!!
> 
> I presume this has something to do with the order in which the
> arguments have to be pushed and popped on the stack...

As various people have pointed out, the order in which arguments are
evaluated is officially undefined, and you are warned not to depend on
it.  This got started mainly, I believe, because the earliest C compiler
used right-to-left evaluation.

The reason for right-to-left is in two parts.  First, functions like
printf are a lot easier to implement if arguments, once pushed, are in
ascending order in memory, e.g. argument 3 follows argument 2.  Second,
many modern machines have stacks that grow downward in memory, i.e. the
latest thing pushed is below the previous thing pushed; the PDP11 did it
that way for complex reasons, and many others have copied the 11.  Putting
these two considerations together, if the leftmost argument is to be at
the lowest location in memory, it must be pushed last.  Hence right-to-left.
-- 
"There's a lot more to do in space   |  Henry Spencer @ U of Toronto Zoology
than sending people to Mars." --Bova | {allegra,ihnp4,decvax,utai}!utzoo!henry