Date: Sat, 12 Oct 1996 12:50:02 +0930 (CST) From: Michael Smith <msmith@atrad.adelaide.edu.au> To: charnier@xp11.frmug.org (Philippe Charnier) Cc: current@FreeBSD.org Subject: Re: gcc's bug. please comment Message-ID: <199610120320.MAA03815@genesis.atrad.adelaide.edu.au> In-Reply-To: <199610111905.VAA01615@xp11.frmug.org> from "Philippe Charnier" at Oct 11, 96 09:05:36 pm
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Philippe Charnier stands accused of saying:
>
> A friend found a bug in gcc-2.7.2.1. The following program fails on
> i386 architecture (both FreeBSD and solaris-x86) and works as expected
> on sparc (both sunos and solaris2.5.1) and on hp (hp-ux9.07). As I'm
> not on the gcc-bug list, please comment before I report the bug. This
> afternoon I sent a bug report to Sun but it was before trying on
> FreeBSD.
>
> The output should be `1 2' but it is `2 1' on i386 computers.
> Here is the code:
>
> ----------mypb2.c-----------------
> #include <stdio.h>
> #include <stdlib.h>
>
> main()
> { FILE *fic;
> int a[2][50], i = 0;
>
> fic=fopen("mypb2.csv","r");
> fscanf(fic, "%d;%d", &a[1][i++], &a[1][i++]);
Arguments to C functions are usually stacked right-to-left (this is
the only way to make functions with variable numbers of arguments work
sensibly)(1). Thus, the args to fscanf are evaluated right-to-left, and
thus your code is working as one might expect.
I would imagine that the Sun and HP compilers calculate the a indices
before stacking them for the call, and thus evaluate left-to-right.
To me, this implies pedantic metrics-and-clockwatching compiler
design, or possibly one or more large customers with large legacy
codebases which depend upon code like the snippet above.
The C standard _explicitly_ forbids you from making any assumptions about
the order of evaluation of function arguments, so the bug is yours.
(1) Yes, I know there are other ways of doing it. No, none of them work
sensibly.
--
]] Mike Smith, Software Engineer msmith@atrad.adelaide.edu.au [[
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