Date: Mon, 14 Oct 1996 10:12:22 +0200 (MESZ) From: "Hr.Ladavac" <lada@ws2301.gud.siemens.co.at> To: charnier@xp11.frmug.org (Philippe Charnier) Cc: current@FreeBSD.org Subject: Re: gcc's bug. please comment Message-ID: <199610140812.AA158850742@ws2301.gud.siemens.co.at> In-Reply-To: <199610111905.VAA01615@xp11.frmug.org> from "Philippe Charnier" at Oct 11, 96 09:05:36 pm
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E-mail message from Philippe Charnier contained:
>
> Hello,
>
> 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++]);
^^^ ^^^
Undefined
Behaviour; see comp.lang.c FAQ
> i = 0;
> while (i < 2) printf("%d ", a[1][i++]);
> printf("\n");
> fclose(fic);
> }
Since you just invoked undefined behavior of the C compiler you should
count yourself happy that your disks weren't wiped off clean.
Specifically, comma in the argument list *is*not*a*sequence*point* and
you cannot tell what the value of i is going to be in those two
references. The compiler is free to optimize it the way it likes it.
As a matter of fact, you cannot tell what should i be even after this
line. What you wrote is just another instance of the well-known:
i = i++;
/Marino
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