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Message-ID: <3280F04E.4D5A@cococo.net>
Date: Wed, 06 Nov 1996 20:08:46 +0000
From: Kelley <kelley@cococo.net>
Reply-To: kelley@cococo.net
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Subject: Re: Compiler bug?
References: <55p7ck$r2l@csgrad.cs.vt.edu>
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Tommy Johnson wrote:
>
> This program doesn't work as I think it should. On line 10, b is not
> incremented between the first and second b++'s, though it is correct
> after the entire expression.
>
> On FreeBSD 2.2-Current as of about Oct 13, the program below produces
> 0x1212 instead of 0x3412 (gcc 2.7.2.1, or gcc 2.6.3). It also happens
> on NetBSD 1.1B as of about May 16 on an HP300 (gcc 2.7.2). It DOES produce
> 0x3412 on a DEC alpha with gcc 2.7.2, or gcc 2.7.2 on a DECstation
> (OSF1 3.2 148, and Ultrix 4.4 rev 69 respectively).
>
> (It also breaks on FreeBSD 2.1.5-Release with gcc 2.6.3)
>
> Am I hallucinating? TIA...
>
> -Tom "Bugger, now I have to wait for someone to wake up." -Marcus
> tjohnson@csgrad.cs.vt.edu "My other computer ALSO runs unix." -me <*>
> http://csgrad.cs.vt.edu/~tjohnson/ Commercial email is unwelcome.
> Message and signature (c) 1996 Tommy O. Johnson, all rights reserved
>
> --- Bite Here --- Bite Here --- Bite Here ---
>
> /* Copyright: test program, do with it as you wish
> ** Tommy Johnson
> */
> main()
> {
> int a,b;
> unsigned char c[2];
>
> b=0;
> c[0]=0x12;
> c[1]=0x34;
>
> a=c[b++]|((c[b++])<<8);
> printf("should be 0x3412 %4x\n",a);
> }
Not sure if it is a bug or not, since the expression (to the best of my
recollection) with a post increment operator isn't evaluated until the
next expression, but try this out on line 10:
a=c[--b] | ((c[++b])<<8);
Very interesting since it is the same variable, but I can tell you that
the Borland C++ 4.5 (on Windoze95)compiler evaluates to 0x1212 also.
Later
Kelley