Re: [PATCH] add strncmp to PowerPC

From: Andreas Schwab <schwab@suse.de>
To: Gabriel Paubert <paubert@iram.es>
Cc: Steven Rostedt <rostedt@goodmis.org>,
	paulus@samba.org, LKML <linux-kernel@vger.kernel.org>,
	linuxppc-dev@ozlabs.org
Subject: Re: [PATCH] add strncmp to PowerPC
Date: Mon, 03 Mar 2008 11:10:27 +0100	[thread overview]
Message-ID: <je4pbow224.fsf@sykes.suse.de> (raw)
In-Reply-To: <20080303095443.GB27105@iram.es> (Gabriel Paubert's message of "Mon\, 3 Mar 2008 10\:54\:43 +0100")

Gabriel Paubert <paubert@iram.es> writes:

> Now that I think a bit more about it, I believe that the C version is
> incorrect: the clrldi/extsb dance takes a value between -255 and +255
> and collapses it into the -128 to 127 range, meaning that the return
> value may be wrong if we rely on the sign of the result. So unless I
> miss something, the problem is much more serious than just stupid code
> (I had just a look at the libc version in C and characters are cast to
> unsigned char before the comparison).

The latter is explicitly required by the C standard.  Ie. even if your
characters are signed they are always compared as unsigned by
strcmp/strncmp/memcmp.

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
SuSE Linux Products GmbH, Maxfeldstraße 5, 90409 Nürnberg, Germany
PGP key fingerprint = 58CA 54C7 6D53 942B 1756  01D3 44D5 214B 8276 4ED5
"And now for something completely different."