Re: c/10339: strncmp generates imPure code

[Andreas Schwab <schwab@suse.de>]

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The following reply was made to PR optimization/10339; it has been noted by GNATS.

From: Andreas Schwab <schwab@suse.de>
To: Michael Ubell <ubell@mindspring.com>
Cc: Timothy C Prince <tprince@myrealbox.com>,
	falk.hueffner@student.uni-tuebingen.de, bangerth@ices.utexas.edu,
	gcc-bugs@gcc.gnu.org, gcc-gnats@gcc.gnu.org
Subject: Re: c/10339: strncmp generates imPure code
Date: Mon, 07 Apr 2003 23:38:18 +0200

 Michael Ubell <ubell@mindspring.com> writes:
 
 |> Andreas Schwab wrote:
 |> > Michael Ubell <ubell@mindspring.com> writes:
 |> > |> Attached is a program that reads 831 unaligned unallocated
 |> > |> bytes.  I can't actually get it to segv on Solaris because
 |> > |> I don't know enough about their memory management, but
 |> > |> I cannot believe this is correct code.
 |> > Yes, you are right, I can reproduce that also on ia64-linux.  The
 |> > conversion to memcmp is really invalid here.
 |> > Andreas.
 |> > 
 |> 
 |> Now I get to argue the other side :-)  Was your string properly null
 |> terminated?  If not, then its not the compiler's fault.  I think
 |> memcmp is valid so long as the strings are well formed.  This will
 |> be true because C strings may not contain embedded nulls.
 
 memcmp does not operate on strings, but on arrays of characters.  That's
 an important difference, because memcmp is allowed, eg, to start comparing
 from the upper end of the arrays, independent of the actual contents of
 them; there is no null termination involved here.  On the other hand
 strncmp must not look behind the first nul character (at least
 conceptionally, the actual implementation may do so anyway if it can
 determine that it is safe).
 
 Andreas.
 
 -- 
 Andreas Schwab, SuSE Labs, schwab@suse.de
 SuSE Linux AG, Deutschherrnstr. 15-19, D-90429 Nürnberg
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