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From: rpw3@redwood.UUCP (Rob Warnock)
Newsgroups: net.lang.c
Subject: Re: String copy idiom.
Message-ID: <189@redwood.UUCP>
Date: Tue, 12-Mar-85 19:46:57 EST
Article-I.D.: redwood.189
Posted: Tue Mar 12 19:46:57 1985
Date-Received: Sat, 16-Mar-85 03:19:01 EST
References: <7044@watdaisy.UUCP> <3448@alice.UUCP> <464@petsd.UUCP>
Organization: [Consultant], Foster City, CA
Lines: 29

Please note that the semantics of

	while (*s++ = *t++) ;
and
	while ((*s = *t) != '\0') {s++; t++;}

are NOT the same; therefore, the generated code CANNOT be the same!
(I noticed this while comparing the code generated on the 68000 compiler
I use.) The first statement leaves "t" pointing at the byte AFTER the null,
while the second leaves "t" pointing to the null. Auto-incrementing cannot
be used in the second case, unless your compiler generates code to "back out"
the final incrementation (an optimization I have on occasion applied by hand
to tight assembly code, but have never seen a compiler use).

The following two ARE equivalent (by the definition of "true" in boolean tests
and due to the "usual conversions" applied to '\0' before the comparison), and
the compiler I use indeed generates the same code for both cases:

	while (*s++ = *t++) ;
and
	while ((*s++ = *t++) != '\0') ;


Rob Warnock
Systems Architecture Consultant

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