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From: Alan_Cote.DlosLV-Comm@Xerox.COM
Newsgroups: comp.lang.c
Subject: Re:   <None>
Message-ID: <8011@brl-adm.ARPA>
Date: Wed, 24-Jun-87 18:26:19 EDT
Article-I.D.: brl-adm.8011
Posted: Wed Jun 24 18:26:19 1987
Date-Received: Fri, 26-Jun-87 06:00:49 EDT
Sender: news@brl-adm.ARPA
Lines: 27

PEBRBV writes,

>Henry Spencer <henry@utzoo.uucp> suggests (assuming a and b have
>no side effects)

>>#define    STREQ(a, b)    (*(a) == *(b) && strcmp((a), (b)) == 0)

>>This makes a considerable speed difference in programs that use
strings
>>a lot, since string comparisons usually fail on the very first
character.

>Except that the compiler is free to do the strcmp call before the
>comparison *(a)==*(b), or even do both tests and AND the results.
>But a good optimizer should do it efficiently.

Kernighan and Ritchie specifies (Appendix A, clause 7.11), "Unlike &, &&
guarantees left-to-right evaluation; moreover, the second operand is not
evaluated if the first operand is 0."

Therefore, the compiler is NOT free to do both tests!

However, STREQ may have only minimal value, as any good strcmp()
implementation will return immediately upon discovering that the first
bytes of its arguments do not match.

	- Alan T. Cote'