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From: ka@hou3c.UUCP (Kenneth Almquist)
Newsgroups: net.lang.c
Subject: Re: & operator
Message-ID: <145@hou3c.UUCP>
Date: Mon, 19-Dec-83 10:02:29 EST
Article-I.D.: hou3c.145
Posted: Mon Dec 19 10:02:29 1983
Date-Received: Tue, 20-Dec-83 06:44:07 EST
References: <481@sdcsla.UUCP> <1064@mit-eddie.UUCP>
Organization: Bell Labs, Holmdel, NJ
Lines: 23

From Steven M. Haflich:
	The variable `in' is indeed modified at the same time that
	the postfix ++ is evaluated, not after the entire statement.
	Consider:
		char *in = "123";
		if (*in++ == *in++) ... ;	/* ought never be true */

	Although except for the `,' `&&' and `||' operators C explicitly does
	not specify order of evaluation within expressions (and therefore
	expressions which use multiple `++' and `--' operators on the same
	variable are usually crocky) I see no reason why the above conditional
	could not be used to check whether the first two characters of a string
	(known to be at least length two) are the same.

Although there may be some support for this view in the C manual, this is not
the interpretation of the people in Murrey Hill.  Be warned that C compilers
may not always perform the increment immediately after fetching the value of
the variable.  Thus the above statement could be compiled as
	temp = in;	/* save value of "in" before incrementing it */
	in += 2;	/* perform both post-increments with one addition */
	if (*temp = *temp) ... ;

Kenneth Almquist