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From: ari@eleazar.dartmouth.edu (Ari Halberstadt)
Newsgroups: comp.lang.c
Subject: Re: use of if (!cptr) and if (cptr) && a programming error
Summary: A VERY serious programming error
Message-ID: <14582@dartvax.Dartmouth.EDU>
Date: 21 Jul 89 16:39:05 GMT
References: <10099@mpx2.mpx.com> <93@microsoft.UUCP> <10100@mpx2.mpx.com> <WJC.89Jul19182602@ho5cad.ho5cad.ATT.COM> <10103@mpx2.mpx.com> <8468@batcomputer.tn.cornell.edu>
Sender: news@dartvax.Dartmouth.EDU
Reply-To: ari@eleazar.dartmouth.edu (Ari Halberstadt)
Organization: Dartmouth College, Hanover, NH
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In article <8468@batcomputer.tn.cornell.edu> lacey@tcgould.tn.cornell.edu (John Lacey) writes:
>In article <10103@mpx2.mpx.com> erik@mpx2.mpx.com (Erik Murrey) writes:
>[Material everybody has seen n times deleted.]
>I am kind of butting in [ :-) ], but how 'bout
>        while ( ( myptr = my_func() )->x != myptr->y )
>                {
>                }

THAT WILL NOT WORK! I quote here from the book "C Traps and Pit Falls" by
Andrew Koenig (Addison-Wesley, 1989), pp. 47:
	Only the four C operators &&, ||, ?:, and , specify an order of
	evaluation.
What this means for the above code is that the machine is free to evaluate
"myptr->y" before "myptr" is assigned a value by my_func(). While it
may work with a certain compiler on a certain machine, you may wake up
tomorrow and discover some very wierd bugs!

-- Ari Halberstadt '91, "Long live succinct signatures"
E-mail: ari@eleazar.dartmouth.edu	Telephone: (603)640-5687
Mailing address: HB1128, Dartmouth College, Hanover NH 03755