Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!dptg!ulysses!andante!alice!ark
From: ark@alice.UUCP (Andrew Koenig)
Newsgroups: comp.lang.c
Subject: Re: use of if (!cptr) and if (cptr) && a programming error
Message-ID: <9720@alice.UUCP>
Date: 1 Aug 89 20:08:51 GMT
References: <10099@mpx2.mpx.com> <93@microsoft.UUCP> <10100@mpx2.mpx.com> <10592@riks.csl.sony.JUNET>
Organization: AT&T Bell Laboratories, Liberty Corner NJ
Lines: 31

In article <10592@riks.csl.sony.JUNET>, diamond@csl.sony.JUNET (Norman Diamond) writes:

> But!  What about the following famous idiom!

>     int ch;  /* everyone knows it cannot be char */
>     while ((ch = getchar()) != EOF) { ... }

> The old value of ch might be compared to EOF?  The loop might be
> executed in incorrect cases (and might not be executed sometimes when
> it should be)?
> IS THIS REALLY TRUE ??????????????????
> Are there really millions of broken C programs due to this idiom?

No, this works fine.

The reason it works is that both operands of an operator
must be completely evaluated before the operator itself is
[unless the operator is && or || or ?:].  Thus, before
the comparison is done, the two subexpressions

	ch = getchar()

and

	EOF

must be evaluated.  There is no guarantee of which one is evaluated
first, but you can't compare two values until you know what they are.
-- 
				--Andrew Koenig
				  ark@europa.att.com