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From: jsdy@hadron.UUCP (Joseph S. D. Yao)
Newsgroups: net.unix-wizards
Subject: Re: Printf sets errno to 25?
Message-ID: <146@hadron.UUCP>
Date: Wed, 25-Dec-85 17:49:07 EST
Article-I.D.: hadron.146
Posted: Wed Dec 25 17:49:07 1985
Date-Received: Sat, 28-Dec-85 01:02:46 EST
References: <119@cpsc6a.UUCP> <212@iclbra.UUCP> <952@brl-tgr.ARPA>
Reply-To: jsdy@hadron.UUCP (Joseph S. D. Yao)
Distribution: net
Organization: Hadron, Inc., Fairfax, VA
Lines: 11
Keywords: isatty, printf, ENOTTY
Summary: printf() does check tty-ness.

As Gwyn and Merritt point out, the value of errno is not guaranteed
to be meaningful except under certain circumstances, e.g., failed
system calls.  The reason it gets set at all inside printf() is that
printf() checks stdout for tty-ness and, if it is not a tty, buffers
output.  Otherwise, depending on the version of UNIX, it line-buffers
it or does not buffer it at all.  Again depending on the version of
UNIX, roughly the same may be true of stdin.  I believe (and I know
Doug'll hit me if I'm wrong) that stderr is always unbuffered.
-- 

	Joe Yao		hadron!jsdy@seismo.{CSS.GOV,ARPA,UUCP}