Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.1.chuqui 4/7/84; site voder.UUCP
Path: utzoo!linus!decvax!ittvax!dcdwest!sdcsvax!sdcrdcf!hplabs!nsc!voder!jeff
From: jeff@voder.UUCP
Newsgroups: net.news.b,net.bugs.4bsd
Subject: alarm() bug in 4.2
Message-ID: <282@voder.UUCP>
Date: Tue, 10-Jul-84 00:05:50 EDT
Article-I.D.: voder.282
Posted: Tue Jul 10 00:05:50 1984
Date-Received: Thu, 12-Jul-84 01:16:18 EDT
Organization: National Semiconductor, Santa Clara
Lines: 32
Xref: 130 342

There is a bug in the 4.2 implementation of alarm(3).  In the following

	oldalarm = alarm(newalarm);

oldalarm gets the current value of the interval timer *rounded down*
to the nearest second.  So ...

	for (...) {
		oldalarm = alarm((unsigned) 0);
		...
		alarm(oldalarm);	
	}

causes the interval timer to be *repeatedly* rounded down.  End
result: the alarm goes off early.  And, of course, if the interval
timer just *happened* to be at some value *less* than a second the
alarm will be turned off completely.

Fortunately, there is an easy workaround; change the previous code
to read:

	for (...) {
		oldmask = sigblock(1<<SIGALRM);
		...
		sigblock(oldmask);
	}

The fix to alarm() is left as an exercise to the interested reader.

Jeff Gilliam

No pain, no gain ...	*sigh*