Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!mit-eddie!husc6!purdue!decwrl!megatest!djones
From: djones@megatest.UUCP (Dave Jones)
Newsgroups: comp.unix.wizards
Subject: Re: sleep(3) uses longjmp and SIGALRM
Message-ID: <844@goofy.megatest.UUCP>
Date: 29 Sep 88 21:30:26 GMT
References: <1988Sep28.135914.25444@lsuc.uucp)
Organization: Megatest Corporation, San Jose, Ca
Lines: 26

From article <1988Sep28.135914.25444@lsuc.uucp), by dave@lsuc.uucp (David Sherman):
) I have a fairly complex CAI system that's used for legal education
) here. It uses pipes, longjmp, SIGUSER and pause(2) to allow the student
) to hit an interrupt key and change the level of instruction at any point.
) It also uses sleep(3) here and there, and SIGALRM to log off idle users.
) With all the various UNIXisms that interact, the programs occasionally
) exhibit strange behaviour, which I'd never been able to pinpoint.
) 
) Today someone asked me why sleep(3) is a library routine rather
) than a system call, which I'd always thought it was.  (This is a
) v7-based UNIX, Perkin-Elmer's Edition VII.)  So I read the code,
) and was startled to find it uses SIGALRM and pause, with a longjmp
) to put you back where you were.

The man-page on the Sun3 (BSD4.2 based) is explicit:

  sleep is implemented by setting an interval timer and  paus-
  ing  until  it expires.  The previous state of this timer is
  saved and restored. 

I would recommend that you process all signals, sleeps, selects, etc,
through a central location.  The "notifier" in Sun's sunview package
would suffice, but I don't know if its source is available.  In
any case, it should not be too difficult to implement it.

Good luck.