Path: utzoo!mnetor!uunet!husc6!bloom-beacon!oberon!cit-vax!cit-vlsi!wen-king
From: wen-king@cit-vlsi.Caltech.Edu (Wen-King Su)
Newsgroups: comp.unix.wizards
Subject: Re: Wait, Select, and a SIGCHLD Race Condition
Message-ID: <4870@cit-vax.Caltech.Edu>
Date: 12 Dec 87 15:59:15 GMT
References: <5105@sol.ARPA> <4857@cit-vax.Caltech.Edu> <496@PT.CS.CMU.EDU>
Sender: news@cit-vax.Caltech.Edu
Reply-To: wen-king@cit-vlsi.UUCP (Wen-King Su)
Organization: California Institute of Technology
Lines: 27

In article <496@PT.CS.CMU.EDU> rpd@F.GP.CS.CMU.EDU (Richard Draves) writes:
<>  on SIGCHLD interrupt, set time_ptr = &ZERO_TIMER;
>...
<>  numfds = select(...,time_ptr);
>...
<>| Wen-King Su  wen-king@vlsi.caltech.edu  Caltech Corp of Cosmic Engineers |
>
<This method doesn't quite work.  The problem is if the signal comes in after
>the select procedure call but before the trap to the kernel, then the
<select will end up blocking because the kernel will get ORIGINAL_TIMER
>instead of ZERO_TIMER as the argument to the trap.  The signal handler
<must atomically zero out the actual timer structure.
>
<Rich

OOPS, you are right.  How about this:

  on SIGCHLD interrupt, set time_struct = ZERO_TIMER;
...
  numfds = select(...,&time_struct);

It should cover all possibilities.  I knew there is a way, I just
couldn't remember all the details.

/*------------------------------------------------------------------------*\
| Wen-King Su  wen-king@vlsi.caltech.edu  Caltech Corp of Cosmic Engineers |
\*------------------------------------------------------------------------*/