Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!rutgers!att!cbnewsm!lfd
From: lfd@cbnewsm.att.com (leland.f.derbenwick)
Newsgroups: comp.unix.wizards
Subject: Re: forking and zombies in SYSV.
Summary: signal(SIGCHLD, SIG_IGN) _is_ in the SVID.
Message-ID: <1990Aug27.170458.20643@cbnewsm.att.com>
Date: 27 Aug 90 17:04:58 GMT
References: <1990Aug20.133107.12516@lth.se> <5980067@hpfcdc.HP.COM>
Organization: AT&T Bell Laboratories
Lines: 41

In article <5980067@hpfcdc.HP.COM>, rml@hpfcdc.HP.COM (Bob Lenk) writes:
> Several people have suggested
> 
> 	signal(SIGCLD, SIG_IGN);
> 
> While this does prevent zombies from being formed in System III, System
> V, and compatible systems, there are some reasons to avoid it:
> 
> 	1) Portability.  This does not work in all implementations
> 	   (notably not in BSD).  It is not required by POSIX, XPG,
> 	   or even the SVID.  

This behavior _is_ required by the SVID: see "signal(BA_OS)" on
page 6-124 of Volume 1 of the Third Edition (8/89):

   If the diposition for SIGCHLD is set to SIG_IGN, the calling
   process's child processes will not create zombie processes
   when they terminate [see exit(BA_OS)].  If the calling process
   subsequently waits for its children, it will block until all
   of its children terminate; it will then return a value of -1
   with errno set to ECHILD ...

> 	2) Possible side-effects.  This can impact any part of your
> 	   program that creates children, including library calls that
> 	   might create children for reasons unknown to you.  It can
> 	   impact the programs exec'd by descendents if they create
> 	   children.  Most code that creates children expects to wait
> 	   for them.  This can cause such code to hang (until all your
> 	   children have died) and/or to get unexpected errors (possibly
> 	   missing out on important status information).

This is correct, but it would be simple to add

   signal(SIGCHLD, SIG_DFL);

to the child processes (in main() if they are invoked via "system",
or just after the fork() if the fork()/exec() is done explicitly).

 -- Speaking strictly for myself,
 --   Lee Derbenwick, AT&T Bell Laboratories, Warren, NJ
 --   lfd@cbnewsm.ATT.COM  or  <wherever>!att!cbnewsm!lfd