waiting on a process group

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

The unix documentation I have read says you can call waitpid with a negitive pid to wait on an entire process group. However, the perl docs for the function of the same name don't mention this.

The following program tests this out:

use strict;

if (my $pid = fork)
{
   # parent
   setpgrp($pid, $pid);

   waitpid($pid, 0);
   while (waitpid(-$pid, 0) != -1) {}
   exit;
}
else
{
   # child
   if (!(my $pid = fork))
   {
      # grandchild
      sleep(1) while 1;
   }
}
[download]

Unfortunately it doesn't seem to work on my system (perl 5.6, Tru64). The parent process exits, without waiting for the grandchild process. Is there another way of waiting on a process group?

Comment on waiting on a process group Download Code

Replies are listed 'Best First'.
Re: waiting on a process group by kschwab (Vicar) on Jul 02, 2001 at 02:40 UTC
The grandchild process' parent is exiting, which causes the grandchild to be inherited by the init() process, and hence, no longer in the grandparent's process group. This "double fork" technique is even used this way on purpose in some code. ( for systems that lack setpgrp ) It's probably easier to demonstrate the process group waiting by spawning multiple children...	[reply]
Re: waiting on a process group by Anonymous Monk on Jul 02, 2001 at 02:57 UTC
The unix documentation I have read says you can call waitpid with a negitive pid to wait on an entire process group. My documentation says something slightly different, it says that a negative pid waits on a child process in the process group, not an entire process group just your children. You should be able to wait on your children with -$pid but not your grandchildren.	[reply]