Re: CHLD signal?

I would do one thing differently from perlmonkey, in that I'd reinstall the signal handler as the first order of business in the REAPER function:

$SIG{CHLD} = \&REAPER;

# waitpid code goes here
[download]

merlyn will correct me if I'm wrong, but I do it this way just in case another SIGCHLD comes in -- if there's nothing in place *within* my signal handler to catch the signal, it could kill the program altogether. Just a little paranoia.

Comment on Re: CHLD signal? Download Code

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RE: Re: CHLD signal? by lhoward (Vicar) on May 08, 2000 at 04:40 UTC
The Perl Cookbook advises being paranoid and always reinstalling your SIG handler as the first step of your signal handler except in the case of SIGCHLD for the following reason: If you re-install the SIGCHLD handler before wait or waitpid gets its information from the OS's process table you risk the child becoming a zombie. Recomended is the following: Loop in the SIGCHLD handler and only re-install the SIGCHLD handler after the loop is complete (code below taken from Perl Cookbook). `$SIG{CHLD}=\&REAPER; sub REAPER{ my $stiff; while(($stiff = waitpid(-1,&WNOHANG))>0){ # do something with $stiff if you want } $SIG{CHLD}=\&REAPER; }` [download] This is necessary because most OSes will not queue up signals so if 2 are sent to your process at aprox the same time your process may behave as if it has received only 1. This may point to a possible solution to the original problem: `$SIG{CHLD}=\&REAPER; sub REAPER{ $SIG{CHLD}='IGNORE'; my $stiff; while(($stiff = waitpid(-1,&WNOHANG))>0){ # do something with $stiff if you want } $SIG{CHLD}=\&REAPER; }` [download] For a detailed discussion see sections 6.17 thru 6.19 (espscially 6.19) of the Perl Cookbook.	[reply] [d/l] [select]

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RE: Re: CHLD signal?
by lhoward (Vicar) on May 08, 2000 at 04:40 UTC

Perl Cookbook

except

If you re-install the SIGCHLD handler before wait or waitpid gets its information from the OS's process table you risk the child becoming a zombie. Recomended is the following: Loop in the SIGCHLD handler and only re-install the SIGCHLD handler after the loop is complete (code below taken from Perl Cookbook).

$SIG{CHLD}=\&REAPER;
sub REAPER{
  my $stiff;
  while(($stiff = waitpid(-1,&WNOHANG))>0){
    # do something with $stiff if you want
  }
  $SIG{CHLD}=\&REAPER;
}
[download]

This may point to a possible solution to the original problem:

$SIG{CHLD}=\&REAPER;
sub REAPER{
  $SIG{CHLD}='IGNORE';
  my $stiff;
  while(($stiff = waitpid(-1,&WNOHANG))>0){
    # do something with $stiff if you want
  }
  $SIG{CHLD}=\&REAPER;
}
[download]

For a detailed discussion see sections 6.17 thru 6.19 (espscially 6.19) of the Perl Cookbook.

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