In the following, extremely basic snippet of code, the idea is to fork off 10 processes all doing just the foo() subroutine and then sleep for 2 seconds.

But it only works if $SIG{CHLD} is not explicitly defined. Right now, even with $SIG{CHLD} defined as an empty anonymous subroutine sleep won't behave properly. If you comment out that first line, the script will behave as normally. Example output follows the script. 

$SIG{CHLD} = sub { };

while (1) {
  for(1..10) {spawn(\&foo)}          # fork off 10 &foo()'s

  my $s = sleep (2);                 # $s will hold the number of
                                     # seconds actually slept
  print "--------------------";
  if ($s) { print "Slept for $s seconds\n"; }# If we slept at all
  else    { print "Did not sleep at all\n"; }# If we didn't
}

sub spawn {
  return if fork();                  # Fork and return the parent
  exit shift->();                    # exec the coderef passed as arg
}

sub foo {
  print '*' for(1..5);
}

[download]

Output with the $SIG{CHLD} line commented out (all output has some asterisks and dashes removed for display purposes): 
**************************************---Slept for 2 seconds
**************************************---Slept for 2 seconds
**************************************---Slept for 2 seconds
**************************************---Slept for 2 seconds
**************************************---Slept for 2 seconds

[download]

And with the $SIG{CHLD} line active. 
**************************************---Slept for 2 seconds
**************************************---Did not sleep at all
**************************************---Did not sleep at all
*********************************---Did not sleep at all
*******************************************---Slept for 2 seconds
**********************************---Did not sleep at all
*******************************************---Did not sleep at all

[download]

Does anyone have any idea what is going on? This is driving me nuts. Does anyone have an alternate sleep or pause routine that works in this situation?
This is obviously just example code that has been whittled down, but the idea behind it is that sleep just isn't working for me (on several machines) when I am dealing with forked children and want to handle them properly.

In reply to Why is SIGCHLD affecting sleep? by jsoverson

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