What's the difference between various $SIG{CHLD} values?

Question

What is the difference between these settings?

$SIG{CHLD} = \'IGNORE\'  
$SIG{CHLD} = \'DEFAULT\'  
$SIG{CHLD} = \'\'  
$SIG{CHLD} = undef

Answer 1

See %SIG.

$SIG{CHLD} = 'IGNORE'; causes your process to ignore SIGCHLD signals.

$SIG{CHLD} = 'DEFAULT'; causes your process to treat SIGCHLD signals as it would had you not messed with $SIG{CHLD} or equivalent. According to kill(1), a process on my system ignores SIGCHLD by default

$SIG{CHLD} = ''; and $SIG{CHLD} = undef; are not valid values.

As for reaping, children of a parent whose SIGCHLD handler is explicitely set to IGNORE will be reaped automatically by the system as soon as it exits.

$ perl -e'
   my $pid = fork;
   if (!$pid) { sleep(1); exit; }
   sleep(2);
   system "ps -o pid,stat,command $pid";
'
  PID STAT COMMAND
14667 ZN+  [perl] 

$ perl -e'
   $SIG{CHLD}="IGNORE";
   my $pid = fork;
   if (!$pid) { sleep(1); exit; }
   sleep(2);
   system "ps -o pid,stat,command $pid";
'
  PID STAT COMMAND

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