Programming Perl, 4th Edition

Reaping Zombies

When a process exits, its parent is sent a CHLD signal by the kernel, and the process becomes a zombie^[156] until the parent calls wait or waitpid. If you start another process in Perl using anything except fork, Perl takes care of reaping your zombied children; but if you use a raw fork, you’re expected to clean up after yourself. On many but not all kernels, a simple hack for autoreaping zombies is to set $SIG{CHLD} to "IGNORE". A more flexible (but tedious) approach is to reap them yourself. Because more than one child may have died before you get around to dealing with them, you must gather your zombies in a loop until there aren’t any more:

use POSIX ":sys_wait_h";
sub REAPER { 1 until waitpid(–1, WNOHANG) == –1 }

To run this code as needed, you can either set a CHLD signal handler for it:

$SIG{CHLD} = \&REAPER;

or, if you’re running in a loop, just arrange to call the reaper every so often.

^[156] Yes, that really is the technical term.