Perl Cookbook

Avoiding Zombie Processes

Problem

Your program forks children, but the dead children accumulate, fill up your process table, and aggravate your system administrator.

Solution

If you don’t need to record the children that have terminated, use:

$SIG{CHLD} = 'IGNORE';

To keep better track of deceased children, install a SIGCHLD handler to call waitpid:

use POSIX ":sys_wait_h";

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

Discussion

When a process exits, the system keeps it in the process table so the parent can check its status—whether it terminated normally or abnormally. Fetching a child’s status (thereby freeing it to drop from the system altogether) is rather grimly called reaping dead children. (This entire recipe is full of ways to harvest your dead children. If this makes you queasy, we understand.) It involves a call to wait or waitpid. Some Perl functions (piped opens, system, and backticks) will automatically reap the children they make, but you must explicitly wait when you use fork to manually start another process.

To avoid accumulating dead children, simply tell the system that you’re not interested in them by setting $SIG{CHLD} to "IGNORE". If you want to know which children die and when, you’ll need to use waitpid.

The waitpid function reaps a single process. Its first argument is the process to wait for—use -1 to mean any process—and ...