Programming Perl, 4th Edition

fork

fork

This function creates two processes out of one by invoking the fork(2) syscall. If it succeeds, the function returns the new child process’s ID to the parent process and 0 to the child process. If the system doesn’t have sufficient resources to allocate a new process, the call fails and returns undef. File descriptors (and sometimes locks on those descriptors) are shared, while everything else is copied—or at least made to look that way.

In ancient versions, unflushed buffers remain unflushed in both processes, which means you might need to set $| on one or more filehandles earlier in the program to avoid duplicate output.

A nearly bulletproof way to launch a child process while checking for “cannot fork” errors would be:

use Errno qw(EAGAIN);
FORK: {
    if ($pid = fork) {
        # parent here
        # child process pid is available in $pid
    }
    elsif (defined $pid) { # $pid is zero here if defined
        # child here
        # parent process pid is available with getppid
    }
    elsif ($! == EAGAIN) {
        # EAGAIN is the supposedly recoverable fork error
        sleep 5;
        redo FORK;
    }
    else {
        # weird fork error
        die "Can't fork: $!";
    }
}

These precautions are not necessary on operations that do an implicit fork(2)—such as system, backticks, or opening a process as a filehandle—because Perl automatically retries a fork on a temporary ...