The fork() Function in Perl

Perl provides a fork() function that corresponds to the Unix system call of the same name. On most Unix-like platforms where the fork() system call is available, Perl's fork() simply calls it. On some platforms such as Windows where the fork() system call is not available, Perl can be built to emulate fork() at the interpreter level.

The fork() function is used to clone a current process. This call create a new process running the same program at the same point. It returns the child pid to the parent process, 0 to the child process, or under if the fork is unsuccessful.

You can use exec() function within a process to launch the requested executable, which will be executed in a separate process area and exec() will wait for it to complete before exiting with the same exit status as that process.

Example

 Live Demo

#!/usr/bin/perl
if(!defined($pid = fork())) {
   # fork returned undef, so unsuccessful
   die "Cannot fork a child: $!";
} elsif ($pid == 0) {
   print "Printed by child process\n";
   exec("date") || die "can't exec date: $!";
} else {
   # fork returned 0 nor undef
   # so this branch is parent
   print "Printed by parent process\n";
   $ret = waitpid($pid, 0);
   print "Completed process id: $ret\n";
}
1;

Output

When above code is executed, it produces the following result −

Printed by parent process
Printed by child process
Tue Sep 17 15:41:08 CDT 2013
Completed process id: 17777

The wait() and waitpid() can be passed as a pseudo-process ID returned by fork(). These calls will properly wait for the termination of the pseudo-process and return its status. If you fork without ever waiting on your children using waitpid() function, you will accumulate zombies. On Unix systems, you can avoid this by setting $SIG{CHLD} to "IGNORE" as follows −

Example

 Live Demo

#!/usr/bin/perl
local $SIG{CHLD} = "IGNORE";
if(!defined($pid = fork())) {
   # fork returned undef, so unsuccessful
   die "Cannot fork a child: $!";
} elsif ($pid == 0) {
   print "Printed by child process\n";
   exec("date") || die "can't exec date: $!";
} else {
   # fork returned 0 nor undef
   # so this branch is parent
   print "Printed by parent process\n";
   $ret = waitpid($pid, 0);
   print "Completed process id: $ret\n";
}
1;

Output

When above code is executed, it produces the following result −

Printed by parent process
Printed by child process
Tue Sep 17 15:44:07 CDT 2013
Completed process id: -1