FAQ 8.12 How do I start a process in the background?


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8.12: How do I start a process in the background?

(contributed by brian d foy)

There's not a single way to run code in the background so you don't have
to wait for it to finish before your program moves on to other tasks.
Process management depends on your particular operating system, and many
of the techniques are in perlipc.

Several CPAN modules may be able to help, including "IPC::Open2" or
"IPC::Open3", "IPC::Run", "Parallel::Jobs", "Parallel::ForkManager",
"POE", "Proc::Background", and "Win32::process". There are many other
modules you might use, so check those namespaces for other options too.

If you are on a Unix-like system, you might be able to get away with a
system call where you put an "&" on the end of the command:

system("cmd &")

You can also try using "fork", as described in perlfunc (although this
is the same thing that many of the modules will do for you).

STDIN, STDOUT, and STDERR are shared
Both the main process and the backgrounded one (the "child" process)
share the same STDIN, STDOUT and STDERR filehandles. If both try to
access them at once, strange things can happen. You may want to
close or reopen these for the child. You can get around this with
"open"ing a pipe (see "open" in perlfunc) but on some systems this
means that the child process cannot outlive the parent.

You'll have to catch the SIGCHLD signal, and possibly SIGPIPE too.
SIGCHLD is sent when the backgrounded process finishes. SIGPIPE is
sent when you write to a filehandle whose child process has closed
(an untrapped SIGPIPE can cause your program to silently die). This
is not an issue with "system("cmd&")".

You have to be prepared to "reap" the child process when it

$SIG{CHLD} = sub { wait };


You can also use a double fork. You immediately "wait()" for your
first child, and the init daemon will "wait()" for your grandchild
once it exits.

unless ($pid = fork) {
unless (fork) {
exec "what you really wanna do";
die "exec failed!";
exit 0;
waitpid($pid, 0);

See "Signals" in perlipc for other examples of code to do this.
Zombies are not an issue with "system("prog &")".


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