Prev: FAQ 4.41 How can I remove duplicate elements from a list or array?
Next: running jar file with multiple arguments in perl
From: PerlFAQ Server on 29 May 2010 06:00
This is an excerpt from the latest version perlfaq8.pod, which
comes with the standard Perl distribution. These postings aim to
reduce the number of repeated questions as well as allow the community
to review and update the answers. The latest version of the complete
perlfaq is at http://faq.perl.org .

--------------------------------------------------------------------

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.

Signals
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&")".

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

$SIG{CHLD} = sub { wait };

$SIG{CHLD} = 'IGNORE';

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 &")".



--------------------------------------------------------------------

The perlfaq-workers, a group of volunteers, maintain the perlfaq. They
are not necessarily experts in every domain where Perl might show up,
so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
Working code is greatly appreciated.

If you'd like to help maintain the perlfaq, see the details in
perlfaq.pod.
 | 
Pages: 1
Prev: FAQ 4.41 How can I remove duplicate elements from a list or array?
Next: running jar file with multiple arguments in perl