| PerlFAQ Server 2005-06-11, 3:57 pm |
| This message is one of several periodic postings to comp.lang.perl.misc
intended to make it easier for perl programmers to find answers to
common questions. The core of this message represents an excerpt
from the documentation provided with Perl.
--------------------------------------------------------------------
8.12: How do I start a process in the background?
Several modules can start other processes that do not block your Perl
program. You can use IPC::Open3, Parallel::Jobs, IPC::Run, and some of
the POE modules. See CPAN for more details.
You could also use
system("cmd &")
or you could use fork as documented in "fork" in perlfunc, with further
examples in perlipc. Some things to be aware of, if you're on a
Unix-like system:
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 &")".
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