Does a fork(2) system call to 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 undef if the fork is unsuccessful. File descriptors (and
sometimes locks on those descriptors) are shared, while everything else is copied. On most
systems supporting fork(), great care has gone into making it extremely efficient (for
example, using copy-on-write technology on data pages), making it the dominant paradigm
for multitasking over the last few decades.
Beginning with v5.6.0, Perl will attempt to flush all files opened for output before
forking the child process, but this may not be supported on some platforms (see perlport). To be safe, you
may need to set $| ($AUTOFLUSH in English) or call the autoflush()
method of IO::Handle on any open handles in order to avoid duplicate output.
If you fork without ever waiting on your children, you will accumulate
zombies. On some systems, you can avoid this by setting $SIG{CHLD} to "IGNORE".
See also perlipc for more
examples of forking and reaping moribund children.
Note that if your forked child inherits system file descriptors like STDIN and STDOUT
that are actually connected by a pipe or socket, even if you exit, then the remote server
(such as, say, a CGI script or a backgrounded job launched from a remote shell) won't
think you're done. You should reopen those to /dev/null if it's any issue.
