perlfork - Perl's fork() emulation
NOTE: As of the 5.8.0 release, fork() emulation has considerably
matured. However, there are still a few known bugs and differences
from real fork() that might affect you. See the "BUGS" and
"CAVEATS AND LIMITATIONS" sections below.
Perl provides a fork() keyword 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. While the emulation is designed to be as compatible as possible with the real fork() at the level of the Perl program, there are certain important differences that stem from the fact that all the pseudo child "processes" created this way live in the same real process as far as the operating system is concerned.
This document provides a general overview of the capabilities and limitations of the fork() emulation. Note that the issues discussed here are not applicable to platforms where a real fork() is available and Perl has been configured to use it.
The fork() emulation is implemented at the level of the Perl interpreter. What this means in general is that running fork() will actually clone the running interpreter and all its state, and run the cloned interpreter in a separate thread, beginning execution in the new thread just after the point where the fork() was called in the parent. We will refer to the thread that implements this child "process" as the pseudo-process.
To the Perl program that called fork(), all this is designed to be
transparent. The parent returns from the fork() with a pseudo-process
ID that can be subsequently used in any process manipulation functions;
the child returns from the fork() with a value of 0 to signify that
it is the child pseudo-process.
Most Perl features behave in a natural way within pseudo-processes.
In the eyes of the operating system, pseudo-processes created via the fork() emulation are simply threads in the same process. This means that any process-level limits imposed by the operating system apply to all pseudo-processes taken together. This includes any limits imposed by the operating system on the number of open file, directory and socket handles, limits on disk space usage, limits on memory size, limits on CPU utilization etc.
If the parent process is killed (either using Perl's kill() builtin, or using some external means) all the pseudo-processes are killed as well, and the whole process exits.
During the normal course of events, the parent process and every pseudo-process started by it will wait for their respective pseudo-children to complete before they exit. This means that the parent and every pseudo-child created by it that is also a pseudo-parent will only exit after their pseudo-children have exited.
A way to mark a pseudo-processes as running detached from their parent (so that the parent would not have to wait() for them if it doesn't want to) will be provided in future.
The fork() emulation will not work entirely correctly when called from within a BEGIN block. The forked copy will run the contents of the BEGIN block, but will not continue parsing the source stream after the BEGIN block. For example, consider the following code:
BEGIN {
fork and exit; # fork child and exit the parent
print "inner\n";
}
print "outer\n";
This will print:
inner
rather than the expected:
inner
outer
This limitation arises from fundamental technical difficulties in cloning and restarting the stacks used by the Perl parser in the middle of a parse.
The open(FOO, "|-") and open(BAR, "-|") constructs are not yet
implemented. This limitation can be easily worked around in new code
by creating a pipe explicitly. The following example shows how to
write to a forked child:
# simulate open(FOO, "|-")
sub pipe_to_fork ($) {
my $parent = shift;
pipe my $child, $parent or die;
my $pid = fork();
die "fork() failed: $!" unless defined $pid;
if ($pid) {
close $child;
}
else {
close $parent;
open(STDIN, "<&=" . fileno($child)) or die;
}
$pid;
}
if (pipe_to_fork('FOO')) {
# parent
print FOO "pipe_to_fork\n";
close FOO;
}
else {
# child
while (<STDIN>) { print; }
close STDIN;
exit(0);
}
And this one reads from the child:
# simulate open(FOO, "-|")
sub pipe_from_fork ($) {
my $parent = shift;
pipe $parent, my $child or die;
my $pid = fork();
die "fork() failed: $!" unless defined $pid;
if ($pid) {
close $child;
}
else {
close $parent;
open(STDOUT, ">&=" . fileno($child)) or die;
}
$pid;
}
if (pipe_from_fork('BAR')) {
# parent
while (<BAR>) { print; }
close BAR;
}
else {
# child
print "pipe_from_fork\n";
close STDOUT;
exit(0);
}
Forking pipe open() constructs will be supported in future.
-1 because the wait() and waitpid() functions treat this number as
being special. The tacit assumption in the current implementation is that
the system never allocates a thread ID of 1 for user threads. A better
representation for pseudo-process IDs will be implemented in future.
Support for concurrent interpreters and the fork() emulation was implemented by ActiveState, with funding from Microsoft Corporation.
This document is authored and maintained by Gurusamy Sarathy <gsar@activestate.com>.