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PROGRAM:

NAME

perlfork - Perl's fork() emulation

SYNOPSIS

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.

DESCRIPTION

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.

Behavior of other Perl features in forked pseudo-processes
Most Perl features behave in a natural way within pseudo-processes.

$$ or $PROCESS_ID
This special variable is correctly set to the pseudo-process ID. It can be used
to identify pseudo-processes within a particular session. Note that this value is
subject to recycling if any pseudo-processes are launched after others have been
wait()-ed on.

%ENV Each pseudo-process maintains its own virtual environment. Modifications to %ENV
affect the virtual environment, and are only visible within that pseudo-process,
and in any processes (or pseudo-processes) launched from it.

chdir() and all other builtins that accept filenames
Each pseudo-process maintains its own virtual idea of the current directory.
Modifications to the current directory using chdir() are only visible within that
pseudo-process, and in any processes (or pseudo-processes) launched from it. All
file and directory accesses from the pseudo-process will correctly map the virtual
working directory to the real working directory appropriately.

wait() and waitpid()
wait() and waitpid() can be passed a pseudo-process ID returned by fork(). These
calls will properly wait for the termination of the pseudo-process and return its
status.

kill() "kill('KILL', ...)" can be used to terminate a pseudo-process by passing it the ID
returned by fork(). The outcome of kill on a pseudo-process is unpredictable and
it should not be used except under dire circumstances, because the operating
system may not guarantee integrity of the process resources when a running thread
is terminated. The process which implements the pseudo-processes can be blocked
and the Perl interpreter hangs. Note that using "kill('KILL', ...)" on a
pseudo-process() may typically cause memory leaks, because the thread that
implements the pseudo-process does not get a chance to clean up its resources.

"kill('TERM', ...)" can also be used on pseudo-processes, but the signal will not
be delivered while the pseudo-process is blocked by a system call, e.g. waiting
for a socket to connect, or trying to read from a socket with no data available.
Starting in Perl 5.14 the parent process will not wait for children to exit once
they have been signalled with "kill('TERM', ...)" to avoid deadlock during process
exit. You will have to explicitly call waitpid() to make sure the child has time
to clean-up itself, but you are then also responsible that the child is not
blocking on I/O either.

exec() Calling exec() within a pseudo-process actually spawns the requested executable in
a separate process and waits for it to complete before exiting with the same exit
status as that process. This means that the process ID reported within the
running executable will be different from what the earlier Perl fork() might have
returned. Similarly, any process manipulation functions applied to the ID
returned by fork() will affect the waiting pseudo-process that called exec(), not
the real process it is waiting for after the exec().

When exec() is called inside a pseudo-process then DESTROY methods and END blocks
will still be called after the external process returns.

exit() exit() always exits just the executing pseudo-process, after automatically
wait()-ing for any outstanding child pseudo-processes. Note that this means that
the process as a whole will not exit unless all running pseudo-processes have
exited. See below for some limitations with open filehandles.

Open handles to files, directories and network sockets
All open handles are dup()-ed in pseudo-processes, so that closing any handles in
one process does not affect the others. See below for some limitations.

Resource limits
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.

Killing the parent process
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.

Lifetime of the parent process and pseudo-processes
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.

Starting with Perl 5.14 a parent will not wait() automatically for any child that has been
signalled with "kill('TERM', ...)" to avoid a deadlock in case the child is blocking on
I/O and never receives the signal.

CAVEATS AND LIMITATIONS

BEGIN blocks
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.

Open filehandles
Any filehandles open at the time of the fork() will be dup()-ed. Thus, the files
can be closed independently in the parent and child, but beware that the dup()-ed
handles will still share the same seek pointer. Changing the seek position in the
parent will change it in the child and vice-versa. One can avoid this by opening
files that need distinct seek pointers separately in the child.

On some operating systems, notably Solaris and Unixware, calling "exit()" from a
child process will flush and close open filehandles in the parent, thereby
corrupting the filehandles. On these systems, calling "_exit()" is suggested
instead. "_exit()" is available in Perl through the "POSIX" module. Please
consult your system's manpages for more information on this.

Open directory handles
Perl will completely read from all open directory handles until they reach the end
of the stream. It will then seekdir() back to the original location and all
future readdir() requests will be fulfilled from the cache buffer. That means
that neither the directory handle held by the parent process nor the one held by
the child process will see any changes made to the directory after the fork()
call.

Note that rewinddir() has a similar limitation on Windows and will not force
readdir() to read the directory again either. Only a newly opened directory
handle will reflect changes to the directory.

Forking pipe open() not yet implemented
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; }
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";
exit(0);
}

Forking pipe open() constructs will be supported in future.

Global state maintained by XSUBs
External subroutines (XSUBs) that maintain their own global state may not work
correctly. Such XSUBs will either need to maintain locks to protect simultaneous
access to global data from different pseudo-processes, or maintain all their state
on the Perl symbol table, which is copied naturally when fork() is called. A
callback mechanism that provides extensions an opportunity to clone their state
will be provided in the near future.

Interpreter embedded in larger application
The fork() emulation may not behave as expected when it is executed in an
application which embeds a Perl interpreter and calls Perl APIs that can evaluate
bits of Perl code. This stems from the fact that the emulation only has knowledge
about the Perl interpreter's own data structures and knows nothing about the
containing application's state. For example, any state carried on the
application's own call stack is out of reach.

Thread-safety of extensions
Since the fork() emulation runs code in multiple threads, extensions calling into
non-thread-safe libraries may not work reliably when calling fork(). As Perl's
threading support gradually becomes more widely adopted even on platforms with a
native fork(), such extensions are expected to be fixed for thread-safety.

PORTABILITY CAVEATS

In portable Perl code, "kill(9, $child)" must not be used on forked processes. Killing a
forked process is unsafe and has unpredictable results. See "kill()", above.

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