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PERLFORK(1) Perl Programmers Reference Guide PERLFORK(1)
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() can be used to terminate a pseudo-process by passing it the ID returned by fork(). This 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. Note that using 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.
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.
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.
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 systems
manpages for more information on this.
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.
BUGS
o Having pseudo-process IDs be negative integers breaks down for the integer "-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.
o In certain cases, the OS-level handles created by the pipe(), socket(), and accept() operators are apparently not
duplicated accurately in pseudo-processes. This only happens in some situations, but where it does happen, it
may result in deadlocks between the read and write ends of pipe handles, or inability to send or receive data
across socket handles.
o This document may be incomplete in some respects.
AUTHOR
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 <gsarATactivestate.com>.
SEE ALSO
"fork" in perlfunc, perlipc
perl v5.12.4 2011-06-07 PERLFORK(1)
