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NAME    [Toc]    [Back]

       perlfork - Perl's fork() emulation

SYNOPSIS    [Toc]    [Back]

           NOTE:  As of the 5.8.0 release, fork()  emulation  has
           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    [Toc]    [Back]

       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    [Toc]    [Back]

       Most Perl features behave in a natural way within

       $$ 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 pseudoprocess
 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().

       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.

       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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

       Lifetime of the parent process and pseudo-processes    [Toc]    [Back]

       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

       CAVEATS AND LIMITATIONS    [Toc]    [Back]

       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 "inner0;
                   print "outer0;

               This will print:


               rather than the expected:


               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

       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:  $!"  unless defined
                       if ($pid) {
                           close $child;
                       else {
                           close $parent;
                           open(STDIN, "<&=" . fileno($child)) or
                   if (pipe_to_fork('FOO')) {
                       # parent
                       print FOO "pipe_to_fork0;
                       close FOO;
                   else {
                       # child
                       while (<STDIN>) { print; }

               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:  $!"  unless defined
                       if ($pid) {
                           close $child;
                       else {
                           close $parent;
                           open(STDOUT, ">&="  .  fileno($child))
or die;

                   if (pipe_from_fork('BAR')) {
                       # parent
                       while (<BAR>) { print; }
                       close BAR;
                   else {
                       # child
                       print "pipe_from_fork0;

               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
       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    [Toc]    [Back]

       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    [Toc]    [Back]

       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>.

SEE ALSO    [Toc]    [Back]

       "fork" in perlfunc, perlipc

perl v5.8.5                 2002-11-06                          6
[ Back ]
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