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

   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


   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:


           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 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

   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;

               if (pipe_to_fork('FOO')) {
                   # parent
                   print FOO "pipe_to_fork\n";
                   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 $pid;
                   if ($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_fork\n";

           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

   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


   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.


   *       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.

   *       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.

   *       This document may be incomplete in some respects.


   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


   "fork" in perlfunc, perlipc


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