perlfork


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.

               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().  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('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,
       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 "sig('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
               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;

               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
       <gsar@activestate.com>.

SEE ALSO
       "fork" in perlfunc, perlipc
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