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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('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 pseudoprocess
               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 pseudochildren
       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 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 non-threadsafe
               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.

BUGS
       •       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.

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



perl v5.16.2                                     2012-10-11                                      PERLFORK(1)