phpman > man > perlfork(1)

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 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 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 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 AT activestate.com>.

SEE ALSO
       "fork" in perlfunc, perlipc



perl v5.34.0                                 2025-07-25                                  PERLFORK(1)