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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-pro-
               cesses 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-pro-
               cesses) 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 cur-
               rent 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 cor-
               rectly 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() 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-pro-
               cess() 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 outstand-
               ing 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

       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 uti-
       lization 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.

       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 future.

       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 inde-
               pendently 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.

       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 noth-
               ing 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
       ?       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 implemen-
               tation 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 <gsarATactivestate.com>.

SEE ALSO
       "fork" in perlfunc, perlipc



perl v5.8.8                       2006-01-07                       PERLFORK(1)