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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 capabili­
       ties 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 config­
       ured to use it.


DESCRIPTION

       The fork() emulation is implemented at the level of the
       Perl interpreter.  What this means in general is that run­
       ning 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.
               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-pro­
               cess 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 circum­
               stances, 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-process() 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 pro­
               cess 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 run­
               ning executable will be different from what the
               earlier Perl fork() might have returned.  Simi­
               larly, any process manipulation functions applied
               to the ID returned by fork() will affect the wait­
               ing pseudo-process that called exec(), not the
               real process it is waiting for after the exec().

       exit()  exit() always exits just the executing pseudo-pro­
               cess, after automatically wait()-ing for any out­
               standing 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 limita­
               tions.

       Resource limits

       In the eyes of the operating system, pseudo-processes cre­
       ated via the fork() emulation are simply threads in the
       same process.  This means that any process-level limits

       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 cor­
               rectly 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, con­
               sider 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
                       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; }
                       close STDIN;
                       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";
                       close STDOUT;

               anism 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

       ·       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 situ­
               ations, 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() emula­
       tion was implemented by ActiveState, with funding from
       Microsoft Corporation.

       This document is authored and maintained by Gurusamy
       Sarathy <gsar@activestate.com>.
  




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