[Python-Dev] Fork on Win32 - was (test_fork1 failing...)

Trent Mick trentm@ActiveState.com
Thu, 27 Jul 2000 16:36:01 -0700

On Fri, Jul 28, 2000 at 09:21:01AM +1000, Neil Hodgson wrote:
>    IIRC ActiveState contributed to Perl a version of fork that works on
> Win32. Has anyone looked at this? Could it be grabbed for Python? This would
> help heal one of the more difficult platform rifts. Emulating fork for Win32
> looks quite difficult to me but if its already done...

I just asked Sarathy about this and he direct me to 'perldoc perlfork' (If
you have ActivePerl 5.6 installed then you can look it up.) I have attached
it here. The emulation of fork() in Win32 is not a perfect solution (e.g.
sockets are not dupped, etc.). 


------------------- snip ---------------------------
    perlfork - Perl's fork() emulation

    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 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.

    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

    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

    %ENV    Each pseudo-process maintains its own virtual enviroment.
            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

    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-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 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 outstanding child
            pseudo-processes. Note that this means that the process as a
            whole will not exit unless all running pseudo-processes have

    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.

    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.


    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:


            rather than the expected:


            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

    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;

                if (pipe_to_fork('FOO')) {
                    # parent
                    print FOO "pipe_to_fork\n";
                    close FOO;
                else {
                    # child
                    while (<STDIN>) { print; }
                    close STDIN;

            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;

                if (pipe_from_fork('BAR')) {
                    # parent
                    while (<BAR>) { print; }
                    close BAR;
                else {
                    # child
                    print "pipe_from_fork\n";
                    close STDOUT;

            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

    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

    *       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.

    *       This document may be incomplete in some respects.

    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

    the section on "fork" in the perlfunc manpage, the perlipc manpage

Trent Mick