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perlfork.pod

=head1 NAME

perlfork - Perl's fork() emulation

=head1 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.

=head1 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 C<0> to signify that
it is the child pseudo-process.

=head2 Behavior of other Perl features in forked pseudo-processes

Most Perl features behave in a natural way within pseudo-processes.

=over 8

=item $$ 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.

=item %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.

=item 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.

=item 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.

=item kill()

C<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 C<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.

C<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 C<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.

=item 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.

=item 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.

=item 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.

=back

=head2 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.

=head2 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.

=head2 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 C<kill('TERM', ...)>
to avoid a deadlock in case the child is blocking on I/O and
never receives the signal.

=head1 CAVEATS AND LIMITATIONS

=over 8

=item 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.

=item 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 C<exit()>
from a child process will flush and close open filehandles in the parent,
thereby corrupting the filehandles.  On these systems, calling C<_exit()>
is suggested instead.  C<_exit()> is available in Perl through the
C<POSIX> module.  Please consult your system's manpages for more information
on this.

=item 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.

=item Forking pipe open() not yet implemented

The C<open(FOO, "|-")> and C<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.

=item 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.

=item 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.

=item 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.

=back

=head1 PORTABILITY CAVEATS

In portable Perl code, C<kill(9, $child)> must not be used on forked processes.
Killing a forked process is unsafe and has unpredictable results.
See L</kill()>, above.

=head1 BUGS

=over 8

=item *

Having pseudo-process IDs be negative integers breaks down for the integer
C<-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 C<1> for user threads.  A better
representation for pseudo-process IDs will be implemented in future.

=item *

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.

=item *

This document may be incomplete in some respects.

=back

=head1 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
E<lt>gsar@activestate.comE<gt>.

=head1 SEE ALSO

L<perlfunc/"fork">, L<perlipc>

=cut

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