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302 lines
10 KiB
Plaintext
=head1 NAME
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perlfork - Perl's fork() emulation
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=head1 SYNOPSIS
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Perl provides a fork() keyword that corresponds to the Unix system call
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of the same name. On most Unix-like platforms where the fork() system
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call is available, Perl's fork() simply calls it.
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On some platforms such as Windows where the fork() system call is not
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available, Perl can be built to emulate fork() at the interpreter level.
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While the emulation is designed to be as compatible as possible with the
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real fork() at the the level of the Perl program, there are certain
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important differences that stem from the fact that all the pseudo child
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"processes" created this way live in the same real process as far as the
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operating system is concerned.
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This document provides a general overview of the capabilities and
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limitations of the fork() emulation. Note that the issues discussed here
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are not applicable to platforms where a real fork() is available and Perl
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has been configured to use it.
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=head1 DESCRIPTION
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The fork() emulation is implemented at the level of the Perl interpreter.
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What this means in general is that running fork() will actually clone the
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running interpreter and all its state, and run the cloned interpreter in
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a separate thread, beginning execution in the new thread just after the
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point where the fork() was called in the parent. We will refer to the
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thread that implements this child "process" as the pseudo-process.
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To the Perl program that called fork(), all this is designed to be
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transparent. The parent returns from the fork() with a pseudo-process
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ID that can be subsequently used in any process manipulation functions;
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the child returns from the fork() with a value of C<0> to signify that
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it is the child pseudo-process.
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=head2 Behavior of other Perl features in forked pseudo-processes
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Most Perl features behave in a natural way within pseudo-processes.
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=over 8
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=item $$ or $PROCESS_ID
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This special variable is correctly set to the pseudo-process ID.
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It can be used to identify pseudo-processes within a particular
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session. Note that this value is subject to recycling if any
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pseudo-processes are launched after others have been wait()-ed on.
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=item %ENV
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Each pseudo-process maintains its own virtual enviroment. Modifications
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to %ENV affect the virtual environment, and are only visible within that
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pseudo-process, and in any processes (or pseudo-processes) launched from
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it.
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=item chdir() and all other builtins that accept filenames
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Each pseudo-process maintains its own virtual idea of the current directory.
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Modifications to the current directory using chdir() are only visible within
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that pseudo-process, and in any processes (or pseudo-processes) launched from
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it. All file and directory accesses from the pseudo-process will correctly
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map the virtual working directory to the real working directory appropriately.
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=item wait() and waitpid()
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wait() and waitpid() can be passed a pseudo-process ID returned by fork().
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These calls will properly wait for the termination of the pseudo-process
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and return its status.
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=item kill()
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kill() can be used to terminate a pseudo-process by passing it the ID returned
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by fork(). This should not be used except under dire circumstances, because
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the operating system may not guarantee integrity of the process resources
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when a running thread is terminated. Note that using kill() on a
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pseudo-process() may typically cause memory leaks, because the thread that
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implements the pseudo-process does not get a chance to clean up its resources.
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=item exec()
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Calling exec() within a pseudo-process actually spawns the requested
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executable in a separate process and waits for it to complete before
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exiting with the same exit status as that process. This means that the
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process ID reported within the running executable will be different from
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what the earlier Perl fork() might have returned. Similarly, any process
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manipulation functions applied to the ID returned by fork() will affect the
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waiting pseudo-process that called exec(), not the real process it is
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waiting for after the exec().
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=item exit()
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exit() always exits just the executing pseudo-process, after automatically
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wait()-ing for any outstanding child pseudo-processes. Note that this means
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that the process as a whole will not exit unless all running pseudo-processes
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have exited.
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=item Open handles to files, directories and network sockets
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All open handles are dup()-ed in pseudo-processes, so that closing
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any handles in one process does not affect the others. See below for
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some limitations.
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=back
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=head2 Resource limits
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In the eyes of the operating system, pseudo-processes created via the fork()
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emulation are simply threads in the same process. This means that any
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process-level limits imposed by the operating system apply to all
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pseudo-processes taken together. This includes any limits imposed by the
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operating system on the number of open file, directory and socket handles,
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limits on disk space usage, limits on memory size, limits on CPU utilization
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etc.
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=head2 Killing the parent process
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If the parent process is killed (either using Perl's kill() builtin, or
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using some external means) all the pseudo-processes are killed as well,
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and the whole process exits.
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=head2 Lifetime of the parent process and pseudo-processes
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During the normal course of events, the parent process and every
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pseudo-process started by it will wait for their respective pseudo-children
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to complete before they exit. This means that the parent and every
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pseudo-child created by it that is also a pseudo-parent will only exit
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after their pseudo-children have exited.
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A way to mark a pseudo-processes as running detached from their parent (so
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that the parent would not have to wait() for them if it doesn't want to)
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will be provided in future.
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=head2 CAVEATS AND LIMITATIONS
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=over 8
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=item BEGIN blocks
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The fork() emulation will not work entirely correctly when called from
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within a BEGIN block. The forked copy will run the contents of the
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BEGIN block, but will not continue parsing the source stream after the
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BEGIN block. For example, consider the following code:
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BEGIN {
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fork and exit; # fork child and exit the parent
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print "inner\n";
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}
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print "outer\n";
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This will print:
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inner
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rather than the expected:
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inner
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outer
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This limitation arises from fundamental technical difficulties in
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cloning and restarting the stacks used by the Perl parser in the
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middle of a parse.
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=item Open filehandles
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Any filehandles open at the time of the fork() will be dup()-ed. Thus,
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the files can be closed independently in the parent and child, but beware
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that the dup()-ed handles will still share the same seek pointer. Changing
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the seek position in the parent will change it in the child and vice-versa.
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One can avoid this by opening files that need distinct seek pointers
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separately in the child.
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=item Forking pipe open() not yet implemented
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The C<open(FOO, "|-")> and C<open(BAR, "-|")> constructs are not yet
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implemented. This limitation can be easily worked around in new code
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by creating a pipe explicitly. The following example shows how to
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write to a forked child:
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# simulate open(FOO, "|-")
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sub pipe_to_fork ($) {
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my $parent = shift;
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pipe my $child, $parent or die;
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my $pid = fork();
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die "fork() failed: $!" unless defined $pid;
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if ($pid) {
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close $child;
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}
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else {
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close $parent;
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open(STDIN, "<&=" . fileno($child)) or die;
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}
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$pid;
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}
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if (pipe_to_fork('FOO')) {
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# parent
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print FOO "pipe_to_fork\n";
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close FOO;
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}
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else {
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# child
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while (<STDIN>) { print; }
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close STDIN;
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exit(0);
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}
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And this one reads from the child:
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# simulate open(FOO, "-|")
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sub pipe_from_fork ($) {
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my $parent = shift;
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pipe $parent, my $child or die;
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my $pid = fork();
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die "fork() failed: $!" unless defined $pid;
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if ($pid) {
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close $child;
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}
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else {
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close $parent;
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open(STDOUT, ">&=" . fileno($child)) or die;
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}
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$pid;
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}
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if (pipe_from_fork('BAR')) {
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# parent
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while (<BAR>) { print; }
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close BAR;
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}
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else {
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# child
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print "pipe_from_fork\n";
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close STDOUT;
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exit(0);
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}
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Forking pipe open() constructs will be supported in future.
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=item Global state maintained by XSUBs
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External subroutines (XSUBs) that maintain their own global state may
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not work correctly. Such XSUBs will either need to maintain locks to
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protect simultaneous access to global data from different pseudo-processes,
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or maintain all their state on the Perl symbol table, which is copied
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naturally when fork() is called. A callback mechanism that provides
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extensions an opportunity to clone their state will be provided in the
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near future.
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=item Interpreter embedded in larger application
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The fork() emulation may not behave as expected when it is executed in an
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application which embeds a Perl interpreter and calls Perl APIs that can
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evaluate bits of Perl code. This stems from the fact that the emulation
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only has knowledge about the Perl interpreter's own data structures and
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knows nothing about the containing application's state. For example, any
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state carried on the application's own call stack is out of reach.
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=item Thread-safety of extensions
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Since the fork() emulation runs code in multiple threads, extensions
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calling into non-thread-safe libraries may not work reliably when
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calling fork(). As Perl's threading support gradually becomes more
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widely adopted even on platforms with a native fork(), such extensions
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are expected to be fixed for thread-safety.
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=back
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=head1 BUGS
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=over 8
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=item *
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Having pseudo-process IDs be negative integers breaks down for the integer
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C<-1> because the wait() and waitpid() functions treat this number as
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being special. The tacit assumption in the current implementation is that
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the system never allocates a thread ID of C<1> for user threads. A better
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representation for pseudo-process IDs will be implemented in future.
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=item *
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This document may be incomplete in some respects.
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=back
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=head1 AUTHOR
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Support for concurrent interpreters and the fork() emulation was implemented
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by ActiveState, with funding from Microsoft Corporation.
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This document is authored and maintained by Gurusamy Sarathy
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E<lt>gsar@activestate.comE<gt>.
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=head1 SEE ALSO
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L<perlfunc/"fork">, L<perlipc>
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=cut
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