PERLFORK(1) Perl Programmers Reference Guide PERLFORK(1)

PERLFORK(1) Perl Programmers Reference Guide PERLFORK(1) #

PERLFORK(1) Perl Programmers Reference Guide PERLFORK(1)

NNAAMMEE #

 perlfork - Perl's fork() emulation

SSYYNNOOPPSSIISS #

     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 ffoorrkk(()) keyword that corresponds to the Unix system call
 of the same name.  On most Unix-like platforms where the ffoorrkk(()) system
 call is available, Perl's ffoorrkk(()) simply calls it.

 On some platforms such as Windows where the ffoorrkk(()) system call is not
 available, Perl can be built to emulate ffoorrkk(()) at the interpreter level.
 While the emulation is designed to be as compatible as possible with the
 real ffoorrkk(()) 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 ffoorrkk(()) emulation.  Note that the issues discussed here
 are not applicable to platforms where a real ffoorrkk(()) is available and Perl
 has been configured to use it.

DDEESSCCRRIIPPTTIIOONN #

 The ffoorrkk(()) emulation is implemented at the level of the Perl interpreter.
 What this means in general is that running ffoorrkk(()) 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 ffoorrkk(()) 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 ffoorrkk(()), all this is designed to be
 transparent.  The parent returns from the ffoorrkk(()) with a pseudo-process ID
 that can be subsequently used in any process-manipulation functions; the
 child returns from the ffoorrkk(()) with a value of 0 to signify that it is the
 child pseudo-process.

BBeehhaavviioorr ooff ootthheerr PPeerrll ffeeaattuurreess iinn ffoorrkkeedd ppsseeuuddoo--pprroocceesssseess 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 wwaaiitt(())-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-processes) launched from it.

 cchhddiirr(()) 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 cchhddiirr(())
         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.

 wwaaiitt(()) and wwaaiittppiidd(())
         wwaaiitt(()) and wwaaiittppiidd(()) can be passed a pseudo-process ID returned
         by ffoorrkk(()).  These calls will properly wait for the termination of
         the pseudo-process and return its status.

 kkiillll(())  "kill('KILL', ...)" can be used to terminate a pseudo-process by
         passing it the ID returned by ffoorrkk(()). 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 "kill('KILL', ...)" on a pseudo-pprroocceessss(()) may
         typically cause memory leaks, because the thread that implements
         the pseudo-process does not get a chance to clean up its
         resources.

         "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 "kill('TERM', ...)" to avoid
         deadlock during process exit.  You will have to explicitly call
         wwaaiittppiidd(()) 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.

 eexxeecc(())  Calling eexxeecc(()) 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
         ffoorrkk(()) might have returned.  Similarly, any process manipulation
         functions applied to the ID returned by ffoorrkk(()) will affect the
         waiting pseudo-process that called eexxeecc(()), not the real process
         it is waiting for after the eexxeecc(()).

         When eexxeecc(()) is called inside a pseudo-process then DESTROY
         methods and END blocks will still be called after the external
         process returns.

 eexxiitt(())  eexxiitt(()) always exits just the executing pseudo-process, after
         automatically wwaaiitt(())-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.

 Open handles to files, directories and network sockets
         All open handles are dduupp(())-ed in pseudo-processes, so that
         closing any handles in one process does not affect the others.
         See below for some limitations.

RReessoouurrccee lliimmiittss In the eyes of the operating system, pseudo-processes created via the ffoorrkk(()) 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.

KKiilllliinngg tthhee ppaarreenntt pprroocceessss If the parent process is killed (either using Perl’s kkiillll(()) builtin, or using some external means) all the pseudo-processes are killed as well, and the whole process exits.

LLiiffeettiimmee ooff tthhee ppaarreenntt pprroocceessss aanndd ppsseeuuddoo--pprroocceesssseess 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 wwaaiitt(()) automatically for any
 child that has been signalled with "kill('TERM', ...)" to avoid a
 deadlock in case the child is blocking on I/O and never receives the
 signal.

CCAAVVEEAATTSS AANNDD LLIIMMIITTAATTIIOONNSS #

 BEGIN blocks
         The ffoorrkk(()) 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 ffoorrkk(()) will be dduupp(())-ed.
         Thus, the files can be closed independently in the parent and
         child, but beware that the dduupp(())-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
         "exit()" from a child process will flush and close open
         filehandles in the parent, thereby corrupting the filehandles.
         On these systems, calling "_exit()" is suggested instead.
         "_exit()" is available in Perl through the "POSIX" module.
         Please consult your system's manpages for more information on
         this.

 Open directory handles
         Perl will completely read from all open directory handles until
         they reach the end of the stream.  It will then sseeeekkddiirr(()) back to
         the original location and all future rreeaaddddiirr(()) 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 ffoorrkk(()) call.

         Note that rreewwiinnddddiirr(()) has a similar limitation on Windows and
         will not force rreeaaddddiirr(()) to read the directory again either.
         Only a newly opened directory handle will reflect changes to the
         directory.

 Forking pipe ooppeenn(()) 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 ooppeenn(()) 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 ffoorrkk(()) 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 ffoorrkk(()) 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 ffoorrkk(()) emulation runs code in multiple threads,
         extensions calling into non-thread-safe libraries may not work
         reliably when calling ffoorrkk(()).  As Perl's threading support
         gradually becomes more widely adopted even on platforms with a
         native ffoorrkk(()), such extensions are expected to be fixed for
         thread-safety.

PPOORRTTAABBIILLIITTYY CCAAVVEEAATTSS #

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

BBUUGGSS #

 •       Having pseudo-process IDs be negative integers breaks down for
         the integer "-1" because the wwaaiitt(()) and wwaaiittppiidd(()) 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 ppiippee(()),
         ssoocckkeett(()), and aacccceepptt(()) 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.

AAUUTTHHOORR #

 Support for concurrent interpreters and the ffoorrkk(()) emulation was
 implemented by ActiveState, with funding from Microsoft Corporation.

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

SSEEEE AALLSSOO #

 "fork" in perlfunc, perlipc

perl v5.36.3 2015-04-25 PERLFORK(1)