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)