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PERLFORK(1)
NAME
perlfork - Perl's fork() emulation (EXPERIMENTAL, subject to change)
SYNOPSIS
WARNING: As of the 5.6.1 release, the fork() emulation continues
to be an experimental feature. Use in production applications is
not recommended. 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.
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 "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 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.
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.
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 exited.
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.
CAVEATS AND LIMITATIONS
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.
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.
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;
}
$pid;
}
if (pipe_to_fork('FOO')) {
# parent
print FOO "pipe_to_fork\n";
close FOO;
}
else {
# child
while (<STDIN>) { print; }
close STDIN;
exit(0);
}
And this one reads from the child:
# simulate open(FOO, "-|")
sub pipe_from_fork ($) {
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";
close STDOUT;
exit(0);
}
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 future.
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
thread-safety.
BUGS
· Perl's regular expression engine currently does not play very
nicely with the fork() emulation. There are known race conditions
arising from the regular expression engine modifying state carried
in the opcode tree at run time (the fork() emulation relies on the
opcode tree being immutable). This typically happens when the
regex contains paren groups or variables interpolated within it
that force a run time recompilation of the regex. Due to this
major bug, the fork() emulation is not recommended for use in
production applications at this time.
· 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.
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
<gsar@activestate.com>.
SEE ALSO
the fork entry in the perlfunc manpage, the perlipc manpage
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