perlfork - Perl's fork() emulation
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.
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 [Toc] [Back]
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 pseudoprocess
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 [Toc] [Back]
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 "inner0;
}
print "outer0;
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;
}
$pid;
}
if (pipe_to_fork('FOO')) {
# parent
print FOO "pipe_to_fork0;
close FOO;
}
else {
# child
while (<STDIN>) { print; }
exit(0);
}
And this one reads from the child:
# simulate open(FOO, "-|")
sub pipe_from_fork ($) {
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_fork0;
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.
o 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.
o 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.
o This document may be incomplete in some respects.
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 <[email protected]>.
"fork" in perlfunc, perlipc
perl v5.8.5 2002-11-06 6 [ Back ] |
