PERLFORK(1) Perl Programmers Reference Guide PERLFORK(1)
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 capabili-
ties 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
Most Perl features behave in a natural way within
pseudo-processes.
$$ or $PROCESS_ID
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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 vir-
tual 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 ter-
minated. 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 pro-
cess 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 run-
ning executable will be different from what the ear-
lier 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().
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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, direc-
tory 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:
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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 indepen-
dently 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, "-|")" con-
structs 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;
}
$pid;
}
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if (pipe_to_fork('FOO')) {
# parent
print FOO "pipe_to_fork\n";
close FOO;
}
else {
# child
while (<STDIN>) { print; }
exit(0);
}
And this one reads from the child:
# simulate open(FOO, "-|")
sub pipe_from_fork ($) {
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 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 simul-
taneous 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 pro-
vides extensions an opportunity to clone their state
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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 exam-
ple, 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.
+ 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 imple-
mentation is that the system never allocates a
thread ID of 1 for user threads. A better represen-
tation for pseudo-process IDs will be implemented in
future.
+ 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 situa-
tions, 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.
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>.
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"fork" in perlfunc, perlipc
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