PERLFORK(1) Perl Programmers Reference Guide PERLFORK(1)
NAME
perlfork - Perl's fork() emulation
SYNOPSIS
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.
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('KILL', ...)" can be used to terminate a pseudo-process by
passing it the ID returned by fork(). 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-process() 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 waitpid() 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.
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().
When exec() is called inside a pseudo-process then DESTROY
methods and END blocks will still be called after the external
process returns.
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.
See below for some limitations with open filehandles.
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.
Starting with Perl 5.14 a parent will not wait() 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.
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.
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 seekdir() back
to the original location and all future readdir() 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 fork() call.
Note that rewinddir() has a similar limitation on Windows and
will not force readdir() to read the directory again either.
Only a newly opened directory handle will reflect changes to the
directory.
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;
}
$pid;
}
if (pipe_to_fork('FOO')) {
# parent
print FOO "pipe_to_fork\n";
close FOO;
}
else {
# child
while (<STDIN>) { print; }
exit(0);
}
And this one reads from the child:
# simulate open(FOO, "-|")
sub pipe_from_fork ($) {
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 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.
PORTABILITY CAVEATS
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 "kill()", above.
BUGS
• 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.
• 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.
• 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
"fork" in perlfunc, perlipc
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