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