3 perlfork - Perl's fork() emulation
7 NOTE: As of the 5.8.0 release, fork() emulation has considerably
8 matured. However, there are still a few known bugs and differences
9 from real fork() that might affect you. See the "BUGS" and
10 "CAVEATS AND LIMITATIONS" sections below.
12 Perl provides a fork() keyword that corresponds to the Unix system call
13 of the same name. On most Unix-like platforms where the fork() system
14 call is available, Perl's fork() simply calls it.
16 On some platforms such as Windows where the fork() system call is not
17 available, Perl can be built to emulate fork() at the interpreter level.
18 While the emulation is designed to be as compatible as possible with the
19 real fork() at the level of the Perl program, there are certain
20 important differences that stem from the fact that all the pseudo child
21 "processes" created this way live in the same real process as far as the
22 operating system is concerned.
24 This document provides a general overview of the capabilities and
25 limitations of the fork() emulation. Note that the issues discussed here
26 are not applicable to platforms where a real fork() is available and Perl
27 has been configured to use it.
31 The fork() emulation is implemented at the level of the Perl interpreter.
32 What this means in general is that running fork() will actually clone the
33 running interpreter and all its state, and run the cloned interpreter in
34 a separate thread, beginning execution in the new thread just after the
35 point where the fork() was called in the parent. We will refer to the
36 thread that implements this child "process" as the pseudo-process.
38 To the Perl program that called fork(), all this is designed to be
39 transparent. The parent returns from the fork() with a pseudo-process
40 ID that can be subsequently used in any process-manipulation functions;
41 the child returns from the fork() with a value of C<0> to signify that
42 it is the child pseudo-process.
44 =head2 Behavior of other Perl features in forked pseudo-processes
46 Most Perl features behave in a natural way within pseudo-processes.
50 =item $$ or $PROCESS_ID
52 This special variable is correctly set to the pseudo-process ID.
53 It can be used to identify pseudo-processes within a particular
54 session. Note that this value is subject to recycling if any
55 pseudo-processes are launched after others have been wait()-ed on.
59 Each pseudo-process maintains its own virtual environment. Modifications
60 to %ENV affect the virtual environment, and are only visible within that
61 pseudo-process, and in any processes (or pseudo-processes) launched from
64 =item chdir() and all other builtins that accept filenames
66 Each pseudo-process maintains its own virtual idea of the current directory.
67 Modifications to the current directory using chdir() are only visible within
68 that pseudo-process, and in any processes (or pseudo-processes) launched from
69 it. All file and directory accesses from the pseudo-process will correctly
70 map the virtual working directory to the real working directory appropriately.
72 =item wait() and waitpid()
74 wait() and waitpid() can be passed a pseudo-process ID returned by fork().
75 These calls will properly wait for the termination of the pseudo-process
76 and return its status.
80 C<kill('KILL', ...)> can be used to terminate a pseudo-process by
81 passing it the ID returned by fork(). The outcome of kill on a pseudo-process
82 is unpredictable and it should not be used except
83 under dire circumstances, because the operating system may not
84 guarantee integrity of the process resources when a running thread is
85 terminated. The process which implements the pseudo-processes can be blocked
86 and the Perl interpreter hangs. Note that using C<kill('KILL', ...)> on a
87 pseudo-process() may typically cause memory leaks, because the thread
88 that implements the pseudo-process does not get a chance to clean up
91 C<kill('TERM', ...)> can also be used on pseudo-processes, but the
92 signal will not be delivered while the pseudo-process is blocked by a
93 system call, e.g. waiting for a socket to connect, or trying to read
94 from a socket with no data available. Starting in Perl 5.14 the
95 parent process will not wait for children to exit once they have been
96 signalled with C<kill('TERM', ...)> to avoid deadlock during process
97 exit. You will have to explicitly call waitpid() to make sure the
98 child has time to clean-up itself, but you are then also responsible
99 that the child is not blocking on I/O either.
103 Calling exec() within a pseudo-process actually spawns the requested
104 executable in a separate process and waits for it to complete before
105 exiting with the same exit status as that process. This means that the
106 process ID reported within the running executable will be different from
107 what the earlier Perl fork() might have returned. Similarly, any process
108 manipulation functions applied to the ID returned by fork() will affect the
109 waiting pseudo-process that called exec(), not the real process it is
110 waiting for after the exec().
112 When exec() is called inside a pseudo-process then DESTROY methods and
113 END blocks will still be called after the external process returns.
117 exit() always exits just the executing pseudo-process, after automatically
118 wait()-ing for any outstanding child pseudo-processes. Note that this means
119 that the process as a whole will not exit unless all running pseudo-processes
120 have exited. See below for some limitations with open filehandles.
122 =item Open handles to files, directories and network sockets
124 All open handles are dup()-ed in pseudo-processes, so that closing
125 any handles in one process does not affect the others. See below for
130 =head2 Resource limits
132 In the eyes of the operating system, pseudo-processes created via the fork()
133 emulation are simply threads in the same process. This means that any
134 process-level limits imposed by the operating system apply to all
135 pseudo-processes taken together. This includes any limits imposed by the
136 operating system on the number of open file, directory and socket handles,
137 limits on disk space usage, limits on memory size, limits on CPU utilization
140 =head2 Killing the parent process
142 If the parent process is killed (either using Perl's kill() builtin, or
143 using some external means) all the pseudo-processes are killed as well,
144 and the whole process exits.
146 =head2 Lifetime of the parent process and pseudo-processes
148 During the normal course of events, the parent process and every
149 pseudo-process started by it will wait for their respective pseudo-children
150 to complete before they exit. This means that the parent and every
151 pseudo-child created by it that is also a pseudo-parent will only exit
152 after their pseudo-children have exited.
154 Starting with Perl 5.14 a parent will not wait() automatically
155 for any child that has been signalled with C<kill('TERM', ...)>
156 to avoid a deadlock in case the child is blocking on I/O and
157 never receives the signal.
159 =head1 CAVEATS AND LIMITATIONS
165 The fork() emulation will not work entirely correctly when called from
166 within a BEGIN block. The forked copy will run the contents of the
167 BEGIN block, but will not continue parsing the source stream after the
168 BEGIN block. For example, consider the following code:
171 fork and exit; # fork child and exit the parent
180 rather than the expected:
185 This limitation arises from fundamental technical difficulties in
186 cloning and restarting the stacks used by the Perl parser in the
189 =item Open filehandles
191 Any filehandles open at the time of the fork() will be dup()-ed. Thus,
192 the files can be closed independently in the parent and child, but beware
193 that the dup()-ed handles will still share the same seek pointer. Changing
194 the seek position in the parent will change it in the child and vice-versa.
195 One can avoid this by opening files that need distinct seek pointers
196 separately in the child.
198 On some operating systems, notably Solaris and Unixware, calling C<exit()>
199 from a child process will flush and close open filehandles in the parent,
200 thereby corrupting the filehandles. On these systems, calling C<_exit()>
201 is suggested instead. C<_exit()> is available in Perl through the
202 C<POSIX> module. Please consult your system's manpages for more information
205 =item Open directory handles
207 Perl will completely read from all open directory handles until they
208 reach the end of the stream. It will then seekdir() back to the
209 original location and all future readdir() requests will be fulfilled
210 from the cache buffer. That means that neither the directory handle held
211 by the parent process nor the one held by the child process will see
212 any changes made to the directory after the fork() call.
214 Note that rewinddir() has a similar limitation on Windows and will not
215 force readdir() to read the directory again either. Only a newly
216 opened directory handle will reflect changes to the directory.
218 =item Forking pipe open() not yet implemented
220 The C<open(FOO, "|-")> and C<open(BAR, "-|")> constructs are not yet
221 implemented. This limitation can be easily worked around in new code
222 by creating a pipe explicitly. The following example shows how to
223 write to a forked child:
225 # simulate open(FOO, "|-")
226 sub pipe_to_fork ($) {
228 pipe my $child, $parent or die;
230 die "fork() failed: $!" unless defined $pid;
236 open(STDIN, "<&=" . fileno($child)) or die;
241 if (pipe_to_fork('FOO')) {
243 print FOO "pipe_to_fork\n";
248 while (<STDIN>) { print; }
252 And this one reads from the child:
254 # simulate open(FOO, "-|")
255 sub pipe_from_fork ($) {
257 pipe $parent, my $child or die;
259 die "fork() failed: $!" unless defined $pid;
265 open(STDOUT, ">&=" . fileno($child)) or die;
270 if (pipe_from_fork('BAR')) {
272 while (<BAR>) { print; }
277 print "pipe_from_fork\n";
281 Forking pipe open() constructs will be supported in future.
283 =item Global state maintained by XSUBs
285 External subroutines (XSUBs) that maintain their own global state may
286 not work correctly. Such XSUBs will either need to maintain locks to
287 protect simultaneous access to global data from different pseudo-processes,
288 or maintain all their state on the Perl symbol table, which is copied
289 naturally when fork() is called. A callback mechanism that provides
290 extensions an opportunity to clone their state will be provided in the
293 =item Interpreter embedded in larger application
295 The fork() emulation may not behave as expected when it is executed in an
296 application which embeds a Perl interpreter and calls Perl APIs that can
297 evaluate bits of Perl code. This stems from the fact that the emulation
298 only has knowledge about the Perl interpreter's own data structures and
299 knows nothing about the containing application's state. For example, any
300 state carried on the application's own call stack is out of reach.
302 =item Thread-safety of extensions
304 Since the fork() emulation runs code in multiple threads, extensions
305 calling into non-thread-safe libraries may not work reliably when
306 calling fork(). As Perl's threading support gradually becomes more
307 widely adopted even on platforms with a native fork(), such extensions
308 are expected to be fixed for thread-safety.
312 =head1 PORTABILITY CAVEATS
314 In portable Perl code, C<kill(9, $child)> must not be used on forked processes.
315 Killing a forked process is unsafe and has unpredictable results.
316 See L</kill()>, above.
324 Having pseudo-process IDs be negative integers breaks down for the integer
325 C<-1> because the wait() and waitpid() functions treat this number as
326 being special. The tacit assumption in the current implementation is that
327 the system never allocates a thread ID of C<1> for user threads. A better
328 representation for pseudo-process IDs will be implemented in future.
332 In certain cases, the OS-level handles created by the pipe(), socket(),
333 and accept() operators are apparently not duplicated accurately in
334 pseudo-processes. This only happens in some situations, but where it
335 does happen, it may result in deadlocks between the read and write ends
336 of pipe handles, or inability to send or receive data across socket
341 This document may be incomplete in some respects.
347 Support for concurrent interpreters and the fork() emulation was implemented
348 by ActiveState, with funding from Microsoft Corporation.
350 This document is authored and maintained by Gurusamy Sarathy
351 E<lt>gsar@activestate.comE<gt>.
355 L<perlfunc/"fork">, L<perlipc>