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(). This should not be used except
82 under dire circumstances, because the operating system may not
83 guarantee integrity of the process resources when a running thread is
84 terminated. Note that using C<kill('KILL', ...)> on a
85 pseudo-process() may typically cause memory leaks, because the thread
86 that implements the pseudo-process does not get a chance to clean up
89 C<kill('TERM', ...)> can also be used on pseudo-processes, but the
90 signal will not be delivered while the pseudo-process is blocked by a
91 system call, e.g. waiting for a socket to connect, or trying to read
92 from a socket with no data available. Starting in Perl 5.14 the
93 parent process will not wait for children to exit once they have been
94 signalled with C<kill('TERM', ...)> to avoid deadlock during process
95 exit. You will have to explicitly call waitpid() to make sure the
96 child has time to clean-up itself, but you are then also responsible
97 that the child is not blocking on I/O either.
101 Calling exec() within a pseudo-process actually spawns the requested
102 executable in a separate process and waits for it to complete before
103 exiting with the same exit status as that process. This means that the
104 process ID reported within the running executable will be different from
105 what the earlier Perl fork() might have returned. Similarly, any process
106 manipulation functions applied to the ID returned by fork() will affect the
107 waiting pseudo-process that called exec(), not the real process it is
108 waiting for after the exec().
110 When exec() is called inside a pseudo-process then DESTROY methods and
111 END blocks will still be called after the external process returns.
115 exit() always exits just the executing pseudo-process, after automatically
116 wait()-ing for any outstanding child pseudo-processes. Note that this means
117 that the process as a whole will not exit unless all running pseudo-processes
118 have exited. See below for some limitations with open filehandles.
120 =item Open handles to files, directories and network sockets
122 All open handles are dup()-ed in pseudo-processes, so that closing
123 any handles in one process does not affect the others. See below for
128 =head2 Resource limits
130 In the eyes of the operating system, pseudo-processes created via the fork()
131 emulation are simply threads in the same process. This means that any
132 process-level limits imposed by the operating system apply to all
133 pseudo-processes taken together. This includes any limits imposed by the
134 operating system on the number of open file, directory and socket handles,
135 limits on disk space usage, limits on memory size, limits on CPU utilization
138 =head2 Killing the parent process
140 If the parent process is killed (either using Perl's kill() builtin, or
141 using some external means) all the pseudo-processes are killed as well,
142 and the whole process exits.
144 =head2 Lifetime of the parent process and pseudo-processes
146 During the normal course of events, the parent process and every
147 pseudo-process started by it will wait for their respective pseudo-children
148 to complete before they exit. This means that the parent and every
149 pseudo-child created by it that is also a pseudo-parent will only exit
150 after their pseudo-children have exited.
152 Starting with Perl 5.14 a parent will not wait() automatically
153 for any child that has been signalled with C<sig('TERM', ...)>
154 to avoid a deadlock in case the child is blocking on I/O and
155 never receives the signal.
157 =head1 CAVEATS AND LIMITATIONS
163 The fork() emulation will not work entirely correctly when called from
164 within a BEGIN block. The forked copy will run the contents of the
165 BEGIN block, but will not continue parsing the source stream after the
166 BEGIN block. For example, consider the following code:
169 fork and exit; # fork child and exit the parent
178 rather than the expected:
183 This limitation arises from fundamental technical difficulties in
184 cloning and restarting the stacks used by the Perl parser in the
187 =item Open filehandles
189 Any filehandles open at the time of the fork() will be dup()-ed. Thus,
190 the files can be closed independently in the parent and child, but beware
191 that the dup()-ed handles will still share the same seek pointer. Changing
192 the seek position in the parent will change it in the child and vice-versa.
193 One can avoid this by opening files that need distinct seek pointers
194 separately in the child.
196 On some operating systems, notably Solaris and Unixware, calling C<exit()>
197 from a child process will flush and close open filehandles in the parent,
198 thereby corrupting the filehandles. On these systems, calling C<_exit()>
199 is suggested instead. C<_exit()> is available in Perl through the
200 C<POSIX> module. Please consult your system's manpages for more information
203 =item Open directory handles
205 Perl will completely read from all open directory handles until they
206 reach the end of the stream. It will then seekdir() back to the
207 original location and all future readdir() requests will be fulfilled
208 from the cache buffer. That means that neither the directory handle held
209 by the parent process nor the one held by the child process will see
210 any changes made to the directory after the fork() call.
212 Note that rewinddir() has a similar limitation on Windows and will not
213 force readdir() to read the directory again either. Only a newly
214 opened directory handle will reflect changes to the directory.
216 =item Forking pipe open() not yet implemented
218 The C<open(FOO, "|-")> and C<open(BAR, "-|")> constructs are not yet
219 implemented. This limitation can be easily worked around in new code
220 by creating a pipe explicitly. The following example shows how to
221 write to a forked child:
223 # simulate open(FOO, "|-")
224 sub pipe_to_fork ($) {
226 pipe my $child, $parent or die;
228 die "fork() failed: $!" unless defined $pid;
234 open(STDIN, "<&=" . fileno($child)) or die;
239 if (pipe_to_fork('FOO')) {
241 print FOO "pipe_to_fork\n";
246 while (<STDIN>) { print; }
250 And this one reads from the child:
252 # simulate open(FOO, "-|")
253 sub pipe_from_fork ($) {
255 pipe $parent, my $child or die;
257 die "fork() failed: $!" unless defined $pid;
263 open(STDOUT, ">&=" . fileno($child)) or die;
268 if (pipe_from_fork('BAR')) {
270 while (<BAR>) { print; }
275 print "pipe_from_fork\n";
279 Forking pipe open() constructs will be supported in future.
281 =item Global state maintained by XSUBs
283 External subroutines (XSUBs) that maintain their own global state may
284 not work correctly. Such XSUBs will either need to maintain locks to
285 protect simultaneous access to global data from different pseudo-processes,
286 or maintain all their state on the Perl symbol table, which is copied
287 naturally when fork() is called. A callback mechanism that provides
288 extensions an opportunity to clone their state will be provided in the
291 =item Interpreter embedded in larger application
293 The fork() emulation may not behave as expected when it is executed in an
294 application which embeds a Perl interpreter and calls Perl APIs that can
295 evaluate bits of Perl code. This stems from the fact that the emulation
296 only has knowledge about the Perl interpreter's own data structures and
297 knows nothing about the containing application's state. For example, any
298 state carried on the application's own call stack is out of reach.
300 =item Thread-safety of extensions
302 Since the fork() emulation runs code in multiple threads, extensions
303 calling into non-thread-safe libraries may not work reliably when
304 calling fork(). As Perl's threading support gradually becomes more
305 widely adopted even on platforms with a native fork(), such extensions
306 are expected to be fixed for thread-safety.
316 Having pseudo-process IDs be negative integers breaks down for the integer
317 C<-1> because the wait() and waitpid() functions treat this number as
318 being special. The tacit assumption in the current implementation is that
319 the system never allocates a thread ID of C<1> for user threads. A better
320 representation for pseudo-process IDs will be implemented in future.
324 In certain cases, the OS-level handles created by the pipe(), socket(),
325 and accept() operators are apparently not duplicated accurately in
326 pseudo-processes. This only happens in some situations, but where it
327 does happen, it may result in deadlocks between the read and write ends
328 of pipe handles, or inability to send or receive data across socket
333 This document may be incomplete in some respects.
339 Support for concurrent interpreters and the fork() emulation was implemented
340 by ActiveState, with funding from Microsoft Corporation.
342 This document is authored and maintained by Gurusamy Sarathy
343 E<lt>gsar@activestate.comE<gt>.
347 L<perlfunc/"fork">, L<perlipc>