1PERLFORK(1)            Perl Programmers Reference Guide            PERLFORK(1)
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NAME

6       perlfork - Perl's fork() emulation
7

SYNOPSIS

9           NOTE:  As of the 5.8.0 release, fork() emulation has considerably
10           matured.  However, there are still a few known bugs and differences
11           from real fork() that might affect you.  See the "BUGS" and
12           "CAVEATS AND LIMITATIONS" sections below.
13
14       Perl provides a fork() keyword that corresponds to the Unix system call
15       of the same name.  On most Unix-like platforms where the fork() system
16       call is available, Perl's fork() simply calls it.
17
18       On some platforms such as Windows where the fork() system call is not
19       available, Perl can be built to emulate fork() at the interpreter
20       level.  While the emulation is designed to be as compatible as possible
21       with the real fork() at the level of the Perl program, there are
22       certain important differences that stem from the fact that all the
23       pseudo child "processes" created this way live in the same real process
24       as far as the operating system is concerned.
25
26       This document provides a general overview of the capabilities and
27       limitations of the fork() emulation.  Note that the issues discussed
28       here are not applicable to platforms where a real fork() is available
29       and Perl has been configured to use it.
30

DESCRIPTION

32       The fork() emulation is implemented at the level of the Perl
33       interpreter.  What this means in general is that running fork() will
34       actually clone the running interpreter and all its state, and run the
35       cloned interpreter in a separate thread, beginning execution in the new
36       thread just after the point where the fork() was called in the parent.
37       We will refer to the thread that implements this child "process" as the
38       pseudo-process.
39
40       To the Perl program that called fork(), all this is designed to be
41       transparent.  The parent returns from the fork() with a pseudo-process
42       ID that can be subsequently used in any process-manipulation functions;
43       the child returns from the fork() with a value of 0 to signify that it
44       is the child pseudo-process.
45
46   Behavior of other Perl features in forked pseudo-processes
47       Most Perl features behave in a natural way within pseudo-processes.
48
49       $$ or $PROCESS_ID
50               This special variable is correctly set to the pseudo-process
51               ID.  It can be used to identify pseudo-processes within a
52               particular session.  Note that this value is subject to
53               recycling if any pseudo-processes are launched after others
54               have been wait()-ed on.
55
56       %ENV    Each pseudo-process maintains its own virtual environment.
57               Modifications to %ENV affect the virtual environment, and are
58               only visible within that pseudo-process, and in any processes
59               (or pseudo-processes) launched from it.
60
61       chdir() and all other builtins that accept filenames
62               Each pseudo-process maintains its own virtual idea of the
63               current directory.  Modifications to the current directory
64               using chdir() are only visible within that pseudo-process, and
65               in any processes (or pseudo-processes) launched from it.  All
66               file and directory accesses from the pseudo-process will
67               correctly map the virtual working directory to the real working
68               directory appropriately.
69
70       wait() and waitpid()
71               wait() and waitpid() can be passed a pseudo-process ID returned
72               by fork().  These calls will properly wait for the termination
73               of the pseudo-process and return its status.
74
75       kill()  "kill('KILL', ...)" can be used to terminate a pseudo-process
76               by passing it the ID returned by fork(). The outcome of kill on
77               a pseudo-process is unpredictable and it should not be used
78               except under dire circumstances, because the operating system
79               may not guarantee integrity of the process resources when a
80               running thread is terminated.  The process which implements the
81               pseudo-processes can be blocked and the Perl interpreter hangs.
82               Note that using "kill('KILL', ...)" on a pseudo-process() may
83               typically cause memory leaks, because the thread that
84               implements the pseudo-process does not get a chance to clean up
85               its resources.
86
87               "kill('TERM', ...)" can also be used on pseudo-processes, but
88               the signal will not be delivered while the pseudo-process is
89               blocked by a system call, e.g. waiting for a socket to connect,
90               or trying to read from a socket with no data available.
91               Starting in Perl 5.14 the parent process will not wait for
92               children to exit once they have been signalled with
93               "kill('TERM', ...)" to avoid deadlock during process exit.  You
94               will have to explicitly call waitpid() to make sure the child
95               has time to clean-up itself, but you are then also responsible
96               that the child is not blocking on I/O either.
97
98       exec()  Calling exec() within a pseudo-process actually spawns the
99               requested executable in a separate process and waits for it to
100               complete before exiting with the same exit status as that
101               process.  This means that the process ID reported within the
102               running executable will be different from what the earlier Perl
103               fork() might have returned.  Similarly, any process
104               manipulation functions applied to the ID returned by fork()
105               will affect the waiting pseudo-process that called exec(), not
106               the real process it is waiting for after the exec().
107
108               When exec() is called inside a pseudo-process then DESTROY
109               methods and END blocks will still be called after the external
110               process returns.
111
112       exit()  exit() always exits just the executing pseudo-process, after
113               automatically wait()-ing for any outstanding child pseudo-
114               processes.  Note that this means that the process as a whole
115               will not exit unless all running pseudo-processes have exited.
116               See below for some limitations with open filehandles.
117
118       Open handles to files, directories and network sockets
119               All open handles are dup()-ed in pseudo-processes, so that
120               closing any handles in one process does not affect the others.
121               See below for some limitations.
122
123   Resource limits
124       In the eyes of the operating system, pseudo-processes created via the
125       fork() emulation are simply threads in the same process.  This means
126       that any process-level limits imposed by the operating system apply to
127       all pseudo-processes taken together.  This includes any limits imposed
128       by the operating system on the number of open file, directory and
129       socket handles, limits on disk space usage, limits on memory size,
130       limits on CPU utilization etc.
131
132   Killing the parent process
133       If the parent process is killed (either using Perl's kill() builtin, or
134       using some external means) all the pseudo-processes are killed as well,
135       and the whole process exits.
136
137   Lifetime of the parent process and pseudo-processes
138       During the normal course of events, the parent process and every
139       pseudo-process started by it will wait for their respective pseudo-
140       children to complete before they exit.  This means that the parent and
141       every pseudo-child created by it that is also a pseudo-parent will only
142       exit after their pseudo-children have exited.
143
144       Starting with Perl 5.14 a parent will not wait() automatically for any
145       child that has been signalled with "kill('TERM', ...)"  to avoid a
146       deadlock in case the child is blocking on I/O and never receives the
147       signal.
148

CAVEATS AND LIMITATIONS

150       BEGIN blocks
151               The fork() emulation will not work entirely correctly when
152               called from within a BEGIN block.  The forked copy will run the
153               contents of the BEGIN block, but will not continue parsing the
154               source stream after the BEGIN block.  For example, consider the
155               following code:
156
157                   BEGIN {
158                       fork and exit;          # fork child and exit the parent
159                       print "inner\n";
160                   }
161                   print "outer\n";
162
163               This will print:
164
165                   inner
166
167               rather than the expected:
168
169                   inner
170                   outer
171
172               This limitation arises from fundamental technical difficulties
173               in cloning and restarting the stacks used by the Perl parser in
174               the middle of a parse.
175
176       Open filehandles
177               Any filehandles open at the time of the fork() will be
178               dup()-ed.  Thus, the files can be closed independently in the
179               parent and child, but beware that the dup()-ed handles will
180               still share the same seek pointer.  Changing the seek position
181               in the parent will change it in the child and vice-versa.  One
182               can avoid this by opening files that need distinct seek
183               pointers separately in the child.
184
185               On some operating systems, notably Solaris and Unixware,
186               calling "exit()" from a child process will flush and close open
187               filehandles in the parent, thereby corrupting the filehandles.
188               On these systems, calling "_exit()" is suggested instead.
189               "_exit()" is available in Perl through the "POSIX" module.
190               Please consult your system's manpages for more information on
191               this.
192
193       Open directory handles
194               Perl will completely read from all open directory handles until
195               they reach the end of the stream.  It will then seekdir() back
196               to the original location and all future readdir() requests will
197               be fulfilled from the cache buffer.  That means that neither
198               the directory handle held by the parent process nor the one
199               held by the child process will see any changes made to the
200               directory after the fork() call.
201
202               Note that rewinddir() has a similar limitation on Windows and
203               will not force readdir() to read the directory again either.
204               Only a newly opened directory handle will reflect changes to
205               the directory.
206
207       Forking pipe open() not yet implemented
208               The "open(FOO, "|-")" and "open(BAR, "-|")" constructs are not
209               yet implemented.  This limitation can be easily worked around
210               in new code by creating a pipe explicitly.  The following
211               example shows how to write to a forked child:
212
213                   # simulate open(FOO, "|-")
214                   sub pipe_to_fork ($) {
215                       my $parent = shift;
216                       pipe my $child, $parent or die;
217                       my $pid = fork();
218                       die "fork() failed: $!" unless defined $pid;
219                       if ($pid) {
220                           close $child;
221                       }
222                       else {
223                           close $parent;
224                           open(STDIN, "<&=" . fileno($child)) or die;
225                       }
226                       $pid;
227                   }
228
229                   if (pipe_to_fork('FOO')) {
230                       # parent
231                       print FOO "pipe_to_fork\n";
232                       close FOO;
233                   }
234                   else {
235                       # child
236                       while (<STDIN>) { print; }
237                       exit(0);
238                   }
239
240               And this one reads from the child:
241
242                   # simulate open(FOO, "-|")
243                   sub pipe_from_fork ($) {
244                       my $parent = shift;
245                       pipe $parent, my $child or die;
246                       my $pid = fork();
247                       die "fork() failed: $!" unless defined $pid;
248                       if ($pid) {
249                           close $child;
250                       }
251                       else {
252                           close $parent;
253                           open(STDOUT, ">&=" . fileno($child)) or die;
254                       }
255                       $pid;
256                   }
257
258                   if (pipe_from_fork('BAR')) {
259                       # parent
260                       while (<BAR>) { print; }
261                       close BAR;
262                   }
263                   else {
264                       # child
265                       print "pipe_from_fork\n";
266                       exit(0);
267                   }
268
269               Forking pipe open() constructs will be supported in future.
270
271       Global state maintained by XSUBs
272               External subroutines (XSUBs) that maintain their own global
273               state may not work correctly.  Such XSUBs will either need to
274               maintain locks to protect simultaneous access to global data
275               from different pseudo-processes, or maintain all their state on
276               the Perl symbol table, which is copied naturally when fork() is
277               called.  A callback mechanism that provides extensions an
278               opportunity to clone their state will be provided in the near
279               future.
280
281       Interpreter embedded in larger application
282               The fork() emulation may not behave as expected when it is
283               executed in an application which embeds a Perl interpreter and
284               calls Perl APIs that can evaluate bits of Perl code.  This
285               stems from the fact that the emulation only has knowledge about
286               the Perl interpreter's own data structures and knows nothing
287               about the containing application's state.  For example, any
288               state carried on the application's own call stack is out of
289               reach.
290
291       Thread-safety of extensions
292               Since the fork() emulation runs code in multiple threads,
293               extensions calling into non-thread-safe libraries may not work
294               reliably when calling fork().  As Perl's threading support
295               gradually becomes more widely adopted even on platforms with a
296               native fork(), such extensions are expected to be fixed for
297               thread-safety.
298

PORTABILITY CAVEATS

300       In portable Perl code, "kill(9, $child)" must not be used on forked
301       processes.  Killing a forked process is unsafe and has unpredictable
302       results.  See "kill()", above.
303

BUGS

305       •       Having pseudo-process IDs be negative integers breaks down for
306               the integer "-1" because the wait() and waitpid() functions
307               treat this number as being special.  The tacit assumption in
308               the current implementation is that the system never allocates a
309               thread ID of 1 for user threads.  A better representation for
310               pseudo-process IDs will be implemented in future.
311
312       •       In certain cases, the OS-level handles created by the pipe(),
313               socket(), and accept() operators are apparently not duplicated
314               accurately in pseudo-processes.  This only happens in some
315               situations, but where it does happen, it may result in
316               deadlocks between the read and write ends of pipe handles, or
317               inability to send or receive data across socket handles.
318
319       •       This document may be incomplete in some respects.
320

AUTHOR

322       Support for concurrent interpreters and the fork() emulation was
323       implemented by ActiveState, with funding from Microsoft Corporation.
324
325       This document is authored and maintained by Gurusamy Sarathy
326       <gsar@activestate.com>.
327

SEE ALSO

329       "fork" in perlfunc, perlipc
330
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332
333perl v5.34.0                      2021-10-18                       PERLFORK(1)
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