1PERLFORK(1)            Perl Programmers Reference Guide            PERLFORK(1)
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3
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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() can be used to terminate a pseudo-process by passing it
76               the ID returned by fork().  This should not be used except
77               under dire circumstances, because the operating system may not
78               guarantee integrity of the process resources when a running
79               thread is terminated.  Note that using kill() on a
80               pseudo-process() may typically cause memory leaks, because the
81               thread that implements the pseudo-process does not get a chance
82               to clean up its resources.
83
84       exec()  Calling exec() within a pseudo-process actually spawns the
85               requested executable in a separate process and waits for it to
86               complete before exiting with the same exit status as that
87               process.  This means that the process ID reported within the
88               running executable will be different from what the earlier Perl
89               fork() might have returned.  Similarly, any process
90               manipulation functions applied to the ID returned by fork()
91               will affect the waiting pseudo-process that called exec(), not
92               the real process it is waiting for after the exec().
93
94               When exec() is called inside a pseudo-process then DESTROY
95               methods and END blocks will still be called after the external
96               process returns.
97
98       exit()  exit() always exits just the executing pseudo-process, after
99               automatically wait()-ing for any outstanding child pseudo-
100               processes.  Note that this means that the process as a whole
101               will not exit unless all running pseudo-processes have exited.
102               See below for some limitations with open filehandles.
103
104       Open handles to files, directories and network sockets
105               All open handles are dup()-ed in pseudo-processes, so that
106               closing any handles in one process does not affect the others.
107               See below for some limitations.
108
109   Resource limits
110       In the eyes of the operating system, pseudo-processes created via the
111       fork() emulation are simply threads in the same process.  This means
112       that any process-level limits imposed by the operating system apply to
113       all pseudo-processes taken together.  This includes any limits imposed
114       by the operating system on the number of open file, directory and
115       socket handles, limits on disk space usage, limits on memory size,
116       limits on CPU utilization etc.
117
118   Killing the parent process
119       If the parent process is killed (either using Perl's kill() builtin, or
120       using some external means) all the pseudo-processes are killed as well,
121       and the whole process exits.
122
123   Lifetime of the parent process and pseudo-processes
124       During the normal course of events, the parent process and every
125       pseudo-process started by it will wait for their respective pseudo-
126       children to complete before they exit.  This means that the parent and
127       every pseudo-child created by it that is also a pseudo-parent will only
128       exit after their pseudo-children have exited.
129
130       A way to mark a pseudo-processes as running detached from their parent
131       (so that the parent would not have to wait() for them if it doesn't
132       want to) will be provided in future.
133
134   CAVEATS AND LIMITATIONS
135       BEGIN blocks
136               The fork() emulation will not work entirely correctly when
137               called from within a BEGIN block.  The forked copy will run the
138               contents of the BEGIN block, but will not continue parsing the
139               source stream after the BEGIN block.  For example, consider the
140               following code:
141
142                   BEGIN {
143                       fork and exit;          # fork child and exit the parent
144                       print "inner\n";
145                   }
146                   print "outer\n";
147
148               This will print:
149
150                   inner
151
152               rather than the expected:
153
154                   inner
155                   outer
156
157               This limitation arises from fundamental technical difficulties
158               in cloning and restarting the stacks used by the Perl parser in
159               the middle of a parse.
160
161       Open filehandles
162               Any filehandles open at the time of the fork() will be
163               dup()-ed.  Thus, the files can be closed independently in the
164               parent and child, but beware that the dup()-ed handles will
165               still share the same seek pointer.  Changing the seek position
166               in the parent will change it in the child and vice-versa.  One
167               can avoid this by opening files that need distinct seek
168               pointers separately in the child.
169
170               On some operating systems, notably Solaris and Unixware,
171               calling "exit()" from a child process will flush and close open
172               filehandles in the parent, thereby corrupting the filehandles.
173               On these systems, calling "_exit()" is suggested instead.
174               "_exit()" is available in Perl through the "POSIX" module.
175               Please consult your systems manpages for more information on
176               this.
177
178       Forking pipe open() not yet implemented
179               The "open(FOO, "|-")" and "open(BAR, "-|")" constructs are not
180               yet implemented.  This limitation can be easily worked around
181               in new code by creating a pipe explicitly.  The following
182               example shows how to write to a forked child:
183
184                   # simulate open(FOO, "|-")
185                   sub pipe_to_fork ($) {
186                       my $parent = shift;
187                       pipe my $child, $parent or die;
188                       my $pid = fork();
189                       die "fork() failed: $!" unless defined $pid;
190                       if ($pid) {
191                           close $child;
192                       }
193                       else {
194                           close $parent;
195                           open(STDIN, "<&=" . fileno($child)) or die;
196                       }
197                       $pid;
198                   }
199
200                   if (pipe_to_fork('FOO')) {
201                       # parent
202                       print FOO "pipe_to_fork\n";
203                       close FOO;
204                   }
205                   else {
206                       # child
207                       while (<STDIN>) { print; }
208                       exit(0);
209                   }
210
211               And this one reads from the child:
212
213                   # simulate open(FOO, "-|")
214                   sub pipe_from_fork ($) {
215                       my $parent = shift;
216                       pipe $parent, my $child or die;
217                       my $pid = fork();
218                       die "fork() failed: $!" unless defined $pid;
219                       if ($pid) {
220                           close $child;
221                       }
222                       else {
223                           close $parent;
224                           open(STDOUT, ">&=" . fileno($child)) or die;
225                       }
226                       $pid;
227                   }
228
229                   if (pipe_from_fork('BAR')) {
230                       # parent
231                       while (<BAR>) { print; }
232                       close BAR;
233                   }
234                   else {
235                       # child
236                       print "pipe_from_fork\n";
237                       exit(0);
238                   }
239
240               Forking pipe open() constructs will be supported in future.
241
242       Global state maintained by XSUBs
243               External subroutines (XSUBs) that maintain their own global
244               state may not work correctly.  Such XSUBs will either need to
245               maintain locks to protect simultaneous access to global data
246               from different pseudo-processes, or maintain all their state on
247               the Perl symbol table, which is copied naturally when fork() is
248               called.  A callback mechanism that provides extensions an
249               opportunity to clone their state will be provided in the near
250               future.
251
252       Interpreter embedded in larger application
253               The fork() emulation may not behave as expected when it is
254               executed in an application which embeds a Perl interpreter and
255               calls Perl APIs that can evaluate bits of Perl code.  This
256               stems from the fact that the emulation only has knowledge about
257               the Perl interpreter's own data structures and knows nothing
258               about the containing application's state.  For example, any
259               state carried on the application's own call stack is out of
260               reach.
261
262       Thread-safety of extensions
263               Since the fork() emulation runs code in multiple threads,
264               extensions calling into non-thread-safe libraries may not work
265               reliably when calling fork().  As Perl's threading support
266               gradually becomes more widely adopted even on platforms with a
267               native fork(), such extensions are expected to be fixed for
268               thread-safety.
269

BUGS

271       ·       Having pseudo-process IDs be negative integers breaks down for
272               the integer "-1" because the wait() and waitpid() functions
273               treat this number as being special.  The tacit assumption in
274               the current implementation is that the system never allocates a
275               thread ID of 1 for user threads.  A better representation for
276               pseudo-process IDs will be implemented in future.
277
278       ·       In certain cases, the OS-level handles created by the pipe(),
279               socket(), and accept() operators are apparently not duplicated
280               accurately in pseudo-processes.  This only happens in some
281               situations, but where it does happen, it may result in
282               deadlocks between the read and write ends of pipe handles, or
283               inability to send or receive data across socket handles.
284
285       ·       This document may be incomplete in some respects.
286

AUTHOR

288       Support for concurrent interpreters and the fork() emulation was
289       implemented by ActiveState, with funding from Microsoft Corporation.
290
291       This document is authored and maintained by Gurusamy Sarathy
292       <gsar@activestate.com>.
293

SEE ALSO

295       "fork" in perlfunc, perlipc
296
297
298
299perl v5.10.1                      2009-02-12                       PERLFORK(1)
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