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1 | =head1 NAME |
2 | ||
c3c83ace | 3 | perlfork - Perl's fork() emulation |
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4 | |
5 | =head1 SYNOPSIS | |
6 | ||
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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. | |
c7fa416b | 11 | |
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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. | |
15 | ||
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 | |
106325ad | 19 | real fork() at the level of the Perl program, there are certain |
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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. | |
23 | ||
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. | |
28 | ||
29 | =head1 DESCRIPTION | |
30 | ||
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. | |
37 | ||
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. | |
43 | ||
44 | =head2 Behavior of other Perl features in forked pseudo-processes | |
45 | ||
46 | Most Perl features behave in a natural way within pseudo-processes. | |
47 | ||
48 | =over 8 | |
49 | ||
50 | =item $$ or $PROCESS_ID | |
51 | ||
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. | |
56 | ||
57 | =item %ENV | |
58 | ||
4375e838 | 59 | Each pseudo-process maintains its own virtual environment. Modifications |
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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 | |
62 | it. | |
63 | ||
64 | =item chdir() and all other builtins that accept filenames | |
65 | ||
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. | |
71 | ||
72 | =item wait() and waitpid() | |
73 | ||
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. | |
77 | ||
78 | =item kill() | |
79 | ||
80 | kill() can be used to terminate a pseudo-process by passing it the ID returned | |
81 | by fork(). This should not be used except under dire circumstances, because | |
82 | the operating system may not guarantee integrity of the process resources | |
83 | when a running thread is terminated. Note that using kill() on a | |
84 | pseudo-process() may typically cause memory leaks, because the thread that | |
85 | implements the pseudo-process does not get a chance to clean up its resources. | |
86 | ||
87 | =item exec() | |
88 | ||
89 | Calling exec() within a pseudo-process actually spawns the requested | |
90 | executable in a separate process and waits for it to complete before | |
91 | exiting with the same exit status as that process. This means that the | |
92 | process ID reported within the running executable will be different from | |
93 | what the earlier Perl fork() might have returned. Similarly, any process | |
94 | manipulation functions applied to the ID returned by fork() will affect the | |
95 | waiting pseudo-process that called exec(), not the real process it is | |
96 | waiting for after the exec(). | |
97 | ||
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98 | When exec() is called inside a pseudo-process then DESTROY methods and |
99 | END blocks will still be called after the external process returns. | |
100 | ||
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101 | =item exit() |
102 | ||
103 | exit() always exits just the executing pseudo-process, after automatically | |
104 | wait()-ing for any outstanding child pseudo-processes. Note that this means | |
105 | that the process as a whole will not exit unless all running pseudo-processes | |
1d335e36 | 106 | have exited. See below for some limitations with open filehandles. |
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107 | |
108 | =item Open handles to files, directories and network sockets | |
109 | ||
110 | All open handles are dup()-ed in pseudo-processes, so that closing | |
111 | any handles in one process does not affect the others. See below for | |
112 | some limitations. | |
113 | ||
114 | =back | |
115 | ||
116 | =head2 Resource limits | |
117 | ||
118 | In the eyes of the operating system, pseudo-processes created via the fork() | |
119 | emulation are simply threads in the same process. This means that any | |
120 | process-level limits imposed by the operating system apply to all | |
121 | pseudo-processes taken together. This includes any limits imposed by the | |
122 | operating system on the number of open file, directory and socket handles, | |
123 | limits on disk space usage, limits on memory size, limits on CPU utilization | |
124 | etc. | |
125 | ||
126 | =head2 Killing the parent process | |
127 | ||
128 | If the parent process is killed (either using Perl's kill() builtin, or | |
129 | using some external means) all the pseudo-processes are killed as well, | |
130 | and the whole process exits. | |
131 | ||
132 | =head2 Lifetime of the parent process and pseudo-processes | |
133 | ||
134 | During the normal course of events, the parent process and every | |
135 | pseudo-process started by it will wait for their respective pseudo-children | |
136 | to complete before they exit. This means that the parent and every | |
137 | pseudo-child created by it that is also a pseudo-parent will only exit | |
138 | after their pseudo-children have exited. | |
139 | ||
140 | A way to mark a pseudo-processes as running detached from their parent (so | |
141 | that the parent would not have to wait() for them if it doesn't want to) | |
142 | will be provided in future. | |
143 | ||
144 | =head2 CAVEATS AND LIMITATIONS | |
145 | ||
146 | =over 8 | |
147 | ||
148 | =item BEGIN blocks | |
149 | ||
150 | The fork() emulation will not work entirely correctly when called from | |
151 | within a BEGIN block. The forked copy will run the contents of the | |
152 | BEGIN block, but will not continue parsing the source stream after the | |
153 | BEGIN block. For example, consider the following code: | |
154 | ||
155 | BEGIN { | |
156 | fork and exit; # fork child and exit the parent | |
157 | print "inner\n"; | |
158 | } | |
159 | print "outer\n"; | |
160 | ||
161 | This will print: | |
162 | ||
163 | inner | |
164 | ||
165 | rather than the expected: | |
166 | ||
167 | inner | |
168 | outer | |
169 | ||
170 | This limitation arises from fundamental technical difficulties in | |
171 | cloning and restarting the stacks used by the Perl parser in the | |
172 | middle of a parse. | |
173 | ||
174 | =item Open filehandles | |
175 | ||
176 | Any filehandles open at the time of the fork() will be dup()-ed. Thus, | |
177 | the files can be closed independently in the parent and child, but beware | |
178 | that the dup()-ed handles will still share the same seek pointer. Changing | |
179 | the seek position in the parent will change it in the child and vice-versa. | |
180 | One can avoid this by opening files that need distinct seek pointers | |
181 | separately in the child. | |
182 | ||
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183 | On some operating systems, notably Solaris and Unixware, calling C<exit()> |
184 | from a child process will flush and close open filehandles in the parent, | |
185 | thereby corrupting the filehandles. On these systems, calling C<_exit()> | |
186 | is suggested instead. C<_exit()> is available in Perl through the | |
187 | C<POSIX> module. Please consult your systems manpages for more information | |
188 | on this. | |
189 | ||
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190 | =item Forking pipe open() not yet implemented |
191 | ||
192 | The C<open(FOO, "|-")> and C<open(BAR, "-|")> constructs are not yet | |
193 | implemented. This limitation can be easily worked around in new code | |
194 | by creating a pipe explicitly. The following example shows how to | |
195 | write to a forked child: | |
196 | ||
197 | # simulate open(FOO, "|-") | |
198 | sub pipe_to_fork ($) { | |
199 | my $parent = shift; | |
200 | pipe my $child, $parent or die; | |
201 | my $pid = fork(); | |
202 | die "fork() failed: $!" unless defined $pid; | |
203 | if ($pid) { | |
204 | close $child; | |
205 | } | |
206 | else { | |
207 | close $parent; | |
208 | open(STDIN, "<&=" . fileno($child)) or die; | |
209 | } | |
210 | $pid; | |
211 | } | |
212 | ||
213 | if (pipe_to_fork('FOO')) { | |
214 | # parent | |
215 | print FOO "pipe_to_fork\n"; | |
216 | close FOO; | |
217 | } | |
218 | else { | |
219 | # child | |
220 | while (<STDIN>) { print; } | |
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221 | exit(0); |
222 | } | |
223 | ||
224 | And this one reads from the child: | |
225 | ||
226 | # simulate open(FOO, "-|") | |
227 | sub pipe_from_fork ($) { | |
228 | my $parent = shift; | |
229 | pipe $parent, my $child or die; | |
230 | my $pid = fork(); | |
231 | die "fork() failed: $!" unless defined $pid; | |
232 | if ($pid) { | |
233 | close $child; | |
234 | } | |
235 | else { | |
236 | close $parent; | |
237 | open(STDOUT, ">&=" . fileno($child)) or die; | |
238 | } | |
239 | $pid; | |
240 | } | |
241 | ||
242 | if (pipe_from_fork('BAR')) { | |
243 | # parent | |
244 | while (<BAR>) { print; } | |
245 | close BAR; | |
246 | } | |
247 | else { | |
248 | # child | |
249 | print "pipe_from_fork\n"; | |
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250 | exit(0); |
251 | } | |
252 | ||
253 | Forking pipe open() constructs will be supported in future. | |
254 | ||
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255 | =item Global state maintained by XSUBs |
256 | ||
257 | External subroutines (XSUBs) that maintain their own global state may | |
258 | not work correctly. Such XSUBs will either need to maintain locks to | |
259 | protect simultaneous access to global data from different pseudo-processes, | |
260 | or maintain all their state on the Perl symbol table, which is copied | |
261 | naturally when fork() is called. A callback mechanism that provides | |
262 | extensions an opportunity to clone their state will be provided in the | |
263 | near future. | |
264 | ||
265 | =item Interpreter embedded in larger application | |
266 | ||
267 | The fork() emulation may not behave as expected when it is executed in an | |
268 | application which embeds a Perl interpreter and calls Perl APIs that can | |
269 | evaluate bits of Perl code. This stems from the fact that the emulation | |
270 | only has knowledge about the Perl interpreter's own data structures and | |
271 | knows nothing about the containing application's state. For example, any | |
272 | state carried on the application's own call stack is out of reach. | |
273 | ||
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274 | =item Thread-safety of extensions |
275 | ||
276 | Since the fork() emulation runs code in multiple threads, extensions | |
277 | calling into non-thread-safe libraries may not work reliably when | |
278 | calling fork(). As Perl's threading support gradually becomes more | |
279 | widely adopted even on platforms with a native fork(), such extensions | |
280 | are expected to be fixed for thread-safety. | |
281 | ||
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282 | =back |
283 | ||
284 | =head1 BUGS | |
285 | ||
286 | =over 8 | |
287 | ||
288 | =item * | |
289 | ||
290 | Having pseudo-process IDs be negative integers breaks down for the integer | |
291 | C<-1> because the wait() and waitpid() functions treat this number as | |
292 | being special. The tacit assumption in the current implementation is that | |
293 | the system never allocates a thread ID of C<1> for user threads. A better | |
294 | representation for pseudo-process IDs will be implemented in future. | |
295 | ||
296 | =item * | |
297 | ||
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298 | In certain cases, the OS-level handles created by the pipe(), socket(), |
299 | and accept() operators are apparently not duplicated accurately in | |
300 | pseudo-processes. This only happens in some situations, but where it | |
301 | does happen, it may result in deadlocks between the read and write ends | |
302 | of pipe handles, or inability to send or receive data across socket | |
303 | handles. | |
304 | ||
305 | =item * | |
306 | ||
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307 | This document may be incomplete in some respects. |
308 | ||
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309 | =back |
310 | ||
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311 | =head1 AUTHOR |
312 | ||
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313 | Support for concurrent interpreters and the fork() emulation was implemented |
314 | by ActiveState, with funding from Microsoft Corporation. | |
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315 | |
316 | This document is authored and maintained by Gurusamy Sarathy | |
317 | E<lt>gsar@activestate.comE<gt>. | |
318 | ||
319 | =head1 SEE ALSO | |
320 | ||
321 | L<perlfunc/"fork">, L<perlipc> | |
322 | ||
323 | =cut |