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1 =head1 NAME 2 3 perlfork - Perl's fork() emulation 4 5 =head1 SYNOPSIS 6 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. 11 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 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. 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 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 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 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. 100 101 =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 106 have exited. See below for some limitations with open filehandles. 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. 182 183 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. 189 190 =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 $pid; 211 } 212 213 if (pipe_to_fork('FOO')) { 214 # parent 215 print FOO "pipe_to_fork\n"; 216 close FOO; 217 } 218 else { 219 # child 220 while (<STDIN>) { print; } 221 exit(0); 222 } 223 224 And this one reads from the child: 225 226 # simulate open(FOO, "-|") 227 sub pipe_from_fork ($) { 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"; 250 exit(0); 251 } 252 253 Forking pipe open() constructs will be supported in future. 254 255 =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. 273 274 =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. 281 282 =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 * 297 298 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 * 306 307 This document may be incomplete in some respects. 308 309 =back 310 311 =head1 AUTHOR 312 313 Support for concurrent interpreters and the fork() emulation was implemented 314 by ActiveState, with funding from Microsoft Corporation. 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
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