9 my $XS_VERSION = $VERSION;
10 $VERSION = eval $VERSION;
12 # Verify this Perl supports threads
14 if (! $Config::Config{useithreads}) {
15 die("This Perl not built to support threads\n");
18 # Complain if 'threads' is loaded after 'threads::shared'
19 if ($threads::shared::threads_shared) {
21 Warning, threads::shared has already been loaded. To
22 enable shared variables, 'use threads' must be called
23 before threads::shared or any module that uses it.
27 # Declare that we have been loaded
28 $threads::threads = 1;
32 XSLoader::load('threads', $XS_VERSION);
39 my $class = shift; # Not used
41 # Exported subroutines
42 my @EXPORT = qw(async);
45 while (my $sym = shift) {
46 if ($sym =~ /^(?:stack|exit)/i) {
47 if (defined(my $arg = shift)) {
48 if ($sym =~ /^stack/i) {
49 threads->set_stack_size($arg);
51 $threads::thread_exit_only = $arg =~ /^thread/i;
55 Carp::croak("threads: Missing argument for option: $sym");
58 } elsif ($sym =~ /^str/i) {
59 import overload ('""' => \&tid);
61 } elsif ($sym =~ /^(?::all|yield)$/) {
62 push(@EXPORT, qw(yield));
66 Carp::croak("threads: Unknown import option: $sym");
70 # Export subroutine names
71 my $caller = caller();
72 foreach my $sym (@EXPORT) {
74 *{$caller.'::'.$sym} = \&{$sym};
77 # Set stack size via environment variable
78 if (exists($ENV{'PERL5_ITHREADS_STACK_SIZE'})) {
79 threads->set_stack_size($ENV{'PERL5_ITHREADS_STACK_SIZE'});
86 # Exit from a thread (only)
89 my ($class, $status) = @_;
90 if (! defined($status)) {
97 Carp::croak('Usage: threads->exit(status)');
100 $class->set_thread_exit_only(1);
104 # 'Constant' args for threads->list()
106 sub threads::running { 1 }
107 sub threads::joinable { 0 }
109 # 'new' is an alias for 'create'
112 # 'async' is a function alias for the 'threads->create()' method
115 unshift(@_, 'threads');
116 # Use "goto" trick to avoid pad problems from 5.8.1 (fixed in 5.8.2)
120 # Thread object equality checking
123 '!=' => sub { ! equal(@_) },
133 threads - Perl interpreter-based threads
137 This document describes threads version 1.92
141 The "interpreter-based threads" provided by Perl are not the fast, lightweight
142 system for multitasking that one might expect or hope for. Threads are
143 implemented in a way that make them easy to misuse. Few people know how to
144 use them correctly or will be able to provide help.
146 The use of interpreter-based threads in perl is officially
147 L<discouraged|perlpolicy/discouraged>.
151 use threads ('yield',
152 'stack_size' => 64*4096,
153 'exit' => 'threads_only',
158 print('Thread started: ', join(' ', @args), "\n");
160 my $thr = threads->create('start_thread', 'argument');
163 threads->create(sub { print("I am a thread\n"); })->join();
165 my $thr2 = async { foreach (@files) { ... } };
167 if (my $err = $thr2->error()) {
168 warn("Thread error: $err\n");
171 # Invoke thread in list context (implicit) so it can return a list
172 my ($thr) = threads->create(sub { return (qw/a b c/); });
173 # or specify list context explicitly
174 my $thr = threads->create({'context' => 'list'},
175 sub { return (qw/a b c/); });
176 my @results = $thr->join();
180 # Get a thread's object
181 $thr = threads->self();
182 $thr = threads->object($tid);
185 $tid = threads->tid();
189 # Give other threads a chance to run
193 # Lists of non-detached threads
194 my @threads = threads->list();
195 my $thread_count = threads->list();
197 my @running = threads->list(threads::running);
198 my @joinable = threads->list(threads::joinable);
200 # Test thread objects
201 if ($thr1 == $thr2) {
205 # Manage thread stack size
206 $stack_size = threads->get_stack_size();
207 $old_size = threads->set_stack_size(32*4096);
209 # Create a thread with a specific context and stack size
210 my $thr = threads->create({ 'context' => 'list',
211 'stack_size' => 32*4096,
212 'exit' => 'thread_only' },
215 # Get thread's context
216 my $wantarray = $thr->wantarray();
218 # Check thread's state
219 if ($thr->is_running()) {
222 if ($thr->is_joinable()) {
226 # Send a signal to a thread
227 $thr->kill('SIGUSR1');
234 Since Perl 5.8, thread programming has been available using a model called
235 I<interpreter threads> which provides a new Perl interpreter for each
236 thread, and, by default, results in no data or state information being shared
239 (Prior to Perl 5.8, I<5005threads> was available through the C<Thread.pm> API.
240 This threading model has been deprecated, and was removed as of Perl 5.10.0.)
242 As just mentioned, all variables are, by default, thread local. To use shared
243 variables, you need to also load L<threads::shared>:
248 When loading L<threads::shared>, you must C<use threads> before you
249 C<use threads::shared>. (C<threads> will emit a warning if you do it the
252 It is strongly recommended that you enable threads via C<use threads> as early
253 as possible in your script.
255 If needed, scripts can be written so as to run on both threaded and
258 my $can_use_threads = eval 'use threads; 1';
259 if ($can_use_threads) {
260 # Do processing using threads
263 # Do it without using threads
269 =item $thr = threads->create(FUNCTION, ARGS)
271 This will create a new thread that will begin execution with the specified
272 entry point function, and give it the I<ARGS> list as parameters. It will
273 return the corresponding threads object, or C<undef> if thread creation failed.
275 I<FUNCTION> may either be the name of a function, an anonymous subroutine, or
278 my $thr = threads->create('func_name', ...);
280 my $thr = threads->create(sub { ... }, ...);
282 my $thr = threads->create(\&func, ...);
284 The C<-E<gt>new()> method is an alias for C<-E<gt>create()>.
288 This will wait for the corresponding thread to complete its execution. When
289 the thread finishes, C<-E<gt>join()> will return the return value(s) of the
290 entry point function.
292 The context (void, scalar or list) for the return value(s) for C<-E<gt>join()>
293 is determined at the time of thread creation.
295 # Create thread in list context (implicit)
296 my ($thr1) = threads->create(sub {
297 my @results = qw(a b c);
301 my $thr1 = threads->create({'context' => 'list'},
303 my @results = qw(a b c);
306 # Retrieve list results from thread
307 my @res1 = $thr1->join();
309 # Create thread in scalar context (implicit)
310 my $thr2 = threads->create(sub {
314 # Retrieve scalar result from thread
315 my $res2 = $thr2->join();
317 # Create a thread in void context (explicit)
318 my $thr3 = threads->create({'void' => 1},
319 sub { print("Hello, world\n"); });
320 # Join the thread in void context (i.e., no return value)
323 See L</"THREAD CONTEXT"> for more details.
325 If the program exits without all threads having either been joined or
326 detached, then a warning will be issued.
328 Calling C<-E<gt>join()> or C<-E<gt>detach()> on an already joined thread will
329 cause an error to be thrown.
333 Makes the thread unjoinable, and causes any eventual return value to be
334 discarded. When the program exits, any detached threads that are still
335 running are silently terminated.
337 If the program exits without all threads having either been joined or
338 detached, then a warning will be issued.
340 Calling C<-E<gt>join()> or C<-E<gt>detach()> on an already detached thread
341 will cause an error to be thrown.
343 =item threads->detach()
345 Class method that allows a thread to detach itself.
347 =item threads->self()
349 Class method that allows a thread to obtain its own I<threads> object.
353 Returns the ID of the thread. Thread IDs are unique integers with the main
354 thread in a program being 0, and incrementing by 1 for every thread created.
358 Class method that allows a thread to obtain its own ID.
362 If you add the C<stringify> import option to your C<use threads> declaration,
363 then using a threads object in a string or a string context (e.g., as a hash
364 key) will cause its ID to be used as the value:
366 use threads qw(stringify);
368 my $thr = threads->create(...);
369 print("Thread $thr started...\n"); # Prints out: Thread 1 started...
371 =item threads->object($tid)
373 This will return the I<threads> object for the I<active> thread associated
374 with the specified thread ID. If C<$tid> is the value for the current thread,
375 then this call works the same as C<-E<gt>self()>. Otherwise, returns C<undef>
376 if there is no thread associated with the TID, if the thread is joined or
377 detached, if no TID is specified or if the specified TID is undef.
379 =item threads->yield()
381 This is a suggestion to the OS to let this thread yield CPU time to other
382 threads. What actually happens is highly dependent upon the underlying
383 thread implementation.
385 You may do C<use threads qw(yield)>, and then just use C<yield()> in your
388 =item threads->list()
390 =item threads->list(threads::all)
392 =item threads->list(threads::running)
394 =item threads->list(threads::joinable)
396 With no arguments (or using C<threads::all>) and in a list context, returns a
397 list of all non-joined, non-detached I<threads> objects. In a scalar context,
398 returns a count of the same.
400 With a I<true> argument (using C<threads::running>), returns a list of all
401 non-joined, non-detached I<threads> objects that are still running.
403 With a I<false> argument (using C<threads::joinable>), returns a list of all
404 non-joined, non-detached I<threads> objects that have finished running (i.e.,
405 for which C<-E<gt>join()> will not I<block>).
407 =item $thr1->equal($thr2)
409 Tests if two threads objects are the same thread or not. This is overloaded
410 to the more natural forms:
412 if ($thr1 == $thr2) {
413 print("Threads are the same\n");
416 if ($thr1 != $thr2) {
417 print("Threads differ\n");
420 (Thread comparison is based on thread IDs.)
424 C<async> creates a thread to execute the block immediately following
425 it. This block is treated as an anonymous subroutine, and so must have a
426 semicolon after the closing brace. Like C<threads-E<gt>create()>, C<async>
427 returns a I<threads> object.
431 Threads are executed in an C<eval> context. This method will return C<undef>
432 if the thread terminates I<normally>. Otherwise, it returns the value of
433 C<$@> associated with the thread's execution status in its C<eval> context.
435 =item $thr->_handle()
437 This I<private> method returns a pointer (i.e., the memory location expressed
438 as an unsigned integer) to the internal thread structure associated with a
439 threads object. For Win32, this is a pointer to the C<HANDLE> value returned
440 by C<CreateThread> (i.e., C<HANDLE *>); for other platforms, it is a pointer
441 to the C<pthread_t> structure used in the C<pthread_create> call (i.e.,
444 This method is of no use for general Perl threads programming. Its intent is
445 to provide other (XS-based) thread modules with the capability to access, and
446 possibly manipulate, the underlying thread structure associated with a Perl
449 =item threads->_handle()
451 Class method that allows a thread to obtain its own I<handle>.
455 =head1 EXITING A THREAD
457 The usual method for terminating a thread is to
458 L<return()|perlfunc/"return EXPR"> from the entry point function with the
459 appropriate return value(s).
463 =item threads->exit()
465 If needed, a thread can be exited at any time by calling
466 C<threads-E<gt>exit()>. This will cause the thread to return C<undef> in a
467 scalar context, or the empty list in a list context.
469 When called from the I<main> thread, this behaves the same as C<exit(0)>.
471 =item threads->exit(status)
473 When called from a thread, this behaves like C<threads-E<gt>exit()> (i.e., the
474 exit status code is ignored).
476 When called from the I<main> thread, this behaves the same as C<exit(status)>.
480 Calling C<die()> in a thread indicates an abnormal exit for the thread. Any
481 C<$SIG{__DIE__}> handler in the thread will be called first, and then the
482 thread will exit with a warning message that will contain any arguments passed
483 in the C<die()> call.
487 Calling L<exit()|perlfunc/"exit EXPR"> inside a thread causes the whole
488 application to terminate. Because of this, the use of C<exit()> inside
489 threaded code, or in modules that might be used in threaded applications, is
490 strongly discouraged.
492 If C<exit()> really is needed, then consider using the following:
494 threads->exit() if threads->can('exit'); # Thread friendly
497 =item use threads 'exit' => 'threads_only'
499 This globally overrides the default behavior of calling C<exit()> inside a
500 thread, and effectively causes such calls to behave the same as
501 C<threads-E<gt>exit()>. In other words, with this setting, calling C<exit()>
502 causes only the thread to terminate.
504 Because of its global effect, this setting should not be used inside modules
507 The I<main> thread is unaffected by this setting.
509 =item threads->create({'exit' => 'thread_only'}, ...)
511 This overrides the default behavior of C<exit()> inside the newly created
514 =item $thr->set_thread_exit_only(boolean)
516 This can be used to change the I<exit thread only> behavior for a thread after
517 it has been created. With a I<true> argument, C<exit()> will cause only the
518 thread to exit. With a I<false> argument, C<exit()> will terminate the
521 The I<main> thread is unaffected by this call.
523 =item threads->set_thread_exit_only(boolean)
525 Class method for use inside a thread to change its own behavior for C<exit()>.
527 The I<main> thread is unaffected by this call.
533 The following boolean methods are useful in determining the I<state> of a
538 =item $thr->is_running()
540 Returns true if a thread is still running (i.e., if its entry point function
541 has not yet finished or exited).
543 =item $thr->is_joinable()
545 Returns true if the thread has finished running, is not detached and has not
546 yet been joined. In other words, the thread is ready to be joined, and a call
547 to C<$thr-E<gt>join()> will not I<block>.
549 =item $thr->is_detached()
551 Returns true if the thread has been detached.
553 =item threads->is_detached()
555 Class method that allows a thread to determine whether or not it is detached.
559 =head1 THREAD CONTEXT
561 As with subroutines, the type of value returned from a thread's entry point
562 function may be determined by the thread's I<context>: list, scalar or void.
563 The thread's context is determined at thread creation. This is necessary so
564 that the context is available to the entry point function via
565 L<wantarray()|perlfunc/"wantarray">. The thread may then specify a value of
566 the appropriate type to be returned from C<-E<gt>join()>.
568 =head2 Explicit context
570 Because thread creation and thread joining may occur in different contexts, it
571 may be desirable to state the context explicitly to the thread's entry point
572 function. This may be done by calling C<-E<gt>create()> with a hash reference
573 as the first argument:
575 my $thr = threads->create({'context' => 'list'}, \&foo);
577 my @results = $thr->join();
579 In the above, the threads object is returned to the parent thread in scalar
580 context, and the thread's entry point function C<foo> will be called in list
581 (array) context such that the parent thread can receive a list (array) from
582 the C<-E<gt>join()> call. (C<'array'> is synonymous with C<'list'>.)
584 Similarly, if you need the threads object, but your thread will not be
585 returning a value (i.e., I<void> context), you would do the following:
587 my $thr = threads->create({'context' => 'void'}, \&foo);
591 The context type may also be used as the I<key> in the hash reference followed
594 threads->create({'scalar' => 1}, \&foo);
596 my ($thr) = threads->list();
597 my $result = $thr->join();
599 =head2 Implicit context
601 If not explicitly stated, the thread's context is implied from the context
602 of the C<-E<gt>create()> call:
604 # Create thread in list context
605 my ($thr) = threads->create(...);
607 # Create thread in scalar context
608 my $thr = threads->create(...);
610 # Create thread in void context
611 threads->create(...);
613 =head2 $thr->wantarray()
615 This returns the thread's context in the same manner as
616 L<wantarray()|perlfunc/"wantarray">.
618 =head2 threads->wantarray()
620 Class method to return the current thread's context. This returns the same
621 value as running L<wantarray()|perlfunc/"wantarray"> inside the current
622 thread's entry point function.
624 =head1 THREAD STACK SIZE
626 The default per-thread stack size for different platforms varies
627 significantly, and is almost always far more than is needed for most
628 applications. On Win32, Perl's makefile explicitly sets the default stack to
629 16 MB; on most other platforms, the system default is used, which again may be
630 much larger than is needed.
632 By tuning the stack size to more accurately reflect your application's needs,
633 you may significantly reduce your application's memory usage, and increase the
634 number of simultaneously running threads.
636 Note that on Windows, address space allocation granularity is 64 KB,
637 therefore, setting the stack smaller than that on Win32 Perl will not save any
642 =item threads->get_stack_size();
644 Returns the current default per-thread stack size. The default is zero, which
645 means the system default stack size is currently in use.
647 =item $size = $thr->get_stack_size();
649 Returns the stack size for a particular thread. A return value of zero
650 indicates the system default stack size was used for the thread.
652 =item $old_size = threads->set_stack_size($new_size);
654 Sets a new default per-thread stack size, and returns the previous setting.
656 Some platforms have a minimum thread stack size. Trying to set the stack size
657 below this value will result in a warning, and the minimum stack size will be
660 Some Linux platforms have a maximum stack size. Setting too large of a stack
661 size will cause thread creation to fail.
663 If needed, C<$new_size> will be rounded up to the next multiple of the memory
664 page size (usually 4096 or 8192).
666 Threads created after the stack size is set will then either call
667 C<pthread_attr_setstacksize()> I<(for pthreads platforms)>, or supply the
668 stack size to C<CreateThread()> I<(for Win32 Perl)>.
670 (Obviously, this call does not affect any currently extant threads.)
672 =item use threads ('stack_size' => VALUE);
674 This sets the default per-thread stack size at the start of the application.
676 =item $ENV{'PERL5_ITHREADS_STACK_SIZE'}
678 The default per-thread stack size may be set at the start of the application
679 through the use of the environment variable C<PERL5_ITHREADS_STACK_SIZE>:
681 PERL5_ITHREADS_STACK_SIZE=1048576
682 export PERL5_ITHREADS_STACK_SIZE
683 perl -e'use threads; print(threads->get_stack_size(), "\n")'
685 This value overrides any C<stack_size> parameter given to C<use threads>. Its
686 primary purpose is to permit setting the per-thread stack size for legacy
687 threaded applications.
689 =item threads->create({'stack_size' => VALUE}, FUNCTION, ARGS)
691 To specify a particular stack size for any individual thread, call
692 C<-E<gt>create()> with a hash reference as the first argument:
694 my $thr = threads->create({'stack_size' => 32*4096}, \&foo, @args);
696 =item $thr2 = $thr1->create(FUNCTION, ARGS)
698 This creates a new thread (C<$thr2>) that inherits the stack size from an
699 existing thread (C<$thr1>). This is shorthand for the following:
701 my $stack_size = $thr1->get_stack_size();
702 my $thr2 = threads->create({'stack_size' => $stack_size}, FUNCTION, ARGS);
706 =head1 THREAD SIGNALLING
708 When safe signals is in effect (the default behavior - see L</"Unsafe signals">
709 for more details), then signals may be sent and acted upon by individual
714 =item $thr->kill('SIG...');
716 Sends the specified signal to the thread. Signal names and (positive) signal
717 numbers are the same as those supported by
718 L<kill()|perlfunc/"kill SIGNAL, LIST">. For example, 'SIGTERM', 'TERM' and
719 (depending on the OS) 15 are all valid arguments to C<-E<gt>kill()>.
721 Returns the thread object to allow for method chaining:
723 $thr->kill('SIG...')->join();
727 Signal handlers need to be set up in the threads for the signals they are
728 expected to act upon. Here's an example for I<cancelling> a thread:
734 # Thread 'cancellation' signal handler
735 $SIG{'KILL'} = sub { threads->exit(); };
741 my $thr = threads->create('thr_func');
745 # Signal the thread to terminate, and then detach
746 # it so that it will get cleaned up automatically
747 $thr->kill('KILL')->detach();
749 Here's another simplistic example that illustrates the use of thread
750 signalling in conjunction with a semaphore to provide rudimentary I<suspend>
751 and I<resume> capabilities:
754 use Thread::Semaphore;
760 # Thread 'suspend/resume' signal handler
762 $sema->down(); # Thread suspended
763 $sema->up(); # Thread resumes
769 # Create a semaphore and pass it to a thread
770 my $sema = Thread::Semaphore->new();
771 my $thr = threads->create('thr_func', $sema);
779 # Allow the thread to continue
782 CAVEAT: The thread signalling capability provided by this module does not
783 actually send signals via the OS. It I<emulates> signals at the Perl-level
784 such that signal handlers are called in the appropriate thread. For example,
785 sending C<$thr-E<gt>kill('STOP')> does not actually suspend a thread (or the
786 whole process), but does cause a C<$SIG{'STOP'}> handler to be called in that
787 thread (as illustrated above).
789 As such, signals that would normally not be appropriate to use in the
790 C<kill()> command (e.g., C<kill('KILL', $$)>) are okay to use with the
791 C<-E<gt>kill()> method (again, as illustrated above).
793 Correspondingly, sending a signal to a thread does not disrupt the operation
794 the thread is currently working on: The signal will be acted upon after the
795 current operation has completed. For instance, if the thread is I<stuck> on
796 an I/O call, sending it a signal will not cause the I/O call to be interrupted
797 such that the signal is acted up immediately.
799 Sending a signal to a terminated/finished thread is ignored.
805 =item Perl exited with active threads:
807 If the program exits without all threads having either been joined or
808 detached, then this warning will be issued.
810 NOTE: If the I<main> thread exits, then this warning cannot be suppressed
811 using C<no warnings 'threads';> as suggested below.
813 =item Thread creation failed: pthread_create returned #
815 See the appropriate I<man> page for C<pthread_create> to determine the actual
816 cause for the failure.
818 =item Thread # terminated abnormally: ...
820 A thread terminated in some manner other than just returning from its entry
821 point function, or by using C<threads-E<gt>exit()>. For example, the thread
822 may have terminated because of an error, or by using C<die>.
824 =item Using minimum thread stack size of #
826 Some platforms have a minimum thread stack size. Trying to set the stack size
827 below this value will result in the above warning, and the stack size will be
830 =item Thread creation failed: pthread_attr_setstacksize(I<SIZE>) returned 22
832 The specified I<SIZE> exceeds the system's maximum stack size. Use a smaller
833 value for the stack size.
837 If needed, thread warnings can be suppressed by using:
839 no warnings 'threads';
841 in the appropriate scope.
847 =item This Perl not built to support threads
849 The particular copy of Perl that you're trying to use was not built using the
850 C<useithreads> configuration option.
852 Having threads support requires all of Perl and all of the XS modules in the
853 Perl installation to be rebuilt; it is not just a question of adding the
854 L<threads> module (i.e., threaded and non-threaded Perls are binary
857 =item Cannot change stack size of an existing thread
859 The stack size of currently extant threads cannot be changed, therefore, the
860 following results in the above error:
862 $thr->set_stack_size($size);
864 =item Cannot signal threads without safe signals
866 Safe signals must be in effect to use the C<-E<gt>kill()> signalling method.
867 See L</"Unsafe signals"> for more details.
869 =item Unrecognized signal name: ...
871 The particular copy of Perl that you're trying to use does not support the
872 specified signal being used in a C<-E<gt>kill()> call.
876 =head1 BUGS AND LIMITATIONS
878 Before you consider posting a bug report, please consult, and possibly post a
879 message to the discussion forum to see if what you've encountered is a known
884 =item Thread-safe modules
886 See L<perlmod/"Making your module threadsafe"> when creating modules that may
887 be used in threaded applications, especially if those modules use non-Perl
890 =item Using non-thread-safe modules
892 Unfortunately, you may encounter Perl modules that are not I<thread-safe>.
893 For example, they may crash the Perl interpreter during execution, or may dump
894 core on termination. Depending on the module and the requirements of your
895 application, it may be possible to work around such difficulties.
897 If the module will only be used inside a thread, you can try loading the
898 module from inside the thread entry point function using C<require> (and
899 C<import> if needed):
903 require Unsafe::Module
904 # Unsafe::Module->import(...);
909 If the module is needed inside the I<main> thread, try modifying your
910 application so that the module is loaded (again using C<require> and
911 C<-E<gt>import()>) after any threads are started, and in such a way that no
912 other threads are started afterwards.
914 If the above does not work, or is not adequate for your application, then file
915 a bug report on L<http://rt.cpan.org/Public/> against the problematic module.
917 =item Memory consumption
919 On most systems, frequent and continual creation and destruction of threads
920 can lead to ever-increasing growth in the memory footprint of the Perl
921 interpreter. While it is simple to just launch threads and then
922 C<-E<gt>join()> or C<-E<gt>detach()> them, for long-lived applications, it is
923 better to maintain a pool of threads, and to reuse them for the work needed,
924 using L<queues|Thread::Queue> to notify threads of pending work. The CPAN
925 distribution of this module contains a simple example
926 (F<examples/pool_reuse.pl>) illustrating the creation, use and monitoring of a
927 pool of I<reusable> threads.
929 =item Current working directory
931 On all platforms except MSWin32, the setting for the current working directory
932 is shared among all threads such that changing it in one thread (e.g., using
933 C<chdir()>) will affect all the threads in the application.
935 On MSWin32, each thread maintains its own the current working directory
938 =item Environment variables
940 Currently, on all platforms except MSWin32, all I<system> calls (e.g., using
941 C<system()> or back-ticks) made from threads use the environment variable
942 settings from the I<main> thread. In other words, changes made to C<%ENV> in
943 a thread will not be visible in I<system> calls made by that thread.
945 To work around this, set environment variables as part of the I<system> call.
949 system("FOO=$msg; echo \$FOO"); # Outputs 'hello' to STDOUT
951 On MSWin32, each thread maintains its own set of environment variables.
953 =item Catching signals
955 Signals are I<caught> by the main thread (thread ID = 0) of a script.
956 Therefore, setting up signal handlers in threads for purposes other than
957 L</"THREAD SIGNALLING"> as documented above will not accomplish what is
960 This is especially true if trying to catch C<SIGALRM> in a thread. To handle
961 alarms in threads, set up a signal handler in the main thread, and then use
962 L</"THREAD SIGNALLING"> to relay the signal to the thread:
964 # Create thread with a task that may time out
965 my $thr = threads->create(sub {
968 $SIG{ALRM} = sub { die("Timeout\n"); };
973 if ($@ =~ /Timeout/) {
974 warn("Task in thread timed out\n");
978 # Set signal handler to relay SIGALRM to thread
979 $SIG{ALRM} = sub { $thr->kill('ALRM') };
981 ... # Main thread continues working
983 =item Parent-child threads
985 On some platforms, it might not be possible to destroy I<parent> threads while
986 there are still existing I<child> threads.
988 =item Creating threads inside special blocks
990 Creating threads inside C<BEGIN>, C<CHECK> or C<INIT> blocks should not be
991 relied upon. Depending on the Perl version and the application code, results
992 may range from success, to (apparently harmless) warnings of leaked scalar, or
993 all the way up to crashing of the Perl interpreter.
997 Since Perl 5.8.0, signals have been made safer in Perl by postponing their
998 handling until the interpreter is in a I<safe> state. See
999 L<perl58delta/"Safe Signals"> and L<perlipc/"Deferred Signals (Safe Signals)">
1002 Safe signals is the default behavior, and the old, immediate, unsafe
1003 signalling behavior is only in effect in the following situations:
1007 =item * Perl has been built with C<PERL_OLD_SIGNALS> (see C<perl -V>).
1009 =item * The environment variable C<PERL_SIGNALS> is set to C<unsafe> (see L<perlrun/"PERL_SIGNALS">).
1011 =item * The module L<Perl::Unsafe::Signals> is used.
1015 If unsafe signals is in effect, then signal handling is not thread-safe, and
1016 the C<-E<gt>kill()> signalling method cannot be used.
1018 =item Returning closures from threads
1020 Returning closures from threads should not be relied upon. Depending on the
1021 Perl version and the application code, results may range from success, to
1022 (apparently harmless) warnings of leaked scalar, or all the way up to crashing
1023 of the Perl interpreter.
1025 =item Returning objects from threads
1027 Returning objects from threads does not work. Depending on the classes
1028 involved, you may be able to work around this by returning a serialized
1029 version of the object (e.g., using L<Data::Dumper> or L<Storable>), and then
1030 reconstituting it in the joining thread. If you're using Perl 5.10.0 or
1031 later, and if the class supports L<shared objects|threads::shared/"OBJECTS">,
1032 you can pass them via L<shared queues|Thread::Queue>.
1034 =item END blocks in threads
1036 It is possible to add L<END blocks|perlmod/"BEGIN, UNITCHECK, CHECK, INIT and
1037 END"> to threads by using L<require|perlfunc/"require VERSION"> or
1038 L<eval|perlfunc/"eval EXPR"> with the appropriate code. These C<END> blocks
1039 will then be executed when the thread's interpreter is destroyed (i.e., either
1040 during a C<-E<gt>join()> call, or at program termination).
1042 However, calling any L<threads> methods in such an C<END> block will most
1043 likely I<fail> (e.g., the application may hang, or generate an error) due to
1044 mutexes that are needed to control functionality within the L<threads> module.
1046 For this reason, the use of C<END> blocks in threads is B<strongly>
1049 =item Open directory handles
1051 In perl 5.14 and higher, on systems other than Windows that do
1052 not support the C<fchdir> C function, directory handles (see
1053 L<opendir|perlfunc/"opendir DIRHANDLE,EXPR">) will not be copied to new
1054 threads. You can use the C<d_fchdir> variable in L<Config.pm|Config> to
1055 determine whether your system supports it.
1057 In prior perl versions, spawning threads with open directory handles would
1058 crash the interpreter.
1059 L<[perl #75154]|http://rt.perl.org/rt3/Public/Bug/Display.html?id=75154>
1061 =item Perl Bugs and the CPAN Version of L<threads>
1063 Support for threads extends beyond the code in this module (i.e.,
1064 F<threads.pm> and F<threads.xs>), and into the Perl interpreter itself. Older
1065 versions of Perl contain bugs that may manifest themselves despite using the
1066 latest version of L<threads> from CPAN. There is no workaround for this other
1067 than upgrading to the latest version of Perl.
1069 Even with the latest version of Perl, it is known that certain constructs
1070 with threads may result in warning messages concerning leaked scalars or
1071 unreferenced scalars. However, such warnings are harmless, and may safely be
1074 You can search for L<threads> related bug reports at
1075 L<http://rt.cpan.org/Public/>. If needed submit any new bugs, problems,
1076 patches, etc. to: L<http://rt.cpan.org/Public/Dist/Display.html?Name=threads>
1086 L<threads> Discussion Forum on CPAN:
1087 L<http://www.cpanforum.com/dist/threads>
1089 L<threads::shared>, L<perlthrtut>
1091 L<http://www.perl.com/pub/a/2002/06/11/threads.html> and
1092 L<http://www.perl.com/pub/a/2002/09/04/threads.html>
1094 Perl threads mailing list:
1095 L<http://lists.perl.org/list/ithreads.html>
1097 Stack size discussion:
1098 L<http://www.perlmonks.org/?node_id=532956>
1102 Artur Bergman E<lt>sky AT crucially DOT netE<gt>
1104 CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>
1108 threads is released under the same license as Perl.
1110 =head1 ACKNOWLEDGEMENTS
1112 Richard Soderberg E<lt>perl AT crystalflame DOT netE<gt> -
1113 Helping me out tons, trying to find reasons for races and other weird bugs!
1115 Simon Cozens E<lt>simon AT brecon DOT co DOT ukE<gt> -
1116 Being there to answer zillions of annoying questions
1118 Rocco Caputo E<lt>troc AT netrus DOT netE<gt>
1120 Vipul Ved Prakash E<lt>mail AT vipul DOT netE<gt> -
1121 Helping with debugging
1123 Dean Arnold E<lt>darnold AT presicient DOT comE<gt> -