thread, this behaves the same as C. =item threads->exit(status) When called from a thread, this behaves like Cexit()> (i.e., the exit status code is ignored). When called from the I

thread, this behaves the same as C. =item die() Calling C in a thread indicates an abnormal exit for the thread. Any C<$SIG{__DIE__}> handler in the thread will be called first, and then the thread will exit with a warning message that will contain any arguments passed in the C call. =item exit(status) Calling L inside a thread causes the whole application to terminate. Because of this, the use of C inside threaded code, or in modules that might be used in threaded applications, is strongly discouraged. If C really is needed, then consider using the following: threads->exit() if threads->can('exit'); # Thread friendly exit(status); =item use threads 'exit' => 'threads_only' This globally overrides the default behavior of calling C inside a thread, and effectively causes such calls to behave the same as Cexit()>. In other words, with this setting, calling C causes only the thread to terminate. Because of its global effect, this setting should not be used inside modules or the like. The I

thread is unaffected by this setting. =item threads->create({'exit' => 'thread_only'}, ...) This overrides the default behavior of C inside the newly created thread only. =item $thr->set_thread_exit_only(boolean) This can be used to change the I behavior for a thread after it has been created. With a I argument, C will cause only the thread to exit. With a I argument, C will terminate the application. The I

thread is unaffected by this call. =item threads->set_thread_exit_only(boolean) Class method for use inside a thread to change its own behavior for C. The I

thread is unaffected by this call. =back =head1 THREAD STATE The following boolean methods are useful in determining the I of a thread. =over =item $thr->is_running() Returns true if a thread is still running (i.e., if its entry point function has not yet finished or exited). =item $thr->is_joinable() Returns true if the thread has finished running, is not detached and has not yet been joined. In other words, the thread is ready to be joined, and a call to C<$thr-Ejoin()> will not I. =item $thr->is_detached() Returns true if the thread has been detached. =item threads->is_detached() Class method that allows a thread to determine whether or not it is detached. =back =head1 THREAD CONTEXT As with subroutines, the type of value returned from a thread's entry point function may be determined by the thread's I: list, scalar or void. The thread's context is determined at thread creation. This is necessary so that the context is available to the entry point function via L. The thread may then specify a value of the appropriate type to be returned from C<-Ejoin()>. =head2 Explicit context Because thread creation and thread joining may occur in different contexts, it may be desirable to state the context explicitly to the thread's entry point function. This may be done by calling C<-Ecreate()> with a hash reference as the first argument: my $thr = threads->create({'context' => 'list'}, \&foo); ... my @results = $thr->join(); In the above, the threads object is returned to the parent thread in scalar context, and the thread's entry point function C will be called in list (array) context such that the parent thread can receive a list (array) from the C<-Ejoin()> call. (C<'array'> is synonymous with C<'list'>.) Similarly, if you need the threads object, but your thread will not be returning a value (i.e., I context), you would do the following: my $thr = threads->create({'context' => 'void'}, \&foo); ... $thr->join(); The context type may also be used as the I in the hash reference followed by a I value: threads->create({'scalar' => 1}, \&foo); ... my ($thr) = threads->list(); my $result = $thr->join(); =head2 Implicit context If not explicitly stated, the thread's context is implied from the context of the C<-Ecreate()> call: # Create thread in list context my ($thr) = threads->create(...); # Create thread in scalar context my $thr = threads->create(...); # Create thread in void context threads->create(...); =head2 $thr->wantarray() This returns the thread's context in the same manner as L. =head2 threads->wantarray() Class method to return the current thread's context. This returns the same value as running L inside the current thread's entry point function. =head1 THREAD STACK SIZE The default per-thread stack size for different platforms varies significantly, and is almost always far more than is needed for most applications. On Win32, Perl's makefile explicitly sets the default stack to 16 MB; on most other platforms, the system default is used, which again may be much larger than is needed. By tuning the stack size to more accurately reflect your application's needs, you may significantly reduce your application's memory usage, and increase the number of simultaneously running threads. Note that on Windows, address space allocation granularity is 64 KB, therefore, setting the stack smaller than that on Win32 Perl will not save any more memory. =over =item threads->get_stack_size(); Returns the current default per-thread stack size. The default is zero, which means the system default stack size is currently in use. =item $size = $thr->get_stack_size(); Returns the stack size for a particular thread. A return value of zero indicates the system default stack size was used for the thread. =item $old_size = threads->set_stack_size($new_size); Sets a new default per-thread stack size, and returns the previous setting. Some platforms have a minimum thread stack size. Trying to set the stack size below this value will result in a warning, and the minimum stack size will be used. Some Linux platforms have a maximum stack size. Setting too large of a stack size will cause thread creation to fail. If needed, C<$new_size> will be rounded up to the next multiple of the memory page size (usually 4096 or 8192). Threads created after the stack size is set will then either call C I<(for pthreads platforms)>, or supply the stack size to C I<(for Win32 Perl)>. (Obviously, this call does not affect any currently extant threads.) =item use threads ('stack_size' => VALUE); This sets the default per-thread stack size at the start of the application. =item $ENV{'PERL5_ITHREADS_STACK_SIZE'} The default per-thread stack size may be set at the start of the application through the use of the environment variable C: PERL5_ITHREADS_STACK_SIZE=1048576 export PERL5_ITHREADS_STACK_SIZE perl -e'use threads; print(threads->get_stack_size(), "\n")' This value overrides any C parameter given to C. Its primary purpose is to permit setting the per-thread stack size for legacy threaded applications. =item threads->create({'stack_size' => VALUE}, FUNCTION, ARGS) To specify a particular stack size for any individual thread, call C<-Ecreate()> with a hash reference as the first argument: my $thr = threads->create({'stack_size' => 32*4096}, \&foo, @args); =item $thr2 = $thr1->create(FUNCTION, ARGS) This creates a new thread (C<$thr2>) that inherits the stack size from an existing thread (C<$thr1>). This is shorthand for the following: my $stack_size = $thr1->get_stack_size(); my $thr2 = threads->create({'stack_size' => $stack_size}, FUNCTION, ARGS); =back =head1 THREAD SIGNALLING When safe signals is in effect (the default behavior - see L for more details), then signals may be sent and acted upon by individual threads. =over 4 =item $thr->kill('SIG...'); Sends the specified signal to the thread. Signal names and (positive) signal numbers are the same as those supported by L. For example, 'SIGTERM', 'TERM' and (depending on the OS) 15 are all valid arguments to C<-Ekill()>. Returns the thread object to allow for method chaining: $thr->kill('SIG...')->join(); =back Signal handlers need to be set up in the threads for the signals they are expected to act upon. Here's an example for I a thread: use threads; sub thr_func { # Thread 'cancellation' signal handler $SIG{'KILL'} = sub { threads->exit(); }; ... } # Create a thread my $thr = threads->create('thr_func'); ... # Signal the thread to terminate, and then detach # it so that it will get cleaned up automatically $thr->kill('KILL')->detach(); Here's another simplistic example that illustrates the use of thread signalling in conjunction with a semaphore to provide rudimentary I and I capabilities: use threads; use Thread::Semaphore; sub thr_func { my $sema = shift; # Thread 'suspend/resume' signal handler $SIG{'STOP'} = sub { $sema->down(); # Thread suspended $sema->up(); # Thread resumes }; ... } # Create a semaphore and pass it to a thread my $sema = Thread::Semaphore->new(); my $thr = threads->create('thr_func', $sema); # Suspend the thread $sema->down(); $thr->kill('STOP'); ... # Allow the thread to continue $sema->up(); CAVEAT: The thread signalling capability provided by this module does not actually send signals via the OS. It I signals at the Perl-level such that signal handlers are called in the appropriate thread. For example, sending C<$thr-Ekill('STOP')> does not actually suspend a thread (or the whole process), but does cause a C<$SIG{'STOP'}> handler to be called in that thread (as illustrated above). As such, signals that would normally not be appropriate to use in the C command (e.g., C) are okay to use with the C<-Ekill()> method (again, as illustrated above). Correspondingly, sending a signal to a thread does not disrupt the operation the thread is currently working on: The signal will be acted upon after the current operation has completed. For instance, if the thread is I on an I/O call, sending it a signal will not cause the I/O call to be interrupted such that the signal is acted up immediately. Sending a signal to a terminated thread is ignored. =head1 WARNINGS =over 4 =item Perl exited with active threads: If the program exits without all threads having either been joined or detached, then this warning will be issued. NOTE: If the I

thread exits, then this warning cannot be suppressed using C as suggested below. =item Thread creation failed: pthread_create returned # See the appropriate I page for C to determine the actual cause for the failure. =item Thread # terminated abnormally: ... A thread terminated in some manner other than just returning from its entry point function, or by using Cexit()>. For example, the thread may have terminated because of an error, or by using C. =item Using minimum thread stack size of # Some platforms have a minimum thread stack size. Trying to set the stack size below this value will result in the above warning, and the stack size will be set to the minimum. =item Thread creation failed: pthread_attr_setstacksize(I) returned 22 The specified I exceeds the system's maximum stack size. Use a smaller value for the stack size. =back If needed, thread warnings can be suppressed by using: no warnings 'threads'; in the appropriate scope. =head1 ERRORS =over 4 =item This Perl not built to support threads The particular copy of Perl that you're trying to use was not built using the C configuration option. Having threads support requires all of Perl and all of the XS modules in the Perl installation to be rebuilt; it is not just a question of adding the L module (i.e., threaded and non-threaded Perls are binary incompatible.) =item Cannot change stack size of an existing thread The stack size of currently extant threads cannot be changed, therefore, the following results in the above error: $thr->set_stack_size($size); =item Cannot signal threads without safe signals Safe signals must be in effect to use the C<-Ekill()> signalling method. See L for more details. =item Unrecognized signal name: ... The particular copy of Perl that you're trying to use does not support the specified signal being used in a C<-Ekill()> call. =back =head1 BUGS AND LIMITATIONS Before you consider posting a bug report, please consult, and possibly post a message to the discussion forum to see if what you've encountered is a known problem. =over =item Thread-safe modules See L when creating modules that may be used in threaded applications, especially if those modules use non-Perl data, or XS code. =item Using non-thread-safe modules Unfortunately, you may encounter Perl modules that are not I. For example, they may crash the Perl interpreter during execution, or may dump core on termination. Depending on the module and the requirements of your application, it may be possible to work around such difficulties. If the module will only be used inside a thread, you can try loading the module from inside the thread entry point function using C (and C if needed): sub thr_func { require Unsafe::Module # Unsafe::Module->import(...); .... } If the module is needed inside the I

thread, try modifying your application so that the module is loaded (again using C and C<-Eimport()>) after any threads are started, and in such a way that no other threads are started afterwards. If the above does not work, or is not adequate for your application, then file a bug report on L against the problematic module. =item Current working directory On all platforms except MSWin32, the setting for the current working directory is shared among all threads such that changing it in one thread (e.g., using C) will affect all the threads in the application. On MSWin32, each thread maintains its own the current working directory setting. =item Environment variables Currently, on all platforms except MSWin32, all I calls (e.g., using C or back-ticks) made from threads use the environment variable settings from the I

thread. In other words, changes made to C<%ENV> in a thread will not be visible in I calls made by that thread. To work around this, set environment variables as part of the I call. For example: my $msg = 'hello'; system("FOO=$msg; echo \$FOO"); # Outputs 'hello' to STDOUT On MSWin32, each thread maintains its own set of environment variables. =item Parent-child threads On some platforms, it might not be possible to destroy I threads while there are still existing I threads. =item Creating threads inside special blocks Creating threads inside C, C or C blocks should not be relied upon. Depending on the Perl version and the application code, results may range from success, to (apparently harmless) warnings of leaked scalar, or all the way up to crashing of the Perl interpreter. =item Unsafe signals Since Perl 5.8.0, signals have been made safer in Perl by postponing their handling until the interpreter is in a I state. See L and L for more details. Safe signals is the default behavior, and the old, immediate, unsafe signalling behavior is only in effect in the following situations: =over 4 =item * Perl has been built with C (see C). =item * The environment variable C is set to C (see L). =item * The module L is used. =back If unsafe signals is in effect, then signal handling is not thread-safe, and the C<-Ekill()> signalling method cannot be used. =item Returning closures from threads Returning closures from threads should not be relied upon. Depending of the Perl version and the application code, results may range from success, to (apparently harmless) warnings of leaked scalar, or all the way up to crashing of the Perl interpreter. =item Returning objects from threads Returning objects from threads does not work. Depending on the classes involved, you may be able to work around this by returning a serialized version of the object (e.g., using L or L), and then reconstituting it in the joining thread. If you're using Perl 5.10.0 or later, and if the class supports L, you can pass them via L. =item END blocks in threads It is possible to add L to threads by using L or L with the appropriate code. These C blocks will then be executed when the thread's interpreter is destroyed (i.e., either during a C<-Ejoin()> call, or at program termination). However, calling any L methods in such an C block will most likely I (e.g., the application may hang, or generate an error) due to mutexes that are needed to control functionality within the L module. For this reason, the use of C blocks in threads is B discouraged. =item Perl Bugs and the CPAN Version of L Support for threads extends beyond the code in this module (i.e., F and F), and into the Perl interpreter itself. Older versions of Perl contain bugs that may manifest themselves despite using the latest version of L from CPAN. There is no workaround for this other than upgrading to the latest version of Perl. Even with the latest version of Perl, it is known that certain constructs with threads may result in warning messages concerning leaked scalars or unreferenced scalars. However, such warnings are harmless, and may safely be ignored. You can search for L related bug reports at L. If needed submit any new bugs, problems, patches, etc. to: L =back =head1 REQUIREMENTS Perl 5.8.0 or later =head1 SEE ALSO L Discussion Forum on CPAN: L Annotated POD for L: L Source repository: L L, L L and L Perl threads mailing list: L Stack size discussion: L =head1 AUTHOR Artur Bergman Esky AT crucially DOT netE CPAN version produced by Jerry D. Hedden =head1 LICENSE threads is released under the same license as Perl. =head1 ACKNOWLEDGEMENTS Richard Soderberg Eperl AT crystalflame DOT netE - Helping me out tons, trying to find reasons for races and other weird bugs! Simon Cozens Esimon AT brecon DOT co DOT ukE - Being there to answer zillions of annoying questions Rocco Caputo Etroc AT netrus DOT netE Vipul Ved Prakash Email AT vipul DOT netE - Helping with debugging Dean Arnold Edarnold AT presicient DOT comE - Stack size API =cut