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threads(3pm)           Perl Programmers Reference Guide           threads(3pm)




NAME

       threads - Perl interpreter-based threads


VERSION

       This document describes threads version 2.07


WARNING

       The "interpreter-based threads" provided by Perl are not the fast,
       lightweight system for multitasking that one might expect or hope for.
       Threads are implemented in a way that make them easy to misuse.  Few
       people know how to use them correctly or will be able to provide help.

       The use of interpreter-based threads in perl is officially discouraged.


SYNOPSIS

           use threads ('yield',
                        'stack_size' => 64*4096,
                        'exit' => 'threads_only',
                        'stringify');

           sub start_thread {
               my @args = @_;
               print('Thread started: ', join(' ', @args), "\n");
           }
           my $thr = threads->create('start_thread', 'argument');
           $thr->join();

           threads->create(sub { print("I am a thread\n"); })->join();

           my $thr2 = async { foreach (@files) { ... } };
           $thr2->join();
           if (my $err = $thr2->error()) {
               warn("Thread error: $err\n");
           }

           # Invoke thread in list context (implicit) so it can return a list
           my ($thr) = threads->create(sub { return (qw/a b c/); });
           # or specify list context explicitly
           my $thr = threads->create({'context' => 'list'},
                                     sub { return (qw/a b c/); });
           my @results = $thr->join();

           $thr->detach();

           # Get a thread's object
           $thr = threads->self();
           $thr = threads->object($tid);

           # Get a thread's ID
           $tid = threads->tid();
           $tid = $thr->tid();
           $tid = "$thr";

           # Give other threads a chance to run
           threads->yield();
           yield();

           # Lists of non-detached threads
           my @threads = threads->list();
           my $thread_count = threads->list();

           my @running = threads->list(threads::running);
           my @joinable = threads->list(threads::joinable);

           # Test thread objects
           if ($thr1 == $thr2) {
               ...
           }

           # Manage thread stack size
           $stack_size = threads->get_stack_size();
           $old_size = threads->set_stack_size(32*4096);

           # Create a thread with a specific context and stack size
           my $thr = threads->create({ 'context'    => 'list',
                                       'stack_size' => 32*4096,
                                       'exit'       => 'thread_only' },
                                     \&foo);

           # Get thread's context
           my $wantarray = $thr->wantarray();

           # Check thread's state
           if ($thr->is_running()) {
               sleep(1);
           }
           if ($thr->is_joinable()) {
               $thr->join();
           }

           # Send a signal to a thread
           $thr->kill('SIGUSR1');

           # Exit a thread
           threads->exit();


DESCRIPTION

       Since Perl 5.8, thread programming has been available using a model
       called interpreter threads which provides a new Perl interpreter for
       each thread, and, by default, results in no data or state information
       being shared between threads.

       (Prior to Perl 5.8, 5005threads was available through the "Thread.pm"
       API.  This threading model has been deprecated, and was removed as of
       Perl 5.10.0.)

       As just mentioned, all variables are, by default, thread local.  To use
       shared variables, you need to also load threads::shared:

           use threads;
           use threads::shared;

       When loading threads::shared, you must "use threads" before you "use
       threads::shared".  ("threads" will emit a warning if you do it the
       other way around.)

       It is strongly recommended that you enable threads via "use threads" as
       early as possible in your script.

       If needed, scripts can be written so as to run on both threaded and
       non-threaded Perls:

           my $can_use_threads = eval 'use threads; 1';
           if ($can_use_threads) {
               # Do processing using threads
               ...
           } else {
               # Do it without using threads
               ...
           }

       $thr = threads->create(FUNCTION, ARGS)
           This will create a new thread that will begin execution with the
           specified entry point function, and give it the ARGS list as
           parameters.  It will return the corresponding threads object, or
           "undef" if thread creation failed.

           FUNCTION may either be the name of a function, an anonymous
           subroutine, or a code ref.

               my $thr = threads->create('func_name', ...);
                   # or
               my $thr = threads->create(sub { ... }, ...);
                   # or
               my $thr = threads->create(\&func, ...);

           The "->new()" method is an alias for "->create()".

       $thr->join()
           This will wait for the corresponding thread to complete its
           execution.  When the thread finishes, "->join()" will return the
           return value(s) of the entry point function.

           The context (void, scalar or list) for the return value(s) for
           "->join()" is determined at the time of thread creation.

               # Create thread in list context (implicit)
               my ($thr1) = threads->create(sub {
                                               my @results = qw(a b c);
                                               return (@results);
                                            });
               #   or (explicit)
               my $thr1 = threads->create({'context' => 'list'},
                                          sub {
                                               my @results = qw(a b c);
                                               return (@results);
                                          });
               # Retrieve list results from thread
               my @res1 = $thr1->join();

               # Create thread in scalar context (implicit)
               my $thr2 = threads->create(sub {
                                               my $result = 42;
                                               return ($result);
                                            });
               # Retrieve scalar result from thread
               my $res2 = $thr2->join();

               # Create a thread in void context (explicit)
               my $thr3 = threads->create({'void' => 1},
                                          sub { print("Hello, world\n"); });
               # Join the thread in void context (i.e., no return value)
               $thr3->join();

           See "THREAD CONTEXT" for more details.

           If the program exits without all threads having either been joined
           or detached, then a warning will be issued.

           Calling "->join()" or "->detach()" on an already joined thread will
           cause an error to be thrown.

       $thr->detach()
           Makes the thread unjoinable, and causes any eventual return value
           to be discarded.  When the program exits, any detached threads that
           are still running are silently terminated.

           If the program exits without all threads having either been joined
           or detached, then a warning will be issued.

           Calling "->join()" or "->detach()" on an already detached thread
           will cause an error to be thrown.

       threads->detach()
           Class method that allows a thread to detach itself.

       threads->self()
           Class method that allows a thread to obtain its own threads object.

       $thr->tid()
           Returns the ID of the thread.  Thread IDs are unique integers with
           the main thread in a program being 0, and incrementing by 1 for
           every thread created.

       threads->tid()
           Class method that allows a thread to obtain its own ID.

       "$thr"
           If you add the "stringify" import option to your "use threads"
           declaration, then using a threads object in a string or a string
           context (e.g., as a hash key) will cause its ID to be used as the
           value:

               use threads qw(stringify);

               my $thr = threads->create(...);
               print("Thread $thr started\n");  # Prints: Thread 1 started

       threads->object($tid)
           This will return the threads object for the active thread
           associated with the specified thread ID.  If $tid is the value for
           the current thread, then this call works the same as "->self()".
           Otherwise, returns "undef" if there is no thread associated with
           the TID, if the thread is joined or detached, if no TID is
           specified or if the specified TID is undef.

       threads->yield()
           This is a suggestion to the OS to let this thread yield CPU time to
           other threads.  What actually happens is highly dependent upon the
           underlying thread implementation.

           You may do "use threads qw(yield)", and then just use "yield()" in
           your code.

       threads->list()
       threads->list(threads::all)
       threads->list(threads::running)
       threads->list(threads::joinable)
           With no arguments (or using "threads::all") and in a list context,
           returns a list of all non-joined, non-detached threads objects.  In
           a scalar context, returns a count of the same.

           With a true argument (using "threads::running"), returns a list of
           all non-joined, non-detached threads objects that are still
           running.

           With a false argument (using "threads::joinable"), returns a list
           of all non-joined, non-detached threads objects that have finished
           running (i.e., for which "->join()" will not block).

       $thr1->equal($thr2)
           Tests if two threads objects are the same thread or not.  This is
           overloaded to the more natural forms:

               if ($thr1 == $thr2) {
                   print("Threads are the same\n");
               }
               # or
               if ($thr1 != $thr2) {
                   print("Threads differ\n");
               }

           (Thread comparison is based on thread IDs.)

       async BLOCK;
           "async" creates a thread to execute the block immediately following
           it.  This block is treated as an anonymous subroutine, and so must
           have a semicolon after the closing brace.  Like
           "threads->create()", "async" returns a threads object.

       $thr->error()
           Threads are executed in an "eval" context.  This method will return
           "undef" if the thread terminates normally.  Otherwise, it returns
           the value of $@ associated with the thread's execution status in
           its "eval" context.

       $thr->_handle()
           This private method returns a pointer (i.e., the memory location
           expressed as an unsigned integer) to the internal thread structure
           associated with a threads object.  For Win32, this is a pointer to
           the "HANDLE" value returned by "CreateThread" (i.e., "HANDLE *");
           for other platforms, it is a pointer to the "pthread_t" structure
           used in the "pthread_create" call (i.e., "pthread_t *").

           This method is of no use for general Perl threads programming.  Its
           intent is to provide other (XS-based) thread modules with the
           capability to access, and possibly manipulate, the underlying
           thread structure associated with a Perl thread.

       threads->_handle()
           Class method that allows a thread to obtain its own handle.


EXITING A THREAD

       The usual method for terminating a thread is to return() from the entry
       point function with the appropriate return value(s).

       threads->exit()
           If needed, a thread can be exited at any time by calling
           "threads->exit()".  This will cause the thread to return "undef" in
           a scalar context, or the empty list in a list context.

           When called from the main thread, this behaves the same as exit(0).

       threads->exit(status)
           When called from a thread, this behaves like "threads->exit()"
           (i.e., the exit status code is ignored).

           When called from the main thread, this behaves the same as
           "exit(status)".

       die()
           Calling "die()" in a thread indicates an abnormal exit for the
           thread.  Any $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 "die()" call.

       exit(status)
           Calling exit() inside a thread causes the whole application to
           terminate.  Because of this, the use of "exit()" inside threaded
           code, or in modules that might be used in threaded applications, is
           strongly discouraged.

           If "exit()" really is needed, then consider using the following:

               threads->exit() if threads->can('exit');   # Thread friendly
               exit(status);

       use threads 'exit' => 'threads_only'
           This globally overrides the default behavior of calling "exit()"
           inside a thread, and effectively causes such calls to behave the
           same as "threads->exit()".  In other words, with this setting,
           calling "exit()" causes only the thread to terminate.

           Because of its global effect, this setting should not be used
           inside modules or the like.

           The main thread is unaffected by this setting.

       threads->create({'exit' => 'thread_only'}, ...)
           This overrides the default behavior of "exit()" inside the newly
           created thread only.

       $thr->set_thread_exit_only(boolean)
           This can be used to change the exit thread only behavior for a
           thread after it has been created.  With a true argument, "exit()"
           will cause only the thread to exit.  With a false argument,
           "exit()" will terminate the application.

           The main thread is unaffected by this call.

       threads->set_thread_exit_only(boolean)
           Class method for use inside a thread to change its own behavior for
           "exit()".

           The main thread is unaffected by this call.


THREAD STATE

       The following boolean methods are useful in determining the state of a
       thread.

       $thr->is_running()
           Returns true if a thread is still running (i.e., if its entry point
           function has not yet finished or exited).

       $thr->is_joinable()
           Returns true if the thread has finished running, is not detached
           and has not yet been joined.  In other words, the thread is ready
           to be joined, and a call to "$thr->join()" will not block.

       $thr->is_detached()
           Returns true if the thread has been detached.

       threads->is_detached()
           Class method that allows a thread to determine whether or not it is
           detached.


THREAD CONTEXT

       As with subroutines, the type of value returned from a thread's entry
       point function may be determined by the thread's context:  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 wantarray().  The thread may then specify a value of the
       appropriate type to be returned from "->join()".

   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
       "->create()" 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 "foo" will be
       called in list (array) context such that the parent thread can receive
       a list (array) from the "->join()" call.  ('array' is synonymous with
       'list'.)

       Similarly, if you need the threads object, but your thread will not be
       returning a value (i.e., void context), you would do the following:

           my $thr = threads->create({'context' => 'void'}, \&foo);
           ...
           $thr->join();

       The context type may also be used as the key in the hash reference
       followed by a true value:

           threads->create({'scalar' => 1}, \&foo);
           ...
           my ($thr) = threads->list();
           my $result = $thr->join();

   Implicit context
       If not explicitly stated, the thread's context is implied from the
       context of the "->create()" 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(...);

   $thr->wantarray()
       This returns the thread's context in the same manner as wantarray().

   threads->wantarray()
       Class method to return the current thread's context.  This returns the
       same value as running wantarray() inside the current thread's entry
       point function.


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.

       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.

       $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.

       $old_size = threads->set_stack_size($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, $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
           "pthread_attr_setstacksize()" (for pthreads platforms), or supply
           the stack size to "CreateThread()" (for Win32 Perl).

           (Obviously, this call does not affect any currently extant
           threads.)

       use threads ('stack_size' => VALUE);
           This sets the default per-thread stack size at the start of the
           application.

       $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
           "PERL5_ITHREADS_STACK_SIZE":

               PERL5_ITHREADS_STACK_SIZE=1048576
               export PERL5_ITHREADS_STACK_SIZE
               perl -e'use threads; print(threads->get_stack_size(), "\n")'

           This value overrides any "stack_size" parameter given to "use
           threads".  Its primary purpose is to permit setting the per-thread
           stack size for legacy threaded applications.

       threads->create({'stack_size' => VALUE}, FUNCTION, ARGS)
           To specify a particular stack size for any individual thread, call
           "->create()" with a hash reference as the first argument:

               my $thr = threads->create({'stack_size' => 32*4096},
                                         \&foo, @args);

       $thr2 = $thr1->create(FUNCTION, ARGS)
           This creates a new thread ($thr2) that inherits the stack size from
           an existing thread ($thr1).  This is shorthand for the following:

               my $stack_size = $thr1->get_stack_size();
               my $thr2 = threads->create({'stack_size' => $stack_size},
                                          FUNCTION, ARGS);


THREAD SIGNALLING

       When safe signals is in effect (the default behavior - see "Unsafe
       signals" for more details), then signals may be sent and acted upon by
       individual threads.

       $thr->kill('SIG...');
           Sends the specified signal to the thread.  Signal names and
           (positive) signal numbers are the same as those supported by
           kill().  For example, 'SIGTERM', 'TERM' and (depending on the OS)
           15 are all valid arguments to "->kill()".

           Returns the thread object to allow for method chaining:

               $thr->kill('SIG...')->join();

       Signal handlers need to be set up in the threads for the signals they
       are expected to act upon.  Here's an example for cancelling 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
       suspend and resume 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 emulates signals at the Perl-
       level such that signal handlers are called in the appropriate thread.
       For example, sending "$thr->kill('STOP')" does not actually suspend a
       thread (or the whole process), but does cause a $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
       "kill()" command (e.g., "kill('KILL', $$)") are okay to use with the
       "->kill()" 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 stuck 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/finished thread is ignored.


WARNINGS

       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 main thread exits, then this warning cannot be
           suppressed using "no warnings 'threads';" as suggested below.

       Thread creation failed: pthread_create returned #
           See the appropriate man page for "pthread_create" to determine the
           actual cause for the failure.

       Thread # terminated abnormally: ...
           A thread terminated in some manner other than just returning from
           its entry point function, or by using "threads->exit()".  For
           example, the thread may have terminated because of an error, or by
           using "die".

       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.

       Thread creation failed: pthread_attr_setstacksize(SIZE) returned 22
           The specified SIZE exceeds the system's maximum stack size.  Use a
           smaller value for the stack size.

       If needed, thread warnings can be suppressed by using:

           no warnings 'threads';

       in the appropriate scope.


ERRORS

       This Perl not built to support threads
           The particular copy of Perl that you're trying to use was not built
           using the "useithreads" 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 threads module (i.e., threaded and non-
           threaded Perls are binary incompatible).

       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);

       Cannot signal threads without safe signals
           Safe signals must be in effect to use the "->kill()" signalling
           method.  See "Unsafe signals" for more details.

       Unrecognized signal name: ...
           The particular copy of Perl that you're trying to use does not
           support the specified signal being used in a "->kill()" call.


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.

       Thread-safe modules
           See "Making your module threadsafe" in perlmod when creating
           modules that may be used in threaded applications, especially if
           those modules use non-Perl data, or XS code.

       Using non-thread-safe modules
           Unfortunately, you may encounter Perl modules that are not thread-
           safe.  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 "require" (and "import" if needed):

               sub thr_func
               {
                   require Unsafe::Module
                   # Unsafe::Module->import(...);

                   ....
               }

           If the module is needed inside the main thread, try modifying your
           application so that the module is loaded (again using "require" and
           "->import()") 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 <http://rt.cpan.org/Public/>
           against the problematic module.

       Memory consumption
           On most systems, frequent and continual creation and destruction of
           threads can lead to ever-increasing growth in the memory footprint
           of the Perl interpreter.  While it is simple to just launch threads
           and then "->join()" or "->detach()" them, for long-lived
           applications, it is better to maintain a pool of threads, and to
           reuse them for the work needed, using queues to notify threads of
           pending work.  The CPAN distribution of this module contains a
           simple example (examples/pool_reuse.pl) illustrating the creation,
           use and monitoring of a pool of reusable threads.

       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 "chdir()") will affect all the threads
           in the application.

           On MSWin32, each thread maintains its own the current working
           directory setting.

       Environment variables
           Currently, on all platforms except MSWin32, all system calls (e.g.,
           using "system()" or back-ticks) made from threads use the
           environment variable settings from the main thread.  In other
           words, changes made to %ENV in a thread will not be visible in
           system calls made by that thread.

           To work around this, set environment variables as part of the
           system 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.

       Catching signals
           Signals are caught by the main thread (thread ID = 0) of a script.
           Therefore, setting up signal handlers in threads for purposes other
           than "THREAD SIGNALLING" as documented above will not accomplish
           what is intended.

           This is especially true if trying to catch "SIGALRM" in a thread.
           To handle alarms in threads, set up a signal handler in the main
           thread, and then use "THREAD SIGNALLING" to relay the signal to the
           thread:

             # Create thread with a task that may time out
             my $thr = threads->create(sub {
                 threads->yield();
                 eval {
                     $SIG{ALRM} = sub { die("Timeout\n"); };
                     alarm(10);
                     ...  # Do work here
                     alarm(0);
                 };
                 if ($@ =~ /Timeout/) {
                     warn("Task in thread timed out\n");
                 }
             };

             # Set signal handler to relay SIGALRM to thread
             $SIG{ALRM} = sub { $thr->kill('ALRM') };

             ... # Main thread continues working

       Parent-child threads
           On some platforms, it might not be possible to destroy parent
           threads while there are still existing child threads.

       Creating threads inside special blocks
           Creating threads inside "BEGIN", "CHECK" or "INIT" 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.

       Unsafe signals
           Since Perl 5.8.0, signals have been made safer in Perl by
           postponing their handling until the interpreter is in a safe state.
           See "Safe Signals" in perl58delta and "Deferred Signals (Safe
           Signals)" in perlipc for more details.

           Safe signals is the default behavior, and the old, immediate,
           unsafe signalling behavior is only in effect in the following
           situations:

           o   Perl has been built with "PERL_OLD_SIGNALS" (see "perl -V").

           o   The environment variable "PERL_SIGNALS" is set to "unsafe" (see
               "PERL_SIGNALS" in perlrun).

           o   The module Perl::Unsafe::Signals is used.

           If unsafe signals is in effect, then signal handling is not thread-
           safe, and the "->kill()" signalling method cannot be used.

       Returning closures from threads
           Returning closures from threads 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.

       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 Data::Dumper or
           Storable), and then reconstituting it in the joining thread.  If
           you're using Perl 5.10.0 or later, and if the class supports shared
           objects, you can pass them via shared queues.

       END blocks in threads
           It is possible to add END blocks to threads by using require or
           eval with the appropriate code.  These "END" blocks will then be
           executed when the thread's interpreter is destroyed (i.e., either
           during a "->join()" call, or at program termination).

           However, calling any threads methods in such an "END" block will
           most likely fail (e.g., the application may hang, or generate an
           error) due to mutexes that are needed to control functionality
           within the threads module.

           For this reason, the use of "END" blocks in threads is strongly
           discouraged.

       Open directory handles
           In perl 5.14 and higher, on systems other than Windows that do not
           support the "fchdir" C function, directory handles (see opendir)
           will not be copied to new threads. You can use the "d_fchdir"
           variable in Config.pm to determine whether your system supports it.

           In prior perl versions, spawning threads with open directory
           handles would crash the interpreter.  [perl #75154]
           <http://rt.perl.org/rt3/Public/Bug/Display.html?id=75154>

       Perl Bugs and the CPAN Version of threads
           Support for threads extends beyond the code in this module (i.e.,
           threads.pm and threads.xs), and into the Perl interpreter itself.
           Older versions of Perl contain bugs that may manifest themselves
           despite using the latest version of threads 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 threads related bug reports at
           <http://rt.cpan.org/Public/>.  If needed submit any new bugs,
           problems, patches, etc. to:
           <http://rt.cpan.org/Public/Dist/Display.html?Name=threads>


REQUIREMENTS

       Perl 5.8.0 or later


SEE ALSO

       threads Discussion Forum on CPAN:
       <http://www.cpanforum.com/dist/threads>

       threads::shared(3), perlthrtut(1)

       <http://www.perl.com/pub/a/2002/06/11/threads.html> and
       <http://www.perl.com/pub/a/2002/09/04/threads.html>

       Perl threads mailing list: <http://lists.perl.org/list/ithreads.html>

       Stack size discussion: <http://www.perlmonks.org/?node_id=532956>


AUTHOR

       Artur Bergman <sky AT crucially DOT net>

       CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>


LICENSE

       threads is released under the same license as Perl.


ACKNOWLEDGEMENTS

       Richard Soderberg <perl AT crystalflame DOT net> - Helping me out tons,
       trying to find reasons for races and other weird bugs!

       Simon Cozens <simon AT brecon DOT co DOT uk> - Being there to answer
       zillions of annoying questions

       Rocco Caputo <troc AT netrus DOT net>

       Vipul Ved Prakash <mail AT vipul DOT net> - Helping with debugging

       Dean Arnold <darnold AT presicient DOT com> - Stack size API



perl v5.24.0                      2016-05-02                      threads(3pm)

perl 5.24 - Generated Sat Nov 26 06:39:11 CST 2016
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