threads(3)	      User Contributed Perl Documentation	    threads(3)



NAME
       threads - Perl interpreter-based threads

VERSION
       This document describes threads version 1.87

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 out: 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 the memory location of 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->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, perlthrtut

       <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 v5.16.3			  2013-05-29			    threads(3)