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

NAME
       threads - Perl interpreter-based	threads

VERSION
       This document describes threads version 2.25

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 makes 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
	   <https://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.

       Locales
	   Prior to Perl 5.28, locales could not be used with threads, due to
	   various race	conditions.  Starting in that release, on systems that
	   implement thread-safe locale	functions, threads can be used,	with
	   some	caveats.  This includes	Windows	starting with Visual Studio
	   2005, and systems compatible	with POSIX 2008.  See "Multi-threaded
	   operation" in perllocale.

	   Each	thread (except the main	thread)	is started using the C locale.
	   The main thread is started like all other Perl programs; see
	   "ENVIRONMENT" in perllocale.	 You can switch	locales	in any thread
	   as often as you like.

	   If you want to inherit the parent thread's locale, you can, in the
	   parent, set a variable like so:

	       $foo = POSIX::setlocale(LC_ALL, NULL);

	   and then pass to threads->create() a	sub that closes	over $foo.
	   Then, in the	child, you say

	       POSIX::setlocale(LC_ALL,	$foo);

	   Or you can use the facilities in threads::shared to pass $foo; or
	   if the environment hasn't changed, in the child, do

	       POSIX::setlocale(LC_ALL,	"");

       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.

       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.

       Identity	of objects returned from threads
	   When	a value	is returned from a thread through a "join" operation,
	   the value and everything that it references is copied across	to the
	   joining thread, in much the same way	that values are	copied upon
	   thread creation.  This works	fine for most kinds of value,
	   including arrays, hashes, and subroutines.  The copying recurses
	   through array elements, reference scalars, variables	closed over by
	   subroutines,	and other kinds	of reference.

	   However, everything referenced by the returned value	is a fresh
	   copy	in the joining thread, even if a returned object had in	the
	   child thread	been a copy of something that previously existed in
	   the parent thread.  After joining, the parent will therefore	have a
	   duplicate of	each such object.  This	sometimes matters, especially
	   if the object gets mutated; this can	especially matter for private
	   data	to which a returned subroutine provides	access.

       Returning blessed objects from threads
	   Returning blessed 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]
	   <https://rt.perl.org/rt3/Public/Bug/Display.html?id=75154>

       Detached	threads	and global destruction
	   If the main thread exits while there	are detached threads which are
	   still running, then Perl's global destruction phase is not executed
	   because otherwise certain global structures that control the
	   operation of	threads	and that are allocated in the main thread's
	   memory may get destroyed before the detached	thread is destroyed.

	   If you are using any	code that requires the execution of the	global
	   destruction phase for clean up (e.g., removing temp files), then do
	   not use detached threads, but rather	join all threads before
	   exiting the program.

       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
	   <https://rt.cpan.org/Public/>.  If needed submit any	new bugs,
	   problems, patches, etc. to:
	   <https://rt.cpan.org/Public/Dist/Display.html?Name=threads>

REQUIREMENTS
       Perl 5.8.0 or later

SEE ALSO
       threads on MetaCPAN: <https://metacpan.org/release/threads>

       Code repository for CPAN	distribution:
       <https://github.com/Dual-Life/threads>

       threads::shared,	perlthrtut

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

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

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

       Sample code in the examples directory of	this distribution on CPAN.

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 v5.32.0			  2020-06-14			    threads(3)

NAME | VERSION | WARNING | SYNOPSIS | DESCRIPTION | EXITING A THREAD | THREAD STATE | THREAD CONTEXT | THREAD STACK SIZE | THREAD SIGNALLING | WARNINGS | ERRORS | BUGS AND LIMITATIONS | REQUIREMENTS | SEE ALSO | AUTHOR | LICENSE | ACKNOWLEDGEMENTS

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