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

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

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

NNAAMMEE #

 threads - Perl interpreter-based threads

VVEERRSSIIOONN #

 This document describes threads version 2.27

WWAARRNNIINNGG #

 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.

SSYYNNOOPPSSIISS #

     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();

DDEESSCCRRIIPPTTIIOONN #

 Since Perl 5.8, thread programming has been available using a model
 called _i_n_t_e_r_p_r_e_t_e_r _t_h_r_e_a_d_s 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, _5_0_0_5_t_h_r_e_a_d_s 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 _A_R_G_S list as
     parameters.  It will return the corresponding threads object, or
     "undef" if thread creation failed.

     _F_U_N_C_T_I_O_N 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->jjooiinn(())
     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->ddeettaacchh(())
     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->ddeettaacchh(())
     Class method that allows a thread to detach itself.

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

 $thr->ttiidd(())
     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->ttiidd(())
     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 _t_h_r_e_a_d_s object for the _a_c_t_i_v_e 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->yyiieelldd(())
     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->lliisstt(())
 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 _t_h_r_e_a_d_s objects.  In a
     scalar context, returns a count of the same.

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

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

 $thr1->equal($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 _t_h_r_e_a_d_s object.

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

 $thr->__hhaannddllee(())
     This _p_r_i_v_a_t_e 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->__hhaannddllee(())
     Class method that allows a thread to obtain its own _h_a_n_d_l_e.

EEXXIITTIINNGG AA TTHHRREEAADD #

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

 threads->eexxiitt(())
     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 _m_a_i_n 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 _m_a_i_n thread, this behaves the same as
     "exit(status)".

 ddiiee(())
     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 eexxiitt(()) 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 _m_a_i_n 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 _e_x_i_t _t_h_r_e_a_d _o_n_l_y behavior for a thread
     after it has been created.  With a _t_r_u_e argument, "exit()" will cause
     only the thread to exit.  With a _f_a_l_s_e argument, "exit()" will
     terminate the application.

     The _m_a_i_n 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 _m_a_i_n thread is unaffected by this call.

TTHHRREEAADD SSTTAATTEE #

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

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

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

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

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

TTHHRREEAADD CCOONNTTEEXXTT #

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

EExxpplliicciitt ccoonntteexxtt 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., _v_o_i_d context), you would do the following:

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

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

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

IImmpplliicciitt ccoonntteexxtt 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(...);

$$tthhrr-->>wwaannttaarrrraayy(()) This returns the thread’s context in the same manner as wwaannttaarrrraayy(()).

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

TTHHRREEAADD SSTTAACCKK SSIIZZEE #

 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->ggeett__ssttaacckk__ssiizzee(());
     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->ggeett__ssttaacckk__ssiizzee(());
     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()" _(_f_o_r _p_t_h_r_e_a_d_s _p_l_a_t_f_o_r_m_s_), or supply the
     stack size to "CreateThread()" _(_f_o_r _W_i_n_3_2 _P_e_r_l_).

     (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); #

TTHHRREEAADD SSIIGGNNAALLLLIINNGG #

 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 kkiillll(()).
     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 _c_a_n_c_e_l_l_i_n_g 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 _s_u_s_p_e_n_d
 and _r_e_s_u_m_e 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 _e_m_u_l_a_t_e_s 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 _s_t_u_c_k 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.

WWAARRNNIINNGGSS #

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

 Thread creation failed: pthread_create returned #
     See the appropriate _m_a_n 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(_S_I_Z_E) returned 22
     The specified _S_I_Z_E 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.

EERRRROORRSS #

 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.

BBUUGGSS AANNDD LLIIMMIITTAATTIIOONNSS #

 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 _t_h_r_e_a_d_-
     _s_a_f_e.  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 _m_a_i_n 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
     (_e_x_a_m_p_l_e_s_/_p_o_o_l___r_e_u_s_e_._p_l) illustrating the creation, use and
     monitoring of a pool of _r_e_u_s_a_b_l_e 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->ccrreeaattee(()) 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 _s_y_s_t_e_m calls (e.g.,
     using "system()" or back-ticks) made from threads use the environment
     variable settings from the _m_a_i_n thread.  In other words, changes made
     to %ENV in a thread will not be visible in _s_y_s_t_e_m calls made by that
     thread.

     To work around this, set environment variables as part of the _s_y_s_t_e_m
     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 _c_a_u_g_h_t 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 _p_a_r_e_n_t threads
     while there are still existing _c_h_i_l_d 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 _s_a_f_e 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:

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

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

     •   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 _f_a_i_l (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 ssttrroonnggllyy
     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.,
     _t_h_r_e_a_d_s_._p_m and _t_h_r_e_a_d_s_._x_s), 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>

RREEQQUUIIRREEMMEENNTTSS #

 Perl 5.8.0 or later

SSEEEE AALLSSOO #

 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 _e_x_a_m_p_l_e_s directory of this distribution on CPAN.

AAUUTTHHOORR #

 Artur Bergman <sky AT crucially DOT net>

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

LLIICCEENNSSEE #

 threads is released under the same license as Perl.

AACCKKNNOOWWLLEEDDGGEEMMEENNTTSS #

 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 v5.36.3 2023-02-15 threads(3p)