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NAME
    threads - Perl interpreter-based threads

VERSION
    This document describes threads version 2.26

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:

        *   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 *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

threads(3pm)
NAME VERSION WARNING SYNOPSIS DESCRIPTION EXITING A THREAD
die() exit(status)
THREAD STATE THREAD CONTEXT
Explicit context Implicit context wantarray() inside the current thread's entry point function.
THREAD STACK SIZE THREAD SIGNALLING WARNINGS ERRORS BUGS AND LIMITATIONS REQUIREMENTS SEE ALSO AUTHOR LICENSE ACKNOWLEDGEMENTS

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