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



NAME
       threads - Perl interpreter-based threads

VERSION
       This document describes threads version 1.86

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, 5__5threads was available through the "Thread.pm" API.  This threading model has
       been deprecated, and was removed as of Perl 5.10.0.)

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

           use threads;
           use threads::shared;

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

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

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

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

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

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

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

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

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

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

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

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

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

           See "THREAD CONTEXT" for more details.

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

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

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

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

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

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

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

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

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

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

               use threads qw(stringify);

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

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

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

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

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

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

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

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

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

           (Thread comparison is based on thread IDs.)

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

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

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

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

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

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

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

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

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

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

       die()
           Calling "die()" in a thread indicates an abnormal exit for the thread.  Any $SIG{__DIE__} handler
           in the thread will be called first, and then the thread will exit with a warning message that
           will contain any arguments passed in the "die()" call.

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

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

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

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

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

           The main thread is unaffected by this setting.

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

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

           The main thread is unaffected by this call.

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

           The main thread is unaffected by this call.

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

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

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

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

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

THREAD CONTEXT
       As with subroutines, the type of value returned from a thread's entry point function may be
       determined by the thread's context:  list, scalar or void.  The thread's context is determined at
       thread creation.  This is necessary so that the context is available to the entry point function via
       wantarray().  The thread may then specify a value of the appropriate type to be returned from
       "->join()".

   Explicit context
       Because thread creation and thread joining may occur in different contexts, it may be desirable to
       state the context explicitly to the thread's entry point function.  This may be done by calling
       "->create()" with a hash reference as the first argument:

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

       In the above, the threads object is returned to the parent thread in scalar context, and the thread's
       entry point function "foo" will be called in list (array) context such that the parent thread can
       receive a list (array) from the "->join()" call.  ('array' is synonymous with 'list'.)

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

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

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

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

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

           # Create thread in list context
           my ($thr) = threads->create(...);

           # Create thread in scalar context
           my $thr = threads->create(...);

           # Create thread in void context
           threads->create(...);

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

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

THREAD STACK SIZE
       The default per-thread stack size for different platforms varies significantly, and is almost always
       far more than is needed for most applications.  On Win32, Perl's makefile explicitly sets the default
       stack to 16 MB; on most other platforms, the system default is used, which again may be much larger
       than is needed.

       By tuning the stack size to more accurately reflect your application's needs, you may significantly
       reduce your application's memory usage, and increase the number of simultaneously running threads.

       Note that on Windows, address space allocation granularity is 64 KB, therefore, setting the stack
       smaller than that on Win32 Perl will not save any more memory.

       threads->get_stack_size();
           Returns the current default per-thread stack size.  The default is zero, which means the system
           default stack size is currently in use.

       $size = $thr->get_stack_size();
           Returns the stack size for a particular thread.  A return value of zero indicates the system
           default stack size was used for the thread.

       $old_size = threads->set_stack_size($new_size);
           Sets a new default per-thread stack size, and returns the previous setting.

           Some platforms have a minimum thread stack size.  Trying to set the stack size below this value
           will result in a warning, and the minimum stack size will be used.

           Some Linux platforms have a maximum stack size.  Setting too large of a stack size will cause
           thread creation to fail.

           If needed, $new_size will be rounded up to the next multiple of the memory page size (usually
           4096 or 8192).

           Threads created after the stack size is set will then either call "pthread_attr_setstacksize()"
           (for pthreads platforms), or supply the stack size to "CreateThread()" (for Win32 Perl).

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

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

       $ENV{'PERL5_ITHREADS_STACK_SIZE'}
           The default per-thread stack size may be set at the start of the application through the use of
           the environment variable "PERL5_ITHREADS_STACK_SIZE":

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

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

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

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

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

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

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

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

           Returns the thread object to allow for method chaining:

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

       Signal handlers need to be set up in the threads for the signals they are expected to act upon.
       Here's an example for cancelling a thread:

           use threads;

           sub thr_func
           {
               # Thread 'cancellation' signal handler
               $SIG{'KILL'} = sub { threads->exit(); };

               ...
           }

           # Create a thread
           my $thr = threads->create('thr_func');

           ...

           # Signal the thread to terminate, and then detach
           # it so that it will get cleaned up automatically
           $thr->kill('KILL')->detach();

       Here's another simplistic example that illustrates the use of thread signalling in conjunction with a
       semaphore to provide rudimentary suspend and resume capabilities:

           use threads;
           use Thread::Semaphore;

           sub thr_func
           {
               my $sema = shift;

               # Thread 'suspend/resume' signal handler
               $SIG{'STOP'} = sub {
                   $sema->down();      # Thread suspended
                   $sema->up();        # Thread resumes
               };

               ...
           }

           # Create a semaphore and pass it to a thread
           my $sema = Thread::Semaphore->new();
           my $thr = threads->create('thr_func', $sema);

           # Suspend the thread
           $sema->down();
           $thr->kill('STOP');

           ...

           # Allow the thread to continue
           $sema->up();

       CAVEAT:  The thread signalling capability provided by this module does not actually send signals via
       the OS.  It emulates signals at the Perl-level such that signal handlers are called in the
       appropriate thread.  For example, sending "$thr->kill('STOP')" does not actually suspend a thread (or
       the whole process), but does cause a $SIG{'STOP'} handler to be called in that thread (as illustrated
       above).

       As such, signals that would normally not be appropriate to use in the "kill()" command (e.g.,
       "kill('KILL', $$)") are okay to use with the "->kill()" method (again, as illustrated above).

       Correspondingly, sending a signal to a thread does not disrupt the operation the thread is currently
       working on:  The signal will be acted upon after the current operation has completed.  For instance,
       if the thread is stuck on an I/O call, sending it a signal will not cause the I/O call to be
       interrupted such that the signal is acted up immediately.

       Sending a signal to a terminated 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 nonthreaded
           threaded Perls are binary incompatible.)

       Cannot change stack size of an existing thread
           The stack size of currently extant threads cannot be changed, therefore, the following results in
           the above error:

               $thr->set_stack_size($size);

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

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

BUGS AND LIMITATIONS
       Before you consider posting a bug report, please consult, and possibly post a message to the
       discussion forum to see if what you've encountered is a known problem.

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

       Using non-thread-safe modules
           Unfortunately, you may encounter Perl modules that are not thread-safe.  For example, they may
           crash the Perl interpreter during execution, or may dump core on termination.  Depending on the
           module and the requirements of your application, it may be possible to work around such
           difficulties.

           If the module will only be used inside a thread, you can try loading the module from inside the
           thread entry point function using "require" (and "import" if needed):

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

                   ....
               }

           If the module is needed inside the main thread, try modifying your application so that the module
           is loaded (again using "require" and "->import()") after any threads are started, and in such a
           way that no other threads are started afterwards.

           If the above does not work, or is not adequate for your application, then file a bug report on
           <http://rt.cpan.org/Public/> against the problematic module.

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

       Current working directory
           On all platforms except MSWin32, the setting for the current working directory is shared among
           all threads such that changing it in one thread (e.g., using "chdir()") will affect all the
           threads in the application.

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

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

           To work around this, set environment variables as part of the system call.  For example:

               my $msg = 'hello';
               system("FOO=$msg; echo \$FOO");   # Outputs 'hello' to STDOUT

           On MSWin32, each thread maintains its own set of environment variables.

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

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

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

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

             ... # Main thread continues working

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

       Creating threads inside special blocks
           Creating threads inside "BEGIN", "CHECK" or "INIT" blocks should not be relied upon.  Depending
           on the Perl version and the application code, results may range from success, to (apparently
           harmless) warnings of leaked scalar, or all the way up to crashing of the Perl interpreter.

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

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

           •   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.

       Returning closures from threads
           Returning closures from threads should not be relied upon.  Depending of the Perl version and the
           application code, results may range from success, to (apparently harmless) warnings of leaked
           scalar, or all the way up to crashing of the Perl interpreter.

       Returning objects from threads
           Returning objects from threads does not work.  Depending on the classes involved, you may be able
           to work around this by returning a serialized version of the object (e.g., using Data::Dumper or
           Storable), and then reconstituting it in the joining thread.  If you're using Perl 5.10.0 or
           later, and if the class supports shared objects, you can pass them via shared queues.

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

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

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

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

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

       Perl Bugs and the CPAN Version of threads
           Support for threads extends beyond the code in this module (i.e., threads.pm and threads.xs), and
           into the Perl interpreter itself.  Older versions of Perl contain bugs that may manifest
           themselves despite using the latest version of threads from CPAN.  There is no workaround for
           this other than upgrading to the latest version of Perl.

           Even with the latest version of Perl, it is known that certain constructs with threads may result
           in warning messages concerning leaked scalars or unreferenced scalars.  However, such warnings
           are harmless, and may safely be ignored.

           You can search for threads related bug reports at <http://rt.cpan.org/Public/>.  If needed submit
           any new bugs, problems, patches, etc. to:
           <http://rt.cpan.org/Public/Dist/Display.html?Name=threads>

REQUIREMENTS
       Perl 5.8.0 or later

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

       threads::shared, perlthrtut

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

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

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

AUTHOR
       Artur Bergman <sky AT crucially DOT net>

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

LICENSE
       threads is released under the same license as Perl.

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

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

       Rocco Caputo <troc AT netrus DOT net>

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

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



perl v5.16.2                                     2012-10-11                                     threads(3pm)