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



NAME
       Thread - Manipulate threads in Perl (for old code only)

DEPRECATED
       The "Thread" module served as the frontend to the old-style thread model, called 5__5threads, that
       was introduced in release 5.005.  That model was deprecated, and has been removed in version 5.10.

       For old code and interim backwards compatibility, the "Thread" module has been reworked to function
       as a frontend for the new interpreter threads (ithreads) model.  However, some previous functionality
       is not available.  Further, the data sharing models between the two thread models are completely
       different, and anything to do with data sharing has to be thought differently.  With ithreads, you
       must explicitly "share()" variables between the threads.

       You are strongly encouraged to migrate any existing threaded code to the new model (i.e., use the
       "threads" and "threads::shared" modules) as soon as possible.

HISTORY
       In Perl 5.005, the thread model was that all data is implicitly shared, and shared access to data has
       to be explicitly synchronized.  This model is called 5__5threads.

       In Perl 5.6, a new model was introduced in which all is was thread local and shared access to data
       has to be explicitly declared.  This model is called ithreads, for "interpreter threads".

       In Perl 5.6, the ithreads model was not available as a public API; only as an internal API that was
       available for extension writers, and to implement fork() emulation on Win32 platforms.

       In Perl 5.8, the ithreads model became available through the "threads" module, and the 5__5threads
       model was deprecated.

       In Perl 5.10, the 5__5threads model was removed from the Perl interpreter.

SYNOPSIS
           use Thread qw(:DEFAULT async yield);

           my $t = Thread->new(\&start_sub, @start_args);

           $result = $t->join;
           $t->detach;

           if ($t->done) {
               $t->join;
           }

           if($t->equal($another_thread)) {
               # ...
           }

           yield();

           my $tid = Thread->self->tid;

           lock($scalar);
           lock(@array);
           lock(%hash);

           my @list = Thread->list;

DESCRIPTION
       The "Thread" module provides multithreading support for Perl.

FUNCTIONS
       $thread = Thread->new(\&start_sub)
       $thread = Thread->new(\&start_sub, LIST)
               "new" starts a new thread of execution in the referenced subroutine. The optional list is
               passed as parameters to the subroutine. Execution continues in both the subroutine and the
               code after the "new" call.

               "Thread->new" returns a thread object representing the newly created thread.

       lock VARIABLE
               "lock" places a lock on a variable until the lock goes out of scope.

               If the variable is locked by another thread, the "lock" call will block until it's available.
               "lock" is recursive, so multiple calls to "lock" are safe--the variable will remain locked
               until the outermost lock on the variable goes out of scope.

               Locks on variables only affect "lock" calls--they do not affect normal access to a variable.
               (Locks on subs are different, and covered in a bit.)  If you really, really want locks to
               block access, then go ahead and tie them to something and manage this yourself.  This is done
               on purpose.  While managing access to variables is a good thing, Perl doesn't force you out
               of its living room...

               If a container object, such as a hash or array, is locked, all the elements of that container
               are not locked. For example, if a thread does a "lock @a", any other thread doing a
               "lock($a[12])" won't block.

               Finally, "lock" will traverse up references exactly one level.  "lock(\$a)" is equivalent to
               "lock($a)", while "lock(\\$a)" is not.

       async BLOCK;
               "async" creates a thread to execute the block immediately following it.  This block is
               treated as an anonymous sub, and so must have a semi-colon after the closing brace. Like
               "Thread->new", "async" returns a thread object.

       Thread->self
               The "Thread->self" function returns a thread object that represents the thread making the
               "Thread->self" call.

       Thread->list
               Returns a list of all non-joined, non-detached Thread objects.

       cond_wait VARIABLE
               The "cond_wait" function takes a locked variable as a parameter, unlocks the variable, and
               blocks until another thread does a "cond_signal" or "cond_broadcast" for that same locked
               variable. The variable that "cond_wait" blocked on is relocked after the "cond_wait" is
               satisfied.  If there are multiple threads "cond_wait"ing on the same variable, all but one
               will reblock waiting to reaquire the lock on the variable.  (So if you're only using
               "cond_wait" for synchronization, give up the lock as soon as possible.)

       cond_signal VARIABLE
               The "cond_signal" function takes a locked variable as a parameter and unblocks one thread
               that's "cond_wait"ing on that variable. If more than one thread is blocked in a "cond_wait"
               on that variable, only one (and which one is indeterminate) will be unblocked.

               If there are no threads blocked in a "cond_wait" on the variable, the signal is discarded.

       cond_broadcast VARIABLE
               The "cond_broadcast" function works similarly to "cond_signal".  "cond_broadcast", though,
               will unblock all the threads that are blocked in a "cond_wait" on the locked variable, rather
               than only one.

       yield   The "yield" function allows another thread to take control of the CPU. The exact results are
               implementation-dependent.

METHODS
       join    "join" waits for a thread to end and returns any values the thread exited with.  "join" will
               block until the thread has ended, though it won't block if the thread has already terminated.

               If the thread being "join"ed "die"d, the error it died with will be returned at this time. If
               you don't want the thread performing the "join" to die as well, you should either wrap the
               "join" in an "eval" or use the "eval" thread method instead of "join".

       detach  "detach" tells a thread that it is never going to be joined i.e.  that all traces of its
               existence can be removed once it stops running.  Errors in detached threads will not be
               visible anywhere - if you want to catch them, you should use $SIG{__DIE__} or something like
               that.

       equal   "equal" tests whether two thread objects represent the same thread and returns true if they
               do.

       tid     The "tid" method returns the tid of a thread. The tid is a monotonically increasing integer
               assigned when a thread is created. The main thread of a program will have a tid of zero,
               while subsequent threads will have tids assigned starting with one.

       done    The "done" method returns true if the thread you're checking has finished, and false
               otherwise.

DEFUNCT
       The following were implemented with 5__5threads, but are no longer available with ithreads.

       lock(\&sub)
               With 5005threads, you could also "lock" a sub such that any calls to that sub from another
               thread would block until the lock was released.

               Also, subroutines could be declared with the ":locked" attribute which would serialize access
               to the subroutine, but allowed different threads non-simultaneous access.

       eval    The "eval" method wrapped an "eval" around a "join", and so waited for a thread to exit,
               passing along any values the thread might have returned and placing any errors into $@.

       flags   The "flags" method returned the flags for the thread - an integer value corresponding to the
               internal flags for the thread.

SEE ALSO
       threads, threads::shared, Thread::Queue, Thread::Semaphore



perl v5.12.5                                     2012-09-11                                      Thread(3pm)