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Thread

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



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

CAVEAT
       Perl has two thread models.

       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 "5005threads".

       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.

       Neither model is configured by default into Perl (except, as mentioned
       above, in Win32 ithreads are always available.)  You can see your
       Perl's threading configuration by running "perl -V" and looking for the
       use...threads variables, or inside script by "use Config;" and testing
       for $Config{use5005threads} and $Config{useithreads}.

       For old code and interim backwards compatibility, the Thread module has
       been reworked to function as a frontend for both 5005threads and
       ithreads.

       Note that the compatibility is not complete: because the data sharing
       models are directly opposed, anything to do with data sharing has to be
       thought differently.  With the ithreads you must explicitly share()
       variables between the threads.

       For new code the use of the "Thread" module is discouraged and the
       direct use of the "threads" and "threads::shared" modules is encouraged
       instead.

       Finally, note that there are many known serious problems with the
       5005threads, one of the least of which is that regular expression match
       variables like $1 are not threadsafe, that is, they easily get cor-
       rupted by competing threads.  Other problems include more insidious
       data corruption and mysterious crashes.  You are seriously urged to use
       ithreads instead.

SYNOPSIS
           use Thread;

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

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

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

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

           yield();

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

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

           lock(\&sub);        # not available with ithreads

           $flags = $t->flags; # not available with ithreads

           my @list = Thread->list;    # not available with ithreads

           use Thread 'async';

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 sub-
               routine. The optional list is passed as parameters to the sub-
               routine. 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 mul-
               tiple 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.

               With 5005threads you may also "lock" a sub, using "lock &sub".
               Any calls to that sub from another thread will block until the
               lock is released. This behaviour is not equivalent to declaring
               the sub with the "locked" attribute.  The "locked" attribute
               serializes access to a subroutine, but allows different threads
               non-simultaneous access. "lock &sub", on the other hand, will
               not allow any other thread access for the duration of the lock.

               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 fol-
               lowing 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 repre-
               sents the thread making the "Thread->self" call.

       cond_wait VARIABLE
               The "cond_wait" function takes a locked variable as a parame-
               ter, unlocks the variable, and blocks until another thread does
               a "cond_signal" or "cond_broadcast" for that same locked vari-
               able. 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 parame-
               ter and unblocks one thread that's "cond_wait"ing on that vari-
               able. 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 vari-
               able, 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 termi-
               nated.

               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".

       eval    The "eval" method wraps an "eval" around a "join", and so waits
               for a thread to exit, passing along any values the thread might
               have returned.  Errors, of course, get placed into $@.  (Not
               available with ithreads.)

       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 cre-
               ated. The main thread of a program will have a tid of zero,
               while subsequent threads will have tids assigned starting with
               one.

       flags   The "flags" method returns the flags for the thread. This is
               the integer value corresponding to the internal flags for the
               thread, and the value may not be all that meaningful to you.
               (Not available with ithreads.)

       done    The "done" method returns true if the thread you're checking
               has finished, and false otherwise.  (Not available with
               ithreads.)

LIMITATIONS
       The sequence number used to assign tids is a simple integer, and no
       checking is done to make sure the tid isn't currently in use.  If a
       program creates more than 2**32 - 1 threads in a single run, threads
       may be assigned duplicate tids.  This limitation may be lifted in a
       future version of Perl.

SEE ALSO
       threads::shared (not available with 5005threads)

       attributes, Thread::Queue, Thread::Semaphore, Thread::Specific (not
       available with ithreads)



perl v5.8.6                       2001-09-21                         Thread(3)