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

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

       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

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

       In Perl 5.6 the ithreads model was not available as a pub-
       lic 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 avail-
       able.)  You can see your Perl's threading configuration by
       running "perl -V" and looking for the use...threads vari-
       ables, 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

       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 thread-
       safe, that is, they easily get corrupted by competing
       threads.  Other problems include more insidious data cor-
       ruption and mysterious crashes.  You are seriously urged
       to use ithreads instead.

perl v5.8.5                 2002-11-06                          1

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

           use Thread;

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

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

           if ($t->done) {

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


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


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

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

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

           use Thread 'async';

       The "Thread" module provides multithreading support for

       $thread = Thread->new(\&start_sub)
       $thread = Thread->new(\&start_sub, LIST)
               "new" starts a new thread of execution in the ref-
               erenced 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 repre-
               senting 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

perl v5.8.5                 2002-11-06                          2

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

               "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

               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

               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 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.

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

       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

perl v5.8.5                 2002-11-06                          3

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

               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 vari-
               able, 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 imple-

       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

       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.

perl v5.8.5                 2002-11-06                          4

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

       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.

       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.)

       The sequence number used to assign tids is a simple inte-
       ger, 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 dupli-
       cate tids.  This limitation may be lifted in a future ver-
       sion of Perl.

       threads::shared (not available with 5005threads)

       attributes, Thread::Queue, Thread::Semaphore, Thread::Spe-
       cific (not available with ithreads)

perl v5.8.5                 2002-11-06                          5