MirBSD manpage: Thread(3p)

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 impli-
     citly 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

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

     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

     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 reg-
     ular expression match variables like $1 are not threadsafe,
     that is, they easily get corrupted by competing threads.
     Other problems include more insidious data corruption and
     mysterious crashes.  You are seriously urged to use ithreads

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         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 refer-
             enced subroutine. The optional list is passed as
             parameters to the subroutine. Execution continues in
             both the subroutine and the code after the "new"

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

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

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             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.

             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 dif-
             ferent 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"

     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

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             after the "cond_wait" is satisfied.  If there are
             multiple threads "cond_wait"ing on the same vari-
             able, 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

             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

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


     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

     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

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             to catch them, you should use $SIG{__DIE__} or some-
             thing 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 pro-
             gram 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

     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 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.


     threads::shared (not available with 5005threads)

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

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