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Thread - manipulate threads in Perl (EXPERIMENTAL, subject to change)


The Thread extension requires Perl to be built in a particular way to enable the older 5.005 threading model. Just to confuse matters, there is an alternate threading model known as "ithreads" that does NOT support this extension. If you are using a binary distribution such as ActivePerl that is built with ithreads support, this extension CANNOT be used.


use Thread;

my $t = new Thread \&start_sub, @start_args;

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

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

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


use Thread 'async';


WARNING: Threading is an experimental feature.  Both the interface
and implementation are subject to change drastically.  In fact, this
documentation describes the flavor of threads that was in version
5.005.  Perl 5.6.0 and later have the beginnings of support for
interpreter threads, which (when finished) is expected to be
significantly different from what is described here.  The information
contained here may therefore soon be obsolete.  Use at your own risk!

The Thread module provides multithreading support for perl.


new \&start_sub
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.

new Thread returns a thread object representing the newly created thread.


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.

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 new Thread, async returns a thread object.


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


Thread->list returns a list of thread objects for all running and finished but un-joined threads.

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_waiting 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_waiting 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.


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



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 joined died, 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.


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 $@.


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


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.


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.


attributes, Thread::Queue, Thread::Semaphore, Thread::Specific.