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 "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 thethreads 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 runningperl -V and looking for theuse...threads variables, or inside script byuse 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 theThread module is discouraged and the direct use of thethreads andthreads::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 corrupted by competing threads. Other problems include more insidious data corruption and mysterious crashes. You are seriously urged to use ithreads instead.
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 = $t->flags;# not available with ithreadsmy @list = Thread->list;# not available with ithreadsuse Thread 'async';TheThread module provides multithreading support for perl.
new starts a new thread of execution in the referenced subroutine. The optional list is passed as parameters to the subroutine. Execution continues in both the subroutine and the code after thenew call.
Thread->new returns a thread object representing the newly created thread.
lock places a lock on a variable until the lock goes out of scope.
If the variable is locked by another thread, thelock call will block until it's available.lock is recursive, so multiple calls tolock are safe--the variable will remain locked until the outermost lock on the variable goes out of scope.
Locks on variables only affectlock calls--they donot affect normal access to a variable. (Locks on subs are different, and covered in a bit.) If you really,really want locks to block access, then go ahead and tie them to something and manage this yourself. This is done on purpose. While managing access to variables is a good thing, Perl doesn't force you out of its living room...
If a container object, such as a hash or array, is locked, all the elements of that container are not locked. For example, if a thread does alock @a, any other thread doing alock($a[12]) won't block.
With 5005threads you may alsolock a sub, usinglock &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 thelocked attribute. Thelocked attribute serializes access to a subroutine, but allows different threads non-simultaneous access.lock &sub, on the other hand, will not allowany other thread access for the duration of the lock.
Finally,lock will traverse up references exactlyone level.lock(\$a) is equivalent tolock($a), whilelock(\\$a) is not.
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. LikeThread->new,async returns a thread object.
TheThread->self function returns a thread object that represents the thread making theThread->self call.
Thecond_wait function takes alocked variable as a parameter, unlocks the variable, and blocks until another thread does acond_signal orcond_broadcast for that same locked variable. The variable thatcond_wait blocked on is relocked after thecond_wait is satisfied. If there are multiple threadscond_waiting on the same variable, all but one will reblock waiting to reaquire the lock on the variable. (So if you're only usingcond_wait for synchronization, give up the lock as soon as possible.)
Thecond_signal function takes a locked variable as a parameter and unblocks one thread that'scond_waiting on that variable. If more than one thread is blocked in acond_wait on that variable, only one (and which one is indeterminate) will be unblocked.
If there are no threads blocked in acond_wait on the variable, the signal is discarded.
Thecond_broadcast function works similarly tocond_signal.cond_broadcast, though, will unblockall the threads that are blocked in acond_wait on the locked variable, rather than only one.
Theyield 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 beingjoineddied, the error it died with will be returned at this time. If you don't want the thread performing thejoin to die as well, you should either wrap thejoin in aneval or use theeval thread method instead ofjoin.
Theeval method wraps aneval around ajoin, 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 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.
Thetid 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.
Theflags 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.)
Thedone 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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