Thread - multithreading
use Thread; my $t = new Thread \&start_sub, @start_args; $t->join; my $tid = Thread->self->tid; my $tlist = Thread->list; lock($scalar); use Thread 'async'; use Thread 'eval';
Thread 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 the
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($a) won't block.
You may also
lock a sub, using
lock &sub. Any calls to that sub from another thread will block until the lock is released. This behaviour is not equvalent to
use attrs qw(locked) in the sub.
use attrs qw(locked) serializes access to a subroutine, but allows different threads non-simultaneous access.
lock &sub, on the other hand, will not allow any other thread access for the duration of the lock.
lock will traverse up references exactly one level.
lock(\$a) is equivalent to
lock(\\$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. Like
async returns a thread object.
Thread->self function returns a thread object that represents the thread making the
Thread->list returns a list of thread objects for all running and finished but un-
cond_wait function takes a locked variable as a parameter, unlocks the variable, and blocks until another thread does a
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 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 function works similarly to
cond_broadcast, though, will unblock all the threads that are blocked in a
cond_wait on the locked variable, rather than only one.
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
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
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
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.