Thread - manipulate threads in Perl

- NAME
- SUPPORTED PLATFORMS
- CAVEAT
- SYNOPSIS
- DESCRIPTION
- FUNCTIONS
- METHODS
- LIMITATIONS
- SEE ALSO

# NAME

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

# SUPPORTED PLATFORMS

none

# CAVEAT

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.

# SYNOPSIS

        use Thread;

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

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

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

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

        lock($scalar);
        yield();

        use Thread 'async';

# DESCRIPTION

        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.

# FUNCTIONS

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 VARIABLE
  `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.

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

Thread->list
  `Thread->list` returns a list of thread objects for all running and finished but un-`join`ed 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_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 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.

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

# METHODS

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 `join`.

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 `$@`.

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.

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.

# LIMITATIONS

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.

# SEE ALSO

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

Thread - manipulate threads in Perl