threads::shared - Perl extension for sharing data structures between threads

  use threads;
  use threads::shared;

  my $var : shared;

  my($scalar, @array, %hash);
  share($scalar);
  share(@array);
  share(%hash);
  my $bar = &share([]);
  $hash{bar} = &share({});

  { lock(%hash); ...  }

  cond_wait($scalar);
  cond_timedwait($scalar, time() + 30);
  cond_broadcast(@array);
  cond_signal(%hash);

  my $lockvar : shared;
  # condition var != lock var
  cond_wait($var, $lockvar);
  cond_timedwait($var, time()+30, $lockvar);

By default, variables are private to each thread, and each newly created thread gets a private copy of each existing variable. This module allows you to share variables across different threads (and pseudoforks on Win32). It is used together with the threads module.

CWshare, CWcond_wait, CWcond_timedwait, CWcond_signal, CWcond_broadcast

Note that if this module is imported when CWthreads has not yet been loaded, then these functions all become no-ops. This makes it possible to write modules that will work in both threaded and non-threaded environments.

share VARIABLE

CWshare takes a value and marks it as shared. You can share a scalar, array, hash, scalar ref, array ref or hash ref. CWshare will return the shared rvalue but always as a reference. CWshare will traverse up references exactly one level. CWshare(\$a) is equivalent to CWshare($a), while CWshare(\\$a) is not. A variable can also be marked as shared at compile time by using the CWshared attribute: CWmy $var : shared. If you want to share a newly created reference unfortunately you need to use CW&share([]) and CW&share({}) syntax due to problems with Perl's prototyping.

lock VARIABLE

CWlock places a lock on a variable until the lock goes out of scope. If the variable is locked by another thread, the CWlock call will block until it's available. CWlock is recursive, so multiple calls to CWlock are safe the variable will remain locked until the outermost lock on the variable goes out of scope. 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 CWlock @a, any other thread doing a CWlock($a[12]) won't block. CWlock will traverse up references exactly one level. CWlock(\$a) is equivalent to CWlock($a), while CWlock(\\$a) is not. Note that you cannot explicitly unlock a variable; you can only wait for the lock to go out of scope. If you need more fine-grained control, see Thread::Semaphore.

cond_wait VARIABLE

cond_wait CONDVAR, LOCKVAR

The CWcond_wait function takes a locked variable as a parameter, unlocks the variable, and blocks until another thread does a CWcond_signal or CWcond_broadcast for that same locked variable. The variable that CWcond_wait blocked on is relocked after the CWcond_wait is satisfied. If there are multiple threads CWcond_waiting on the same variable, all but one will reblock waiting to reacquire the lock on the variable. (So if you're only using CWcond_wait for synchronisation, give up the lock as soon as possible). The two actions of unlocking the variable and entering the blocked wait state are atomic, the two actions of exiting from the blocked wait state and relocking the variable are not. In its second form, CWcond_wait takes a shared, unlocked variable followed by a shared, locked variable. The second variable is unlocked and thread execution suspended until another thread signals the first variable. It is important to note that the variable can be notified even if no thread CWcond_signal or CWcond_broadcast on the variable. It is therefore important to check the value of the variable and go back to waiting if the requirement is not fulfilled. For example, to pause until a shared counter drops to zero:

    { lock($counter); cond_wait($count) until $counter == 0; }

cond_timedwait VARIABLE, ABS_TIMEOUT

cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR

In its two-argument form, CWcond_timedwait takes a locked variable and an absolute timeout as parameters, unlocks the variable, and blocks until the timeout is reached or another thread signals the variable. A false value is returned if the timeout is reached, and a true value otherwise. In either case, the variable is re-locked upon return. Like CWcond_wait, this function may take a shared, locked variable as an additional parameter; in this case the first parameter is an unlocked condition variable protected by a distinct lock variable. Again like CWcond_wait, waking up and reacquiring the lock are not atomic, and you should always check your desired condition after this function returns. Since the timeout is an absolute value, however, it does not have to be recalculated with each pass:

    lock($var);
    my $abs = time() + 15;
    until ($ok = desired_condition($var)) {
      last if !cond_timedwait($var, $abs);
    }
    # we got it if $ok, otherwise we timed out!

cond_signal VARIABLE

The CWcond_signal function takes a locked variable as a parameter and unblocks one thread that's CWcond_waiting on that variable. If more than one thread is blocked in a CWcond_wait on that variable, only one (and which one is indeterminate) will be unblocked. If there are no threads blocked in a CWcond_wait on the variable, the signal is discarded. By always locking before signaling, you can (with care), avoid signaling before another thread has entered cond_wait(). CWcond_signal will normally generate a warning if you attempt to use it on an unlocked variable. On the rare occasions where doing this may be sensible, you can skip the warning with

    { no warnings 'threads'; cond_signal($foo) }

cond_broadcast VARIABLE

The CWcond_broadcast function works similarly to CWcond_signal. CWcond_broadcast, though, will unblock all the threads that are blocked in a CWcond_wait on the locked variable, rather than only one.

threads::shared is designed to disable itself silently if threads are not available. If you want access to threads, you must CWuse threads before you CWuse threads::shared. threads will emit a warning if you use it after threads::shared.

CWbless is not supported on shared references. In the current version, CWbless will only bless the thread local reference and the blessing will not propagate to the other threads. This is expected to be implemented in a future version of Perl.

Does not support splice on arrays!

Taking references to the elements of shared arrays and hashes does not autovivify the elements, and neither does slicing a shared array/hash over non-existent indices/keys autovivify the elements.

share() allows you to CWshare $hashref->{key} without giving any error message. But the CW$hashref->{key} is not shared, causing the error locking can only be used on shared values to occur when you attempt to CWlock $hasref->{key}.

Arthur Bergman <arthur at contiller.se>

threads::shared is released under the same license as Perl

Documentation borrowed from the old Thread.pm

threads, perlthrtut, <http://www.perl.com/pub/a/2002/06/11/threads.html>