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_broadcast(@array); cond_signal(%hash);
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.
share, lock, cond_wait, cond_signal, cond_broadcast
Note that if this module is imported when threads 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 takes a value and marks it as shared. You can share a scalar,
array, hash, scalar ref, array ref or hash ref. share will return
the shared rvalue.
share will traverse up references exactly one level.
share(\$a) is equivalent to share($a), while share(\\$a) is not.
A variable can also be marked as shared at compile time by using the
shared attribute: my $var : shared.
If you want to share a newly created reference unfortunately you
need to use &share([]) and &share({}) syntax due to problems
with Perl's prototyping.
lock places a lock on a variable until the lock goes out of scope.
If the variable is locked by another thread, the lock call will
block until it's available. lock is recursive, so multiple calls
to lock are safe -- the variable will remain locked until the
outermost lock on the variable goes out of scope.
If a container object, such as a hash or array, is locked, all the
elements of that container are not locked. For example, if a thread
does a lock @a, any other thread doing a lock($a[12]) won't block.
lock will traverse up references exactly one level.
lock(\$a) is equivalent to lock($a), while lock(\\$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.
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 reacquire the lock on the variable. (So if you're only using
cond_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.
It is important to note that the variable can be notified even if
no thread cond_signal or cond_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.
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. By always locking before signaling, you can
(with care), avoid signaling before another thread has entered cond_wait().
cond_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 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.
threads::shared is designed to disable itself silently if threads are
not available. If you want access to threads, you must use threads
before you use threads::shared. threads will emit a warning if you
use it after threads::shared.
bless is not supported on shared references. In the current version,
bless 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 share $hashref-{key}> without giving any error
message. But the $hashref-{key}> is not shared, causing the error
"locking can only be used on shared values" to occur when you attempt to
lock $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