threads::shared - Perl extension for sharing data structures between threads
- use threads;
- use threads::shared;
- my $var : shared;
- $var = $scalar_value;
- $var = $shared_ref_value;
- $var = &share($simple_unshared_ref_value);
- $var = &share(new Foo);
- my($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, $lockvar);
By default, variables are private to each thread, and each newly created thread gets a private copy of each existing variable. This module allows you to share variables across different threads (and pseudoforks on Win32). It is used together with the threads module.
share
, cond_wait
, cond_timedwait
, cond_signal
, cond_broadcast
Note that if this module is imported when threads
has not yet been
loaded, then these functions all become no-ops. This makes it possible
to write modules that will work in both threaded and non-threaded
environments.
share
takes a value and marks it as shared. You can share a scalar,
array, hash, scalar ref, array ref or hash ref. share
will return
the shared rvalue but always as a reference.
share
will traverse up references exactly one level.
share(\$a)
is equivalent to share($a)
, while share(\\$a)
is not.
This means that you must create nested shared data structures by first
creating individual shared leaf notes, then adding them to a shared hash
or array.
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.
The only values that can be assigned to a shared scalar are other scalar values, or shared refs, eg
- my $var : shared;
- $var = 1; # ok
- $var = &share([]); # ok
- $var = []; # error
- $var = A->new; # error
- $var = &share(A->new); # ok as long as the A object is not nested
Note that it is often not wise to share an object unless the class itself has been written to support sharing; for example, an object's destructor may get called multiple times, one for each thread's scope exit.
lock places a lock on a variable until the lock goes out of scope.
If the variable is locked by another thread, the lock call will
block until it's available. lock is recursive, so multiple calls
to lock are safe -- the variable will remain locked until the
outermost lock on the variable goes out of scope.
If a container object, such as a hash or array, is locked, all the
elements of that container are not locked. For example, if a thread
does a lock @a
, any other thread doing a lock($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_wait
ing 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.
In its second form, cond_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 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. For example,
to pause until a shared counter drops to zero:
- { lock($counter); cond_wait($count) until $counter == 0; }
In its two-argument form, cond_timedwait
takes a locked variable
and an absolute timeout as parameters, unlocks the variable, and blocks
until the timeout is reached or another thread signals the variable. A
false value is returned if the timeout is reached, and a true value
otherwise. In either case, the variable is re-locked upon return.
Like cond_wait
, this function may take a shared, locked variable
as an additional parameter; in this case the first parameter is an
unlocked condition variable protected by a distinct lock variable.
Again like cond_wait
, waking up and reacquiring the lock are not
atomic, and you should always check your desired condition after this
function returns. Since the timeout is an absolute value, however, it
does not have to be recalculated with each pass:
The cond_signal
function takes a locked variable as a parameter
and unblocks one thread that's cond_wait
ing on that variable. If
more than one thread is blocked in a cond_wait
on that variable,
only one (and which one is indeterminate) will be unblocked.
If there are no threads blocked in a cond_wait
on the variable,
the signal is discarded. By always locking before signaling, you can
(with care), avoid signaling before another thread has entered cond_wait().
cond_signal
will normally generate a warning if you attempt to use it
on an unlocked variable. On the rare occasions where doing this may be
sensible, you can skip the warning with
- { no warnings 'threads'; cond_signal($foo) }
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.
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