NAME
threads::shared - Perl extension for sharing data structures between threads
SYNOPSIS
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, $lockvar);
DESCRIPTION
By default, variables are private to each thread, and each newly created thread gets a private copy of each existing variable. This module allows you to share variables across different threads (and pseudoforks on Win32). It is used together with the threads module.
EXPORT
share
, cond_wait
, cond_timedwait
, cond_signal
, cond_broadcast
Note that if this module is imported when threads
has not yet been
loaded, then these functions all become no-ops. This makes it possible
to write modules that will work in both threaded and non-threaded
environments.
FUNCTIONS
- share VARIABLE
share
takes a value and marks it as shared. You can share a scalar, array, hash, scalar ref, array ref or hash ref.share
will return the shared rvalue but always as a reference.share
will traverse up references exactly one level.share(\$a)
is equivalent toshare($a)
, whileshare(\\$a)
is not.A variable can also be marked as shared at compile time by using the
shared
attribute:my $var : shared
.If you want to share a newly created reference unfortunately you need to use
&share([])
and&share({})
syntax due to problems with Perl's prototyping. - lock VARIABLE
lock
places a lock on a variable until the lock goes out of scope. If the variable is locked by another thread, thelock
call will block until it's available.lock
is recursive, so multiple calls tolock
are safe -- the variable will remain locked until the outermost lock on the variable goes out of scope.If a container object, such as a hash or array, is locked, all the elements of that container are not locked. For example, if a thread does a
lock @a
, any other thread doing alock($a[12])
won't block.lock
will traverse up references exactly one level.lock(\$a)
is equivalent tolock($a)
, whilelock(\\$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
cond_wait
function takes a locked variable as a parameter, unlocks the variable, and blocks until another thread does acond_signal
orcond_broadcast
for that same locked variable. The variable thatcond_wait
blocked on is relocked after thecond_wait
is satisfied. If there are multiple threadscond_wait
ing on the same variable, all but one will reblock waiting to reacquire the lock on the variable. (So if you're only usingcond_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
orcond_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 $counter == 0; }
- cond_timedwait VARIABLE, ABS_TIMEOUT
- cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR
In its two-argument form,
cond_timedwait
takes a locked variable and an absolute timeout as parameters, unlocks the variable, and blocks until the timeout is reached or another thread signals the variable. A false value is returned if the timeout is reached, and a true value otherwise. In either case, the variable is re-locked upon return.Like
cond_wait
, this function may take a shared, locked variable as an additional parameter; in this case the first parameter is an unlocked condition variable protected by a distinct lock variable.Again like
cond_wait
, waking up and reacquiring the lock are not atomic, and you should always check your desired condition after this function returns. Since the timeout is an absolute value, however, it does not have to be recalculated with each pass:lock($var); my $abs = time() + 15; until ($ok = desired_condition($var)) { last if !cond_timedwait($var, $abs); } # we got it if $ok, otherwise we timed out!
- cond_signal VARIABLE
The
cond_signal
function takes a locked variable as a parameter and unblocks one thread that'scond_wait
ing on that variable. If more than one thread is blocked in acond_wait
on that variable, only one (and which one is indeterminate) will be unblocked.If there are no threads blocked in a
cond_wait
on the variable, the signal is discarded. By always locking before signaling, you can (with care), avoid signaling before another thread has entered cond_wait().cond_signal
will normally generate a warning if you attempt to use it on an unlocked variable. On the rare occasions where doing this may be sensible, you can skip the warning with{ no warnings 'threads'; cond_signal($foo) }
- cond_broadcast VARIABLE
The
cond_broadcast
function works similarly tocond_signal
.cond_broadcast
, though, will unblock all the threads that are blocked in acond_wait
on the locked variable, rather than only one.
NOTES
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.
BUGS
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}
.
AUTHOR
Arthur Bergman <arthur at contiller.se>
threads::shared is released under the same license as Perl
Documentation borrowed from the old Thread.pm
SEE ALSO
threads, perlthrtut, http://www.perl.com/pub/a/2002/06/11/threads.html