ext::threads::shaPerl:Programmersext::threads::shared::shared(3p)NAMEthreads::shared - Perl extension for sharing data structures
between threads
SYNOPSIS
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, $lockvar);
DESCRIPTION
By default, variables are private to each thread, and each
newly created thread gets a private copy of each existing
variable. This module allows you to share variables across
different threads (and pseudoforks on Win32). It is used
together with the threads module.
EXPORT
"share", "cond_wait", "cond_timedwait", "cond_signal",
"cond_broadcast"
Note that if this module is imported when "threads" has not
yet been loaded, then these functions all become no-ops.
This makes it possible to write modules that will work in
both threaded and non-threaded environments.
FUNCTIONS
share VARIABLE
"share" takes a value and marks it as shared. You can
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share a scalar, array, hash, scalar ref, array ref or
hash ref. "share" will return the shared rvalue but
always as a reference.
"share" will traverse up references exactly one level.
"share(\$a)" is equivalent to "share($a)", while
"share(\\$a)" is not. This means that you must create
nested shared data structures by first creating indivi-
dual 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 unfor-
tunately 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
$var = &share(A->new); # ok as long as the A object is not nested
Note that it is often not wise to share an object unless
the class itself has been written to support sharing;
for example, an object's destructor may get called mul-
tiple times, one for each thread's scope exit.
lock VARIABLE
"lock" places a lock on a variable until the lock goes
out of scope. If the variable is locked by another
thread, the "lock" call will block until it's available.
"lock" is recursive, so multiple calls to "lock" are
safe -- the variable will remain locked until the outer-
most lock on the variable goes out of scope.
If a container object, such as a hash or array, is
locked, all the elements of that container are not
locked. For example, if a thread does a "lock @a", any
other thread doing a "lock($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
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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 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 synchronisa-
tion, 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 exit-
ing from the blocked wait state and relocking the vari-
able 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 vari-
able.
It is important to note that the variable can be noti-
fied 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 $counter == 0; }
cond_timedwait VARIABLE, ABS_TIMEOUT
cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR
In its two-argument form, "cond_timedwait" takes a
locked variable and an absolute timeout as parameters,
unlocks the variable, and blocks until the timeout is
reached or another thread signals the variable. A false
value is returned if the timeout is reached, and a true
value otherwise. In either case, the variable is re-
locked upon return.
Like "cond_wait", this function may take a shared,
locked variable as an additional parameter; in this case
the first parameter is an unlocked condition variable
protected by a distinct lock variable.
Again like "cond_wait", waking up and reacquiring the
lock are not atomic, and you should always check your
desired condition after this function returns. Since
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the timeout is an absolute value, however, it does not
have to be recalculated with each pass:
lock($var);
my $abs = time() + 15;
until ($ok = desired_condition($var)) {
last if !cond_timedwait($var, $abs);
}
# we got it if $ok, otherwise we timed out!
cond_signal VARIABLE
The "cond_signal" function takes a locked variable as a
parameter and unblocks one thread that's "cond_wait"ing
on that variable. If more than one thread is blocked in
a "cond_wait" on that variable, only one (and which one
is indeterminate) will be unblocked.
If there are no threads blocked in a "cond_wait" on the
variable, the signal is discarded. By always locking
before signaling, you can (with care), avoid signaling
before another thread has entered cond_wait().
"cond_signal" will normally generate a warning if you
attempt to use it on an unlocked variable. On the rare
occasions where doing this may be sensible, you can skip
the warning with
{ no warnings 'threads'; cond_signal($foo) }
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.
NOTESthreads::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 ver-
sion of Perl.
Does not support splice on arrays!
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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>
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