threads::shared(3) Perl Programmers Reference Guide threads::shared(3)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, $lockvar);
DESCRIPTION
By default, variables are private to each thread, and each newly cre-
ated thread gets a private copy of each existing variable. This module
allows you to share variables across different threads (and pseudoforks
on Win32). It is used together with the threads module.
EXPORT
"share", "cond_wait", "cond_timedwait", "cond_signal", "cond_broadcast"
Note that if this module is imported when "threads" has not yet been
loaded, then these functions all become no-ops. This makes it possible
to write modules that will work in both threaded and non-threaded envi-
ronments.
FUNCTIONS
share VARIABLE
"share" takes a value and marks it as shared. You can share a
scalar, array, hash, scalar ref, array ref or hash ref. "share"
will return the shared rvalue but always as a reference.
"share" will traverse up references exactly one level.
"share(\$a)" is equivalent to "share($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
$var = &share(A->new); # ok as long as the A object is not nested
Note that it is often not wise to share an object unless the class
itself has been written to support sharing; for example, an
object's destructor may get called multiple times, one for each
thread's scope exit.
lock VARIABLE
"lock" places a lock on a variable until the lock goes out of
scope. If the variable is locked by another thread, the "lock"
call will block until it's available. "lock" is recursive, so mul-
tiple calls to "lock" are safe -- the variable will remain locked
until the outermost lock on the variable goes out of scope.
If a container object, such as a hash or array, is locked, all the
elements of that container are not locked. For example, if a thread
does a "lock @a", any other thread doing a "lock($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.
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 wait-
ing to reacquire the lock on the variable. (So if you're only using
"cond_wait" for synchronisation, give up the lock as soon as possi-
ble). 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 sig-
nals 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 $counter == 0; }
cond_timedwait VARIABLE, ABS_TIMEOUT
cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR
In its two-argument form, "cond_timedwait" takes a locked variable
and an absolute timeout as parameters, unlocks the variable, and
blocks until the timeout is reached or another thread signals the
variable. A false value is returned if the timeout is reached, and
a true value otherwise. In either case, the variable is re-locked
upon return.
Like "cond_wait", this function may take a shared, locked variable
as an additional parameter; in this case the first parameter is an
unlocked condition variable protected by a distinct lock variable.
Again like "cond_wait", waking up and reacquiring the lock are not
atomic, and you should always check your desired condition after
this function returns. Since the timeout is an absolute value,
however, it does not have to be recalculated with each pass:
lock($var);
my $abs = time() + 15;
until ($ok = desired_condition($var)) {
last if !cond_timedwait($var, $abs);
}
# we got it if $ok, otherwise we timed out!
cond_signal VARIABLE
The "cond_signal" function takes a locked variable as a parameter
and unblocks one thread that's "cond_wait"ing on that variable. If
more than one thread is blocked in a "cond_wait" on that variable,
only one (and which one is indeterminate) will be unblocked.
If there are no threads blocked in a "cond_wait" on the variable,
the signal is discarded. By always locking before signaling, you
can (with care), avoid signaling before another thread has entered
cond_wait().
"cond_signal" will normally generate a warning if you attempt to
use it on an unlocked variable. On the rare occasions where doing
this may be sensible, you can skip the warning with
{ no warnings 'threads'; cond_signal($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 imple-
mented 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>
perl v5.8.8 2006-06-14 threads::shared(3)
