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threads::shared(3pm) Perl Programmers Reference Guide threads::shared(3pm)
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, $lockvar);
DESCRIPTION
By default, variables are private to each thread, and each newly created thread
gets a private copy of each existing variable. This module allows you to share
variables across different threads (and pseudoforks on Win32). It is used together
with the threads module.
EXPORT
"share", "cond_wait", "cond_timedwait", "cond_signal", "cond_broadcast"
Note that if this module is imported when "threads" has not yet been loaded, then
these functions all become no-ops. This makes it possible to write modules that
will work in both threaded and non-threaded environments.
FUNCTIONS
share VARIABLE
"share" takes a value and marks it as shared. You can share a scalar, array,
hash, scalar ref, array ref or hash ref. "share" will return the shared rvalue
but always as a reference.
"share" will traverse up references exactly one level. "share(\$a)" is equiva-
lent to "share($a)", while "share(\\$a)" is not.
A variable can also be marked as shared at compile time by using the "shared"
attribute: "my $var : shared".
If you want to share a newly created reference unfortunately you need to use
"&share([])" and "&share({})" syntax due to problems with Perl’s prototyping.
lock VARIABLE
"lock" places a lock on a variable until the lock goes out of scope. If the
variable is locked by another thread, the "lock" call will block until it’s
available. "lock" is recursive, so multiple calls to "lock" are safe -- the
variable will remain locked until the outermost lock on the variable goes out
of scope.
If a container object, such as a hash or array, is locked, all the elements of
that container are not locked. For example, if a thread does a "lock @a", any
other thread doing a "lock($a[12])" won’t block.
"lock" will traverse up references exactly one level. "lock(\$a)" is equiva-
lent 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_broad-
cast" 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 reac-
quire the lock on the variable. (So if you’re only using "cond_wait" for syn-
chronisation, give up the lock as soon as possible). The two actions of unlock-
ing 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 $counter == 0; }
cond_timedwait VARIABLE, ABS_TIMEOUT
cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR
In its two-argument form, "cond_timedwait" takes a locked variable and an abso-
lute timeout as parameters, unlocks the variable, and blocks until the timeout
is reached or another thread signals the variable. A false value is returned
if the timeout is reached, and a true value otherwise. In either case, the
variable is re-locked upon return.
Like "cond_wait", this function may take a shared, locked variable as an addi-
tional parameter; in this case the first parameter is an unlocked condition
variable protected by a distinct lock variable.
Again like "cond_wait", waking up and reacquiring the lock are not atomic, and
you should always check your desired condition after this function returns.
Since the timeout is an absolute value, however, it does not have to be recal-
culated with each pass:
lock($var);
my $abs = time() + 15;
until ($ok = desired_condition($var)) {
last if !cond_timedwait($var, $abs);
}
# we got it if $ok, otherwise we timed out!
cond_signal VARIABLE
The "cond_signal" function takes a locked variable as a parameter and unblocks
one thread that’s "cond_wait"ing on that variable. If more than one thread is
blocked in a "cond_wait" on that variable, only one (and which one is indeter-
minate) will be unblocked.
If there are no threads blocked in a "cond_wait" on the variable, the signal is
discarded. By always locking before signaling, you can (with care), avoid sig-
naling before another thread has entered cond_wait().
"cond_signal" will normally generate a warning if you attempt to use it on an
unlocked variable. On the rare occasions where doing this may be sensible, you
can skip the warning with
{ no warnings ’threads’; cond_signal($foo) }
cond_broadcast VARIABLE
The "cond_broadcast" function works similarly to "cond_signal". "cond_broad-
cast", though, will unblock all the threads that are blocked in a "cond_wait"
on the locked variable, rather than only one.
NOTES
threads::shared is designed to disable itself silently if threads are not avail-
able. 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>
perl v5.8.6 2001-09-21 threads::shared(3pm)
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