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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
VERSION
This document describes threads::shared version 1.32
SYNOPSIS
use threads;
use threads::shared;
my $var :shared;
my %hsh :shared;
my @ary :shared;
my ($scalar, @array, %hash);
share($scalar);
share(@array);
share(%hash);
$var = $scalar_value;
$var = $shared_ref_value;
$var = shared_clone($non_shared_ref_value);
$var = shared_clone({'foo' => [qw/foo bar baz/]});
$hsh{'foo'} = $scalar_value;
$hsh{'bar'} = $shared_ref_value;
$hsh{'baz'} = shared_clone($non_shared_ref_value);
$hsh{'quz'} = shared_clone([1..3]);
$ary[0] = $scalar_value;
$ary[1] = $shared_ref_value;
$ary[2] = shared_clone($non_shared_ref_value);
$ary[3] = shared_clone([ {}, [] ]);
{ lock(%hash); ... }
cond_wait($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, $lockvar);
DESCRIPTION
By default, variables are private to each thread, and each newly created thread gets a private copy of each existing
variable. This module allows you to share variables across different threads (and pseudo-forks on Win32). It is used
together with the threads module.
This module supports the sharing of the following data types only: scalars and scalar refs, arrays and array refs, and
hashes and hash refs.
EXPORT
The following functions are exported by this module: "share", "shared_clone", "is_shared", "cond_wait", "cond_timedwait",
"cond_signal" and "cond_broadcast"
Note that if this module is imported when threads has not yet been loaded, then these functions all become no-ops. This
makes it possible to write modules that will work in both threaded and non-threaded environments.
FUNCTIONS
share VARIABLE
"share" takes a variable and marks it as shared:
my ($scalar, @array, %hash);
share($scalar);
share(@array);
share(%hash);
"share" will return the shared rvalue, but always as a reference.
Variables can also be marked as shared at compile time by using the ":shared" attribute:
my ($var, %hash, @array) :shared;
Shared variables can only store scalars, refs of shared variables, or refs of shared data (discussed in next
section):
my ($var, %hash, @array) :shared;
my $bork;
# Storing scalars
$var = 1;
$hash{'foo'} = 'bar';
$array[0] = 1.5;
# Storing shared refs
$var = \%hash;
$hash{'ary'} = \@array;
$array[1] = \$var;
# The following are errors:
# $var = \$bork; # ref of non-shared variable
# $hash{'bork'} = []; # non-shared array ref
# push(@array, { 'x' => 1 }); # non-shared hash ref
shared_clone REF
"shared_clone" takes a reference, and returns a shared version of its argument, performing a deep copy on any non-
shared elements. Any shared elements in the argument are used as is (i.e., they are not cloned).
my $cpy = shared_clone({'foo' => [qw/foo bar baz/]});
Object status (i.e., the class an object is blessed into) is also cloned.
my $obj = {'foo' => [qw/foo bar baz/]};
bless($obj, 'Foo');
my $cpy = shared_clone($obj);
print(ref($cpy), "\n"); # Outputs 'Foo'
For cloning empty array or hash refs, the following may also be used:
$var = &share([]); # Same as $var = shared_clone([]);
$var = &share({}); # Same as $var = shared_clone({});
is_shared VARIABLE
"is_shared" checks if the specified variable is shared or not. If shared, returns the variable's internal ID
(similar to refaddr()). Otherwise, returns "undef".
if (is_shared($var)) {
print("\$var is shared\n");
} else {
print("\$var is not shared\n");
}
When used on an element of an array or hash, "is_shared" checks if the specified element belongs to a shared array or
hash. (It does not check the contents of that element.)
my %hash :shared;
if (is_shared(%hash)) {
print("\%hash is shared\n");
}
$hash{'elem'} = 1;
if (is_shared($hash{'elem'})) {
print("\$hash{'elem'} is in a shared hash\n");
}
lock VARIABLE
"lock" places a advisory 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. Multiple calls to "lock" by the same thread from within
dynamically nested scopes are safe -- the variable will remain locked until the outermost lock on the variable goes
out of scope.
"lock" follows references exactly one level:
my %hash :shared;
my $ref = \%hash;
lock($ref); # This is equivalent to lock(%hash)
Note that you cannot explicitly unlock a variable; you can only wait for the lock to go out of scope. This is most
easily accomplished by locking the variable inside a block.
my $var :shared;
{
lock($var);
# $var is locked from here to the end of the block
...
}
# $var is now unlocked
As locks are advisory, they do not prevent data access or modification by another thread that does not itself attempt
to obtain a lock on the variable.
You cannot lock the individual elements of a container variable:
my %hash :shared;
$hash{'foo'} = 'bar';
#lock($hash{'foo'}); # Error
lock(%hash); # Works
If you need more fine-grained control over shared variable access, 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 re-block 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 re-locking
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($counter); cond_wait($counter) 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 $ok, otherwise we timed out!
cond_signal VARIABLE
The "cond_signal" function takes a locked variable as a parameter and unblocks one thread that's "cond_wait"ing on
that variable. If more than one thread is blocked in a "cond_wait" on that variable, only one (and which one is
indeterminate) will be unblocked.
If there are no threads blocked in a "cond_wait" on the variable, the signal is discarded. By always locking before
signaling, you can (with care), avoid signaling before another thread has entered cond_wait().
"cond_signal" will normally generate a warning if you attempt to use it on an unlocked variable. On the rare
occasions where doing this may be sensible, you can suppress the warning with:
{ no warnings 'threads'; cond_signal($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.
OBJECTS
threads::shared exports a version of bless() that works on shared objects such that blessings propagate across threads.
# Create a shared 'Foo' object
my $foo :shared = shared_clone({});
bless($foo, 'Foo');
# Create a shared 'Bar' object
my $bar :shared = shared_clone({});
bless($bar, 'Bar');
# Put 'bar' inside 'foo'
$foo->{'bar'} = $bar;
# Rebless the objects via a thread
threads->create(sub {
# Rebless the outer object
bless($foo, 'Yin');
# Cannot directly rebless the inner object
#bless($foo->{'bar'}, 'Yang');
# Retrieve and rebless the inner object
my $obj = $foo->{'bar'};
bless($obj, 'Yang');
$foo->{'bar'} = $obj;
})->join();
print(ref($foo), "\n"); # Prints 'Yin'
print(ref($foo->{'bar'}), "\n"); # Prints 'Yang'
print(ref($bar), "\n"); # Also prints 'Yang'
NOTES
threads::shared is designed to disable itself silently if threads are not available. This allows you to write modules
and packages that can be used in both threaded and non-threaded applications.
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 AND LIMITATIONS
When "share" is used on arrays, hashes, array refs or hash refs, any data they contain will be lost.
my @arr = qw(foo bar baz);
share(@arr);
# @arr is now empty (i.e., == ());
# Create a 'foo' object
my $foo = { 'data' => 99 };
bless($foo, 'foo');
# Share the object
share($foo); # Contents are now wiped out
print("ERROR: \$foo is empty\n")
if (! exists($foo->{'data'}));
Therefore, populate such variables after declaring them as shared. (Scalar and scalar refs are not affected by this
problem.)
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, once for each thread's scope exit. Another danger is that the
contents of hash-based objects will be lost due to the above mentioned limitation. See examples/class.pl (in the CPAN
distribution of this module) for how to create a class that supports object sharing.
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($hashref->{key})" and "share($arrayref->[idx])" without giving any error message. But the
"$hashref->{key}" or "$arrayref->[idx]" is not shared, causing the error "lock can only be used on shared values" to
occur when you attempt to "lock($hasref->{key})" or "lock($arrayref->[idx])" in another thread.
Using refaddr()) is unreliable for testing whether or not two shared references are equivalent (e.g., when testing for
circular references). Use "is_shared VARIABLE" in is_shared(), instead:
use threads;
use threads::shared;
use Scalar::Util qw(refaddr);
# If ref is shared, use threads::shared's internal ID.
# Otherwise, use refaddr().
my $addr1 = is_shared($ref1) || refaddr($ref1);
my $addr2 = is_shared($ref2) || refaddr($ref2);
if ($addr1 == $addr2) {
# The refs are equivalent
}
each() does not work properly on shared references embedded in shared structures. For example:
my %foo :shared;
$foo{'bar'} = shared_clone({'a'=>'x', 'b'=>'y', 'c'=>'z'});
while (my ($key, $val) = each(%{$foo{'bar'}})) {
...
}
Either of the following will work instead:
my $ref = $foo{'bar'};
while (my ($key, $val) = each(%{$ref})) {
...
}
foreach my $key (keys(%{$foo{'bar'}})) {
my $val = $foo{'bar'}{$key};
...
}
View existing bug reports at, and submit any new bugs, problems, patches, etc. to:
http://rt.cpan.org/Public/Dist/Display.html?Name=threads-shared
<http://rt.cpan.org/Public/Dist/Display.html?Name=threads-shared>;
SEE ALSO
threads::shared Discussion Forum on CPAN: http://www.cpanforum.com/dist/threads-shared
<http://www.cpanforum.com/dist/threads-shared>;
Annotated POD for threads::shared: http://annocpan.org/~JDHEDDEN/threads-shared-1.32/shared.pm
<http://annocpan.org/~JDHEDDEN/threads-shared-1.32/shared.pm>;
Source repository: http://code.google.com/p/threads-shared/ <http://code.google.com/p/threads-shared/>;
threads, perlthrtut
<http://www.perl.com/pub/a/2002/06/11/threads.html>; and <http://www.perl.com/pub/a/2002/09/04/threads.html>;
Perl threads mailing list: <http://lists.cpan.org/showlist.cgi?name=iThreads>;
AUTHOR
Artur Bergman <sky AT crucially DOT net>
Documentation borrowed from the old Thread.pm.
CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>.
LICENSE
threads::shared is released under the same license as Perl.
perl v5.12.4 2011-06-20 threads::shared(3pm)
