threads::shared(3pm) Perl Programmers Reference Guide threads::shared(3pm)NAMEthreads::shared - Perl extension for sharing data structures between
threads
VERSION
This document describes threads::shared version 1.40
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.
OBJECTSthreads::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'
NOTESthreads::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.
Destructors may not be called on objects if those objects still exist
at global destruction time. If the destructors must be called, make
sure there are no circular references and that nothing is referencing
the objects, before the program ends.
Does not support "splice" on arrays. Does not support explicitly
changing array lengths via $#array -- use "push" and "pop" instead.
Taking references to the elements of shared arrays and hashes does not
autovivify the elements, and neither does slicing a shared array/hash
over non-existent indices/keys autovivify the elements.
"share()" allows you to "share($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(), 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 ALSOthreads::shared Discussion Forum on CPAN:
http://www.cpanforum.com/dist/threads-shared
<http://www.cpanforum.com/dist/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.perl.org/list/ithreads.html>
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>.
LICENSEthreads::shared is released under the same license as Perl.
perl v5.16.3 2013-03-04 threads::shared(3pm)
