threads::shared - Perl extension for sharing data structures between threads
This document describes threads::shared version 1.70
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 $lockvar :shared;# condition var != lock varcond_wait($var, $lockvar);cond_timedwait($var, time()+30, $lockvar);By default, variables are private to each thread, and each newly created thread gets a private copy of each existing variable. This module allows you to share variables across different threads (and pseudo-forks on Win32). It is used together with thethreads module.
This module supports the sharing of the following data types only: scalars and scalar refs, arrays and array refs, and hashes and hash refs.
The following functions are exported by this module:share,shared_clone,is_shared,cond_wait,cond_timedwait,cond_signal andcond_broadcast
Note that if this module is imported whenthreads has not yet been loaded, then these functions all become no-ops. This makes it possible to write modules that will work in both threaded and non-threaded environments.
share takes a variable and marks it as shared:
my ($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 refshared_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({});Not all Perl data types can be cloned (e.g., globs, code refs). By default,shared_clone willcroak if it encounters such items. To change this behaviour to a warning, then set the following:
$threads::shared::clone_warn = 1;In this case,undef will be substituted for the item to be cloned. If set to zero:
$threads::shared::clone_warn = 0;then theundef substitution will be performed silently.
is_shared checks if the specified variable is shared or not. If shared, returns the variable's internal ID (similar torefaddr() (seeScalar::Util). Otherwise, returnsundef.
if (is_shared($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 places aadvisory lock on a variable until the lock goes out of scope. If the variable is locked by another thread, thelock call will block until it's available. Multiple calls tolock 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 exactlyone 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 ...}# $var is now unlockedAs locks are advisory, they do not prevent data access or modification by another thread that does not itself attempt to obtain a lock on the variable.
You cannot lock the individual elements of a container variable:
my %hash :shared;$hash{'foo'} = 'bar';#lock($hash{'foo'}); # Errorlock(%hash); # WorksIf you need more fine-grained control over shared variable access, seeThread::Semaphore.
Thecond_wait function takes alocked variable as a parameter, unlocks the variable, and blocks until another thread does acond_signal orcond_broadcast for that same locked variable. The variable thatcond_wait blocked on is re-locked after thecond_wait is satisfied. If there are multiple threadscond_waiting on the same variable, all but one will re-block waiting to reacquire the lock on the variable. (So if you're only usingcond_wait for synchronization, give up the lock as soon as possible). The two actions of unlocking the variable and entering the blocked wait state are atomic, the two actions of exiting from the blocked wait state and re-locking the variable are not.
In its second form,cond_wait takes a shared,unlocked variable followed by a shared,locked variable. The second variable is unlocked and thread execution suspended until another thread signals the first variable.
It is important to note that the variable can be notified even if no threadcond_signal orcond_broadcast on the variable. It is therefore important to check the value of the variable and go back to waiting if the requirement is not fulfilled. For example, to pause until a shared counter drops to zero:
{ lock($counter); cond_wait($counter) until $counter == 0; }In its two-argument form,cond_timedwait takes alocked variable and an absolute timeout inepoch seconds (seetime() in perlfunc for more) as parameters, unlocks the variable, and blocks until the timeout is reached or another thread signals the variable. A false value is returned if the timeout is reached, and a true value otherwise. In either case, the variable is re-locked upon return.
Likecond_wait, this function may take a shared,locked variable as an additional parameter; in this case the first parameter is anunlocked condition variable protected by a distinct lock variable.
Again likecond_wait, waking up and reacquiring the lock are not atomic, and you should always check your desired condition after this function returns. Since the timeout is an absolute value, however, it does not have to be recalculated with each pass:
lock($var);my $abs = time() + 15;until ($ok = desired_condition($var)) { last if !cond_timedwait($var, $abs);}# we got it if $ok, otherwise we timed out!Thecond_signal function takes alocked variable as a parameter and unblocks one thread that'scond_waiting on that variable. If more than one thread is blocked in acond_wait on that variable, only one (and which one is indeterminate) will be unblocked.
If there are no threads blocked in acond_wait on the variable, the signal is discarded. By always locking before signaling, you can (with care), avoid signaling before another thread has entered cond_wait().
cond_signal will normally generate a warning if you attempt to use it on an unlocked variable. On the rare occasions where doing this may be sensible, you can suppress the warning with:
{ no warnings 'threads'; cond_signal($foo); }Thecond_broadcast function works similarly tocond_signal.cond_broadcast, though, will unblockall the threads that are blocked in acond_wait on the locked variable, rather than only one.
threads::shared exports a version ofbless() that works on shared objects such thatblessings propagate across threads.
# Create a shared 'Foo' objectmy $foo :shared = shared_clone({});bless($foo, 'Foo');# Create a shared 'Bar' objectmy $bar :shared = shared_clone({});bless($bar, 'Bar');# Put 'bar' inside 'foo'$foo->{'bar'} = $bar;# Rebless the objects via a threadthreads->create(sub { # Rebless the outer object bless($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'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 mustuse threads before youuse threads::shared.threads will emit a warning if you use it afterthreads::shared.
See"cond_signal VARIABLE", above.
Whenshare 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' objectmy $foo = { 'data' => 99 };bless($foo, 'foo');# Share the objectshare($foo); # Contents are now wiped outprint("ERROR: \$foo is empty\n") if (! exists($foo->{'data'}));Therefore, populate such variablesafter declaring them as shared. (Scalar and scalar refs are not affected by this problem.)
Blessing a shared item after it has been nested in another shared item does not propagate the blessing to the shared reference:
my $foo = &share({});my $bar = &share({});$bar->{foo} = $foo;bless($foo, 'baz'); # $foo is now of class 'baz', # but $bar->{foo} is unblessed.Therefore, you should bless objects before sharing them.
It is often not wise to share an object unless the class itself has been written to support sharing. For example, a shared object's destructor may get called multiple times, once for each thread's scope exit, or may not get called at all if it is embedded inside another shared object. Another issue is that the contents of hash-based objects will be lost due to the above mentioned limitation. Seeexamples/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 supportsplice on arrays. Does not support explicitly changing array lengths via $#array -- usepush andpop instead.
Taking references to the elements of shared arrays and hashes does not autovivify the elements, and neither does slicing a shared array/hash over non-existent indices/keys autovivify the elements.
share() allows you toshare($hashref->{key}) andshare($arrayref->[idx]) without giving any error message. But the$hashref->{key} or$arrayref->[idx] isnot shared, causing the error "lock can only be used on shared values" to occur when you attempt tolock($hashref->{key}) orlock($arrayref->[idx]) in another thread.
Usingrefaddr() is unreliable for testing whether or not two shared references are equivalent (e.g., when testing for circular references). Useis_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}; ...}This module supports dual-valued variables created usingdualvar() fromScalar::Util. However, while$! acts like a dualvar, it is implemented as a tied SV. To propagate its value, use the follow construct, if needed:
my $errno :shared = dualvar($!,$!);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
threads::shared on MetaCPAN:https://metacpan.org/release/threads-shared
Code repository for CPAN distribution:https://github.com/Dual-Life/threads-shared
http://www.perl.com/pub/a/2002/06/11/threads.html andhttp://www.perl.com/pub/a/2002/09/04/threads.html
Perl threads mailing list:http://lists.perl.org/list/ithreads.html
Sample code in theexamples directory of this distribution on CPAN.
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>.
threads::shared is released under the same license as Perl.
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