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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 1.40

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 nonthreaded
       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 relocking
           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.

       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 ALSO
       threads::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>.

LICENSE
       threads::shared is released under the same license as Perl.



perl v5.16.2                                     2012-10-25                             threads::shared(3pm)