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 1.43

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), "
");	     # 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" will croak 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 the "undef" substitution will be performed silently.

       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
");
	     } else {
		 print("$var is not shared
");
	     }

	   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
");
	     }

	     $hash{'elem'} = 1;
	     if (is_shared($hash{'elem'})) {
		 print("$hash{'elem'} is in a shared hash
");
	     }

       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 re-locked 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 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 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 in epoch seconds (see time() 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.

	   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($foo),	   "
");    # Prints 'Yin'
	 print(ref($foo->{'bar'}), "
");    # Prints 'Yang'
	 print(ref($bar),	   "
");    # 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: $foo is empty
")
	     if (! exists($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($hashref->{key})" or "lock($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};
	       ...
	   }

       This module supports dual-valued variables created using dualvar() from Scalar::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>

SEE ALSO

       threads::shared Discussion Forum on CPAN: <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 v5.18.2							    2014-01-06						      threads::shared(3pm)