  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 createdthread gets a private copy of each existing variable. This module allows youto share variables across different threads (and pseudo-forks on Win32). Itis used together with the threads module.

This module supports the sharing of the following data types only: scalarsand 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_signaland 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 writemodules 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, orrefs 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 itsargument, performing a deep copy on any non-shared elements. Any sharedelements 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 torefaddr()). 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 thespecified element belongs to a shared array or hash. (It does not checkthe 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 ofscope. If the variable is locked by another thread, the lock call willblock until it's available. Multiple calls to lock by the same thread fromwithin dynamically nested scopes are safe -- the variable will remain lockeduntil 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 thelock to go out of scope. This is most easily accomplished by locking thevariable 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 byanother 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, seeThread::Semaphore.
cond_wait VARIABLE
cond_wait CONDVAR, LOCKVAR
The cond_wait function takes a locked variable as a parameter, unlocksthe variable, and blocks until another thread does a cond_signal orcond_broadcast for that same locked variable. The variable thatcond_wait blocked on is relocked after the cond_wait is satisfied. Ifthere are multiple threads cond_waiting on the same variable, all but onewill re-block waiting to reacquire the lock on the variable. (So if you're onlyusing cond_wait for synchronisation, give up the lock as soon as possible).The two actions of unlocking the variable and entering the blocked wait stateare atomic, the two actions of exiting from the blocked wait state andre-locking the variable are not.
In its second form, cond_wait takes a shared, unlocked variable followedby a shared, locked variable. The second variable is unlocked and threadexecution suspended until another thread signals the first variable.
It is important to note that the variable can be notified even if no threadcond_signal or cond_broadcast on the variable. It is thereforeimportant to check the value of the variable and go back to waiting if therequirement is not fulfilled. For example, to pause until a shared counterdrops 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 anabsolute timeout as parameters, unlocks the variable, and blocks until thetimeout is reached or another thread signals the variable. A false value isreturned if the timeout is reached, and a true value otherwise. In eithercase, the variable is re-locked upon return.
Like cond_wait, this function may take a shared, locked variable as anadditional parameter; in this case the first parameter is an unlockedcondition 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 functionreturns. Since the timeout is an absolute value, however, it does not have tobe 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 andunblocks one thread that's cond_waiting on that variable. If more than onethread is blocked in a cond_wait on that variable, only one (and which oneis indeterminate) will be unblocked.
If there are no threads blocked in a cond_wait on the variable, the signalis discarded. By always locking before signaling, you can (with care), avoidsignaling before another thread has entered cond_wait().
cond_signal will normally generate a warning if you attempt to use it on anunlocked variable. On the rare occasions where doing this may be sensible, youcan 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 ina cond_wait on the locked variable, rather than only one.

OBJECTS

threads::shared exports a version of bless REF thatworks 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 notavailable. This allows you to write modules and packages that can be usedin both threaded and non-threaded applications.

If you want access to threads, you must use threads before youuse threads::shared. threads will emit a warning if you use it afterthreads::shared.

BUGS AND LIMITATIONS

When share is used on arrays, hashes, array refs or hash refs, any datathey 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. (Scalarand scalar refs are not affected by this problem.)

It is often not wise to share an object unless the class itself has beenwritten to support sharing. For example, an object's destructor may getcalled multiple times, once for each thread's scope exit. Another danger isthat the contents of hash-based objects will be lost due to the abovementioned limitation. See examples/class.pl (in the CPAN distribution ofthis module) for how to create a class that supports object sharing.

Destructors may not be called on objects if those objects still exist atglobal destruction time. If the destructors must be called, make surethere are no circular references and that nothing is referencing theobjects, before the program ends.

Does not support splice on arrays. Does not support explicitly changingarray lengths via $#array -- use push and pop instead.

Taking references to the elements of shared arrays and hashes does notautovivify the elements, and neither does slicing a shared array/hash overnon-existent indices/keys autovivify the elements.

share() allows you to share($hashref->{key}) andshare($arrayref->[idx]) without giving any error message. But the$hashref->{key} or $arrayref->[idx] is not shared, causingthe error "lock can only be used on shared values" to occur when you attemptto lock($hasref->{key}) or lock($arrayref->[idx]) in anotherthread.

Using refaddr()) is unreliable for testingwhether or not two shared references are equivalent (e.g., when testing forcircular 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 HASH does not work properly on shared referencesembedded 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

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.

Page index

NAME
VERSION
SYNOPSIS
DESCRIPTION
EXPORT
FUNCTIONS
OBJECTS
NOTES
BUGS AND LIMITATIONS
SEE ALSO
AUTHOR
LICENSE

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