threads::shared − Perl extension for sharing data structures between threads


This document describes threads::shared version 1.37


  use threads;
  use threads::shared;
  my $var :shared;
  my %hsh :shared;
  my @ary :shared;
  my ($scalar, @array, %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);
  $ary[3] = shared_clone([ {}, [] ]);
  { lock(%hash); ...  }
  cond_timedwait($scalar, time() + 30);
  my $lockvar :shared;
  # condition var != lock var
  cond_wait($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 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.


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.



"share" takes a variable and marks it as shared:

  my ($scalar, @array, %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" 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;
      # $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

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

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:

  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.


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'} = $obj;
  print(ref($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 must "use threads" before you "use threads::shared". threads will emit a warning if you use it after threads::shared.


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);
  # @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/ (in the CPAN distribution of this module) for how to create a class that supports object sharing.

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 VARIABLE " in 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:−shared <>


threads::shared Discussion Forum on CPAN:−shared <>

threads, perlthrtut

<> and <>

Perl threads mailing list: <>


Artur Bergman <sky AT crucially DOT net>

Documentation borrowed from the old

CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>.


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


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.