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threads::shared(3pm)   Perl Programmers Reference Guide   threads::shared(3pm)


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


       This document describes threads::shared version 1.56


         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


       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


       share VARIABLE
           "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

             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

             $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()"
           (see Scalar::Util).  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;
                 # $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:

             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_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


       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


       cond_broadcast() called on unlocked variable
       cond_signal() called on unlocked variable
           See "cond_signal VARIABLE", above.


       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.)

       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.  See
       examples/ (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:


       threads::shared on MetaCPAN:

       Code repository for CPAN distribution:

       threads(3), perlthrtut(1)

       <> and

       Perl threads mailing list: <>

       Sample code in the examples directory of this distribution on CPAN.


       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.

perl v5.26.1                      2017-07-18              threads::shared(3pm)

perl 5.26.1 - Generated Tue Nov 7 16:14:56 CST 2017
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