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       threads::shared - Perl extension for sharing data struc-
       tures between threads

         use threads;
         use threads::shared;

         my $var : shared;

         my($scalar, @array, %hash);
         my $bar = &share([]);
         $hash{bar} = &share({});

         { 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 pseudoforks on Win32).  It
       is used together with the threads module.

       "share", "cond_wait", "cond_timedwait", "cond_signal",

       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 VARIABLE
           "share" takes a value and marks it as shared. You can
           share a scalar, array, hash, scalar ref, array ref or
           hash ref.  "share" will return the shared rvalue but
           always as a reference.

           "share" will traverse up references exactly one level.
           "share(\$a)" is equivalent to "share($a)", while

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           "share(\\$a)" is not.

           A variable can also be marked as shared at compile
           time by using the "shared" attribute: "my $var :

           If you want to share a newly created reference unfor-
           tunately you need to use "&share([])" and "&share({})"
           syntax due to problems with Perl's prototyping.

       lock VARIABLE
           "lock" places a 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 avail-
           able. "lock" is recursive, so multiple calls to "lock"
           are safe -- the variable will remain locked until the
           outermost lock on the variable goes out of scope.

           If a container object, such as a hash or array, is
           locked, all the elements of that container are not
           locked. For example, if a thread does a "lock @a", any
           other thread doing a "lock($a[12])" won't block.

           "lock" will traverse up references exactly one level.
           "lock(\$a)" is equivalent to "lock($a)", while
           "lock(\\$a)" is not.

           Note that you cannot explicitly unlock a variable; you
           can only wait for the lock to go out of scope. If you
           need more fine-grained control, 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_broad-
           cast" 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 reblock 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 possi-
           ble). 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
           relocking the variable are not.

           In its second form, "cond_wait" takes a shared,
           unlocked variable followed by a shared, locked vari-
           able.  The second variable is unlocked and thread exe-
           cution suspended until another thread signals the
           first variable.

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           It is important to note that the variable can be noti-
           fied even if no thread "cond_signal" or "cond_broad-
           cast" 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($counter); cond_wait($count) 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:

               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

           If there are no threads blocked in a "cond_wait" on
           the variable, the signal is discarded. By always lock-
           ing before signaling, you can (with care), avoid sig-
           naling 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
           skip the warning with

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               { 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 is designed to disable itself silently if
       threads are not available. 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

       "bless" is not supported on shared references. In the cur-
       rent version, "bless" will only bless the thread local
       reference and the blessing will not propagate to the other
       threads. This is expected to be implemented in a future
       version of Perl.

       Does not support splice on arrays!

       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 $hashref->{key}" without giv-
       ing any error message.  But the "$hashref->{key}" is not
       shared, causing the error "locking can only be used on
       shared values" to occur when you attempt to "lock $has-

       Arthur Bergman <arthur at contiller.se>

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

       Documentation borrowed from the old Thread.pm

       threads, perlthrtut,

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