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threads::shared(3) - threads::shared - Perl extension for sharing data structures between threads - man 3 threads::shared

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

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

         use threads;
         use threads::shared;

         my $var : shared;

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

         { lock(%hash); ...  }

         cond_timedwait($scalar, time(1,2,n)() + 30);

         my $lockvar : shared;
         # condition var != lock var
         cond_wait($var, $lockvar);
         cond_timedwait($var, time(1,2,n)()+30, $lockvar);

       By default, variables are private to each thread, and each newly cre-
       ated thread gets(3,n) 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", "cond_broadcast"

       Note that if(3,n) this module is imported when "threads" has not yet been
       loaded, then these functions all become no-ops. This makes it possible
       to write(1,2) modules that will work in(1,8) both threaded and non-threaded envi-

       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($a)", while "share(\\$a)" is

           A variable can also be marked as shared at compile time(1,2,n) by using
           the "shared" attribute: "my $var : shared".

           If you want to share a newly created reference unfortunately 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 available. "lock" is recursive, so mul-
           tiple 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(3,n) a thread
           does a "lock @a", any other thread doing a "lock($a[12])" won't

           "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_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 reblock wait-
           ing to reacquire the lock on the variable. (So if(3,n) 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 variable.  The second variable is
           unlocked and thread execution suspended until another thread sig-
           nals the first variable.

           It is important to note that the variable can be notified even if(3,n)
           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(3,n) 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(1,3x,3x cbreak) as parameters, unlocks the variable, and
           blocks until the timeout(1,3x,3x cbreak) is reached or another thread signals the
           variable.  A false value is returned if(3,n) the timeout(1,3x,3x cbreak) 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(1,8) 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(1,3x,3x cbreak) is an absolute value,
           however, it does not have to be recalculated with each pass:

               my $abs = time(1,2,n)() + 15;
               until ($ok = desired_condition($var)) {
                 last if(3,n) !cond_timedwait($var, $abs);
               # we got it if(3,n) $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(1,8) a "cond_wait" on that variable,
           only one (and which one is indeterminate) will be unblocked.

           If there are no threads blocked in(1,8) a "cond_wait" on the variable,
           the signal(2,7) 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(3,n) 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

               { 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(1,8) a "cond_wait" on the locked variable, rather than only

       threads::shared is designed to disable itself silently if(3,n) threads are
       not available. If you want access(2,5) to threads, you must "use threads"
       before you "use threads::shared".  threads will emit a warning if(3,n) you
       use it after threads::shared.

       "bless" is not supported on shared references. In the current version(1,3,5),
       "bless" will only bless the thread local reference and the blessing
       will not propagate to the other threads. This is expected to be imple-
       mented in(1,8) a future version(1,3,5) 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 $hashref->{key}" without giving any error(8,n)
       message.  But the "$hashref->{key}" is not shared, causing the error(8,n)
       "locking can only be used on shared values" to occur when you attempt
       to "lock $hasref->{key}".

       Arthur Bergman <arthur at>

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

       Documentation borrowed from the old

       threads, perlthrtut,

perl v5.8.5                       2001-09-21                threads::shared(3)

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