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

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

       This document describes threads::shared version 1.61

	 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,	perlthrtut

       <> 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 v5.32.0			  2020-06-14		    threads::shared(3)


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