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use threads; use threads::shared; my $var :shared; $var = $scalar_value; $var = $shared_ref_value; $var = share($simple_unshared_ref_value); my ($scalar, @array, %hash); share($scalar); share(@array); share(%hash); my $bar = &share([]); $hash{bar} = &share({}); { lock(%hash); ... } cond_wait($scalar); cond_timedwait($scalar, time() + 30); cond_broadcast(@array); cond_signal(%hash); my $lockvar :shared; # condition var != lock var cond_wait($var, $lockvar); cond_timedwait($var, time()+30, $lockvar);
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.
A variable can also be marked as shared at compile time by using the ":shared" attribute: "my $var :shared;".
Due to problems with Perl's prototyping, if you want to share a newly created reference, you need to use the "&share([])" and "&share({})" syntax.
The only values that can be assigned to a shared scalar are other scalar values, or shared refs:
my $var :shared; $var = 1; # ok $var = []; # error $var = &share([]); # ok
"share" will traverse up references exactly one level. "share(\$a)" is equivalent to "share($a)", while "share(\\$a)" is not. This means that you must create nested shared data structures by first creating individual shared leaf nodes, and then adding them to a shared hash or array.
my %hash :shared; $hash{'meaning'} = &share([]); $hash{'meaning'}[0] = &share({}); $hash{'meaning'}[0]{'life'} = 42;
if (is_shared($var)) { print("\$var is shared\n"); } else { print("\$var is not shared\n"); }
Locking a container object, such as a hash or array, doesn't lock the elements of that container. For example, if a thread does a "lock(@a)", any other thread doing a "lock($a[12])" won't block.
"lock()" follows references exactly one level. "lock(\$a)" is equivalent to "lock($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. This is most easily accomplished by locking the variable inside a block.
my $var :shared; { lock($var); # $var is locked from here to the end of the block ... } # $var is now unlocked
If you need more fine-grained control over shared variable access, see Thread::Semaphore.
In its second form, "cond_wait" takes a shared, unlocked variable followed by a shared, locked variable. The second variable is unlocked and thread execution suspended until another thread signals the first variable.
It is important to note that the variable can be notified even if no thread "cond_signal" or "cond_broadcast" on the variable. It is therefore important to check the value of the variable and go back to waiting if the requirement is not fulfilled. For example, to pause until a shared counter drops to zero:
{ lock($counter); cond_wait($count) until $counter == 0; }
Like "cond_wait", this function may take a shared, locked variable as an additional parameter; in this case the first parameter is an unlocked condition variable protected by a distinct lock variable.
Again like "cond_wait", waking up and reacquiring the lock are not atomic, and you should always check your desired condition after this function returns. Since the timeout is an absolute value, however, it does not have to be recalculated with each pass:
lock($var); my $abs = time() + 15; until ($ok = desired_condition($var)) { last if !cond_timedwait($var, $abs); } # we got it if $ok, otherwise we timed out!
If there are no threads blocked in a "cond_wait" on the variable, the signal is discarded. By always locking before signaling, you can (with care), avoid signaling before another thread has entered cond_wait().
"cond_signal" will normally generate a warning if you attempt to use it on an unlocked variable. On the rare occasions where doing this may be sensible, you can suppress the warning with:
{ no warnings 'threads'; cond_signal($foo); }
# Create a shared 'foo' object my $foo; share($foo); $foo = &share({}); bless($foo, 'foo'); # Create a shared 'bar' object my $bar; share($bar); $bar = &share({}); bless($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'} = $obj; })->join(); print(ref($foo), "\n"); # Prints 'yin' print(ref($foo->{'bar'}), "\n"); # Prints 'yang' print(ref($bar), "\n"); # Also prints 'yang'
my @arr = qw(foo bar baz); share(@arr); # @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/class.pl (in the CPAN distribution of this module) for how to create a class that supports object sharing.
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($hashref->{key})" without giving any error message. But the "$hashref->{key}" is not shared, causing the error ``locking can only be used on shared values'' to occur when you attempt to "lock($hasref->{key})".
View existing bug reports at, and submit any new bugs, problems, patches, etc. to: <http://rt.cpan.org/NoAuth/Bugs.html?Dist=threads-shared>
Annotated POD for threads::shared: <http://annocpan.org/~JDHEDDEN/threads-shared-1.14/shared.pm>
Source repository: <http://code.google.com/p/threads-shared/>
threads, perlthrtut
<http://www.perl.com/pub/a/2002/06/11/threads.html> and <http://www.perl.com/pub/a/2002/09/04/threads.html>
Perl threads mailing list: <http://lists.cpan.org/showlist.cgi?name=iThreads>
threads::shared is released under the same license as Perl.
Documentation borrowed from the old Thread.pm.
CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>.