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NAME
threads::shared - Perl extension for sharing data structures between threads
VERSION
This document describes threads::shared version 1.43
SYNOPSIS
use threads;
use threads::shared;
my $var :shared;
my %hsh :shared;
my @ary :shared;
my ($scalar, @array, %hash);
share($scalar);
share(@array);
share(%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);
$ary[3] = shared_clone([ {}, [] ]);
{ lock(%hash); ... }
cond_wait($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, $lockvar);
DESCRIPTION
By default, variables are private to each thread, and each newly created thread gets a private copy of
each existing variable. This module allows you to share variables across different threads (and pseudo-
forks on Win32). It is used together with the threads module.
This module supports the sharing of the following data types only: scalars and scalar refs, arrays and
array refs, and hashes and hash refs.
EXPORT
The following functions are exported by this module: "share", "shared_clone", "is_shared", "cond_wait",
"cond_timedwait", "cond_signal" and "cond_broadcast"
Note that if this module is imported when threads has not yet been loaded, then these functions all
become no-ops. This makes it possible to write modules that will work in both threaded and non-threaded
environments.
FUNCTIONS
share VARIABLE
"share" takes a variable and marks it as shared:
my ($scalar, @array, %hash);
share($scalar);
share(@array);
share(%hash);
"share" will return the shared rvalue, but always as a reference.
Variables can also be marked as shared at compile time by using the ":shared" attribute:
my ($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 cloned.
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 used:
$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:
$threads::shared::clone_warn = 0;
then the "undef" substitution will be performed silently.
is_shared VARIABLE
"is_shared" checks if the specified variable is shared or not. If shared, returns the variable's
internal ID (similar to refaddr()). Otherwise, returns "undef".
if (is_shared($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 $var :shared;
{
lock($var);
# $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 $counter == 0; }
cond_timedwait VARIABLE, ABS_TIMEOUT
cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR
In its two-argument form, "cond_timedwait" takes a locked variable and an absolute timeout in epoch
seconds (see time() in perlfunc for more) as parameters, unlocks the variable, and blocks until the
timeout is reached or another thread signals the variable. A false value is returned if the timeout
is reached, and a true value otherwise. In either case, the variable is re-locked upon return.
Like "cond_wait", this function may take a shared, locked variable as an additional parameter; in
this case the first parameter is an unlocked condition variable protected by a distinct lock
variable.
Again like "cond_wait", waking up and reacquiring the lock are not atomic, and you should always
check your desired condition after this function returns. Since the timeout is an absolute value,
however, it does not have to be recalculated with each pass:
lock($var);
my $abs = time() + 15;
until ($ok = desired_condition($var)) {
last if !cond_timedwait($var, $abs);
}
# we got it if $ok, otherwise we timed out!
cond_signal VARIABLE
The "cond_signal" function takes a locked variable as a parameter and unblocks one thread that's
"cond_wait"ing on that variable. If more than one thread is blocked in a "cond_wait" on that
variable, only one (and which one is indeterminate) will be unblocked.
If there are no threads blocked in a "cond_wait" on the variable, the signal is discarded. By always
locking before signaling, you can (with care), avoid signaling before another thread has entered
cond_wait().
"cond_signal" will normally generate a warning if you attempt to use it on an unlocked variable. On
the rare occasions where doing this may be sensible, you can suppress the warning with:
{ no warnings 'threads'; cond_signal($foo); }
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.
OBJECTS
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'} = $obj;
})->join();
print(ref($foo), "\n"); # Prints 'Yin'
print(ref($foo->{'bar'}), "\n"); # Prints 'Yang'
print(ref($bar), "\n"); # Also prints 'Yang'
NOTES
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 threads::shared.
BUGS AND LIMITATIONS
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);
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.
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:
<http://rt.cpan.org/Public/Dist/Display.html?Name=threads-shared>
SEE ALSO
threads::shared Discussion Forum on CPAN: <http://www.cpanforum.com/dist/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.perl.org/list/ithreads.html>
AUTHOR
Artur Bergman <sky AT crucially DOT net>
Documentation borrowed from the old Thread.pm.
CPAN version produced by Jerry D. Hedden <jdhedden AT cpan DOT org>.
LICENSE
threads::shared is released under the same license as Perl.
perl v5.18.2 2014-01-06 threads::shared(3perl)