NAME¶
threads::shared - Perl extension for sharing data structures between threads
VERSION¶
This document describes threads::shared version 1.46
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:
$threads::shared::clone_warn = 0;
then the "undef" substitution will be performed silently.
- is_shared VARIABLE
- "is_shared" checks if the specified variable is shared or not.
If shared, returns the variable's internal ID (similar to
"refaddr()" (see Scalar::Util). Otherwise, returns
"undef".
if (is_shared($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 $var :shared;
{
lock($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.
WARNINGS¶
- cond_broadcast() called on unlocked variable
- cond_signal() called on unlocked variable
- See "cond_signal VARIABLE", above.
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.