threads::shared(3p) Perl Programmers Reference Guide threads::shared(3p) #
threads::shared(3p) Perl Programmers Reference Guide threads::shared(3p)
NNAAMMEE #
threads::shared - Perl extension for sharing data structures between
threads
VVEERRSSIIOONN #
This document describes threads::shared version 1.64
SSYYNNOOPPSSIISS #
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, $lockvar);
DDEESSCCRRIIPPTTIIOONN #
By default, variables are private to each thread, and each newly created
thread gets a private copy of each existing variable. This module allows
you to share variables across different threads (and pseudo-forks on
Win32). It is used together with the threads module.
This module supports the sharing of the following data types only:
scalars and scalar refs, arrays and array refs, and hashes and hash refs.
EEXXPPOORRTT #
The following functions are exported by this module: "share",
"shared_clone", "is_shared", "cond_wait", "cond_timedwait", "cond_signal"
and "cond_broadcast"
Note that if this module is imported when threads has not yet been
loaded, then these functions all become no-ops. This makes it possible
to write modules that will work in both threaded and non-threaded
environments.
FFUUNNCCTTIIOONNSS #
share VARIABLE
"share" takes a variable and marks it as shared:
my ($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 aaddvviissoorryy 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 _o_n_e 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 lloocckkeedd 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, uunnlloocckkeedd variable
followed by a shared, lloocckkeedd 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 $counter == 0; }
cond_timedwait VARIABLE, ABS_TIMEOUT
cond_timedwait CONDVAR, ABS_TIMEOUT, LOCKVAR
In its two-argument form, "cond_timedwait" takes a lloocckkeedd variable
and an absolute timeout in _e_p_o_c_h seconds (see ttiimmee(()) in perlfunc for
more) as parameters, unlocks the variable, and blocks until the
timeout is reached or another thread signals the variable. A false
value is returned if the timeout is reached, and a true value
otherwise. In either case, the variable is re-locked upon return.
Like "cond_wait", this function may take a shared, lloocckkeedd variable as
an additional parameter; in this case the first parameter is an
uunnlloocckkeedd condition variable protected by a distinct lock variable.
Again like "cond_wait", waking up and reacquiring the lock are not
atomic, and you should always check your desired condition after this
function returns. Since the timeout is an absolute value, however,
it does not have to be recalculated with each pass:
lock($var);
my $abs = time() + 15;
until ($ok = desired_condition($var)) {
last if !cond_timedwait($var, $abs);
}
# we got it if $ok, otherwise we timed out!
cond_signal VARIABLE
The "cond_signal" function takes a lloocckkeedd variable as a parameter and
unblocks one thread that's "cond_wait"ing on that variable. If more
than one thread is blocked in a "cond_wait" on that variable, only
one (and which one is indeterminate) will be unblocked.
If there are no threads blocked in a "cond_wait" on the variable, the
signal is discarded. By always locking before signaling, you can
(with care), avoid signaling before another thread has entered
ccoonndd__wwaaiitt(()).
"cond_signal" will normally generate a warning if you attempt to use
it on an unlocked variable. On the rare occasions where doing this
may be sensible, you can suppress the warning with:
{ no warnings 'threads'; cond_signal($foo); }
cond_broadcast VARIABLE
The "cond_broadcast" function works similarly to "cond_signal".
"cond_broadcast", though, will unblock aallll the threads that are
blocked in a "cond_wait" on the locked variable, rather than only
one.
OOBBJJEECCTTSS #
threads::shared exports a version of bblleessss(()) that works on shared objects
such that _b_l_e_s_s_i_n_g_s 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'
NNOOTTEESS #
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.
WWAARRNNIINNGGSS #
ccoonndd__bbrrooaaddccaasstt(()) called on unlocked variable
ccoonndd__ssiiggnnaall(()) called on unlocked variable
See "cond_signal VARIABLE", above.
BBUUGGSS AANNDD LLIIMMIITTAATTIIOONNSS #
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 aafftteerr 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 _e_x_a_m_p_l_e_s_/_c_l_a_s_s_._p_l (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 "$arrayref->[idx]" is nnoott shared, causing the error
"lock can only be used on shared values" to occur when you attempt to
"lock($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 iiss__sshhaarreedd(()), 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
}
eeaacchh(()) 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>
SSEEEE AALLSSOO #
threads::shared on MetaCPAN:
<https://metacpan.org/release/threads-shared>
Code repository for CPAN distribution:
<https://github.com/Dual-Life/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>
Sample code in the _e_x_a_m_p_l_e_s directory of this distribution on CPAN.
AAUUTTHHOORR #
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>.
LLIICCEENNSSEE #
threads::shared is released under the same license as Perl.
perl v5.36.3 2023-02-15 threads::shared(3p)