| Thread::Queue - Thread-safe queues |
Thread::Queue - Thread-safe queues
This document describes Thread::Queue version 3.13
use strict;
use warnings;
use threads;
use Thread::Queue;
my $q = Thread::Queue->new(); # A new empty queue
# Worker thread
my $thr = threads->create(
sub {
# Thread will loop until no more work
while (defined(my $item = $q->dequeue())) {
# Do work on $item
...
}
}
);
# Send work to the thread
$q->enqueue($item1, ...);
# Signal that there is no more work to be sent
$q->end();
# Join up with the thread when it finishes
$thr->join();
...
# Count of items in the queue
my $left = $q->pending();
# Non-blocking dequeue
if (defined(my $item = $q->dequeue_nb())) {
# Work on $item
}
# Blocking dequeue with 5-second timeout
if (defined(my $item = $q->dequeue_timed(5))) {
# Work on $item
}
# Set a size for a queue
$q->limit = 5;
# Get the second item in the queue without dequeuing anything
my $item = $q->peek(1);
# Insert two items into the queue just behind the head
$q->insert(1, $item1, $item2);
# Extract the last two items on the queue
my ($item1, $item2) = $q->extract(-2, 2);
This module provides thread-safe FIFO queues that can be accessed safely by any number of threads.
Any data types supported by the threads::shared manpage can be passed via queues:
Ordinary scalars are added to queues as they are.
If not already thread-shared, the other complex data types will be cloned
(recursively, if needed, and including any blessings and read-only
settings) into thread-shared structures before being placed onto a queue.
For example, the following would cause the Thread::Queue manpage to create a empty,
shared array reference via &shared([]), copy the elements 'foo', 'bar'
and 'baz' from @ary into it, and then place that shared reference onto
the queue:
my @ary = qw/foo bar baz/; $q->enqueue(\@ary);
However, for the following, the items are already shared, so their references are added directly to the queue, and no cloning takes place:
my @ary :shared = qw/foo bar baz/; $q->enqueue(\@ary);
my $obj = &shared({});
$$obj{'foo'} = 'bar';
$$obj{'qux'} = 99;
bless($obj, 'My::Class');
$q->enqueue($obj);
See LIMITATIONS for caveats related to passing objects via queues.
new()new(LIST)
The following methods deal with queues on a FIFO basis.
enqueue(LIST)dequeue()dequeue(COUNT)enqueue more items).
dequeue_nb()dequeue_nb(COUNT)undef is
returned.
dequeue_timed(TIMEOUT)undef if the
queue is empty.
The timeout may be a number of seconds relative to the current time (e.g., 5 seconds from when the call is made), or may be an absolute timeout in epoch seconds the same as would be used with cond_timedwait(). Fractional seconds (e.g., 2.5 seconds) are also supported (to the extent of the underlying implementation).
If TIMEOUT is missing, undef, or less than or equal to 0, then this call
behaves the same as dequeue_nb.
pending()undef if the queue
has been ended (see below), and there are no more items in the queue.
enqueue() will block until
the number of pending items in the queue drops below the limit. The
limit does not prevent enqueuing items beyond that count:
my $q = Thread::Queue->new(1, 2);
$q->limit = 4;
$q->enqueue(3, 4, 5); # Does not block
$q->enqueue(6); # Blocks until at least 2 items are
# dequeued
my $size = $q->limit; # Returns the current limit (may return
# 'undef')
$q->limit = 0; # Queue size is now unlimited
Calling any of the dequeue methods with COUNT greater than a queue's
limit will generate an error.
end()All threads blocking on dequeue() calls will be unblocked with any
remaining items in the queue and/or undef being returned. Any subsequent
calls to dequeue() will behave like dequeue_nb().
Once ended, no more items may be placed in the queue.
The following methods can be used to manipulate items anywhere in a queue.
To prevent the contents of a queue from being modified by another thread while it is being examined and/or changed, lock the queue inside a local block:
{
lock($q); # Keep other threads from changing the queue's contents
my $item = $q->peek();
if ($item ...) {
...
}
}
# Queue is now unlocked
peek()peek(INDEX)If no items exists at the specified index (i.e., the queue is empty, or the
index is beyond the number of items on the queue), then undef is returned.
Remember, the returned item is not removed from the queue, so manipulating a
peeked at reference affects the item on the queue.
$q->enqueue(1, 2, 3, 4); $q->insert(1, qw/foo bar/); # Queue now contains: 1, foo, bar, 2, 3, 4
Specifying an index position greater than the number of items in the queue just adds the list to the end.
Negative index positions are supported:
$q->enqueue(1, 2, 3, 4); $q->insert(-2, qw/foo bar/); # Queue now contains: 1, 2, foo, bar, 3, 4
Specifying a negative index position greater than the number of items in the queue adds the list to the head of the queue.
extract()extract(INDEX)extract operates the same as dequeue_nb.
This method is non-blocking, and will return only as many items as are available to fulfill the request:
$q->enqueue(1, 2, 3, 4);
my $item = $q->extract(2) # Returns 3
# Queue now contains: 1, 2, 4
my @items = $q->extract(1, 3) # Returns (2, 4)
# Queue now contains: 1
Specifying an index position greater than the number of items in the
queue results in undef or an empty list being returned.
$q->enqueue('foo');
my $nada = $q->extract(3) # Returns undef
my @nada = $q->extract(1, 3) # Returns ()
Negative index positions are supported. Specifying a negative index position
greater than the number of items in the queue may return items from the head
of the queue (similar to dequeue_nb) if the count overlaps the head of the
queue from the specified position (i.e. if queue size + index + count is
greater than zero):
$q->enqueue(qw/foo bar baz/);
my @nada = $q->extract(-6, 2); # Returns () - (3+(-6)+2) <= 0
my @some = $q->extract(-6, 4); # Returns (foo) - (3+(-6)+4) > 0
# Queue now contains: bar, baz
my @rest = $q->extract(-3, 4); # Returns (bar, baz) -
# (2+(-3)+4) > 0
Queues created by the Thread::Queue manpage can be used in both threaded and non-threaded applications.
Passing objects on queues may not work if the objects' classes do not support sharing. See BUGS AND LIMITATIONS in the threads::shared manpage for more.
Passing array/hash refs that contain objects may not work for Perl prior to 5.10.0.
Thread::Queue on MetaCPAN: https://metacpan.org/release/Thread-Queue
Code repository for CPAN distribution: https://github.com/Dual-Life/Thread-Queue
the threads manpage, the threads::shared manpage
Sample code in the examples directory of this distribution on CPAN.
Jerry D. Hedden, <jdhedden AT cpan DOT org>
This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.
| Thread::Queue - Thread-safe queues |