| Thread::Queue(3pm) | Perl Programmers Reference Guide | Thread::Queue(3pm) |
Thread::Queue - Thread-safe queues
This document describes Thread::Queue version 3.14
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 threads::shared 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 "bless"ings and read-only settings) into thread-shared structures before being placed onto a queue.
For example, the following would cause Thread::Queue 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.
The following methods deal with queues on a FIFO basis.
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".
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')
$q->limit = 0; # Queue size is now unlimited
Calling any of the dequeue methods with "COUNT" greater than a queue's "limit" will generate an error.
All threads blocking on "dequeue()" calls will be unblocked with any remaining items in the queue and/or "undef" being returned. Any subsequent calls to "dequeue()" will behave like "dequeue_nb()".
Once ended, no more items may be placed in the queue.
The following methods can be used to manipulate items anywhere in a queue.
To prevent the contents of a queue from being modified by another thread while it is being examined and/or changed, lock the queue inside a local block:
{
lock($q); # Keep other threads from changing the queue's contents
my $item = $q->peek();
if ($item ...) {
...
}
}
# Queue is now unlocked
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 "peek"ed 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.
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 Thread::Queue 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 threads::shared 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>
threads, threads::shared
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.
| 2022-02-19 | perl v5.34.1 |