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
-
- Array refs
-
- Hash refs
-
- Scalar refs
-
- Objects based on the above
-
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 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()
-
Creates a new empty queue.
- ->
new(LIST) ->new(LIST)
-
Creates a new queue pre-populated with the provided list of items.
The following methods deal with queues on a FIFO basis.
- ->enqueue(LIST)
-
Adds a list of items onto the end of the queue.
- ->dequeue()
-
- ->
dequeue(COUNT) ->dequeue(COUNT)
-
Removes the requested number of items (default is 1) from the head of the
queue, and returns them. If the queue contains fewer than the requested
number of items, then the thread will be blocked until the requisite number
of items are available (i.e., until other threads
enqueue more items).
- ->dequeue_nb()
-
- ->
dequeue_nb(COUNT) ->dequeue_nb(COUNT)
-
Removes the requested number of items (default is 1) from the head of the
queue, and returns them. If the queue contains fewer than the requested
number of items, then it immediately (i.e., non-blocking) returns whatever
items there are on the queue. If the queue is empty, then
undef is
returned.
- ->dequeue_timed(TIMEOUT)
-
- ->dequeue_timed(TIMEOUT, COUNT)->dequeue_timed(TIMEOUT, COUNT)
-
Removes the requested number of items (default is 1) from the head of the
queue, and returns them. If the queue contains fewer than the requested
number of items, then the thread will be blocked until the requisite number of
items are available, or until the timeout is reached. If the timeout is
reached, it returns whatever items there are on the queue, or
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()
-
Returns the number of items still in the queue. Returns
undef if the queue
has been ended (see below), and there are no more items in the queue.
- ->limit
-
Sets the size of the queue. If set, calls to
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')
$q->limit = 0; # Queue size is now unlimited
Calling any of the dequeue methods with COUNT greater than a queue's
limit will generate an error.
- ->end()
-
Declares that no more items will be added to the queue.
All threads blocking on dequeue() calls will be unblocked with any
remaining items in the queue and/or undef being returned. Any subsequent
calls to dequeue() will behave like dequeue_nb() .
Once ended, no more items may be placed in the queue.
The following methods can be used to manipulate items anywhere in a queue.
To prevent the contents of a queue from being modified by another thread
while it is being examined and/or changed, lock the queue inside a local block:
{
lock($q); # Keep other threads from changing the queue's contents
my $item = $q->peek();
if ($item ...) {
...
}
}
# Queue is now unlocked
- ->peek()
-
- ->
peek(INDEX) ->peek(INDEX)
-
Returns an item from the queue without dequeuing anything. Defaults to the
the head of queue (at index position 0) if no index is specified. Negative
index values are supported as with arrays (i.e., -1
is the end of the queue, -2 is next to last, and so on).
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.
- ->insert(INDEX, LIST)
-
Adds the list of items to the queue at the specified index position (0
is the head of the list). Any existing items at and beyond that position are
pushed back past the newly added items:
$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(INDEX)
-
- ->extract(INDEX, COUNT)->extract(INDEX, COUNT)
-
Removes and returns the specified number of items (defaults to 1) from the
specified index position in the queue (0 is the head of the queue). When
called with no arguments,
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
threads, 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.
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