Perl 5 version 22.4 documentation

Thread::Queue

NAME

Thread::Queue - Thread-safe queues

VERSION

This document describes Thread::Queue version 3.05

SYNOPSIS

  1.     use strict;
  2.     use warnings;
  4.     use threads;
  5.     use Thread::Queue;
  7.     my $q = Thread::Queue->new();    # A new empty queue
  9.     # Worker thread
  10.     my $thr = threads->create(
  11.         sub {
  12.             # Thread will loop until no more work
  13.             while (defined(my $item = $q->dequeue())) {
  14.                 # Do work on $item
  15.                 ...
  16.             }
  17.         }
  18.     );
  20.     # Send work to the thread
  21.     $q->enqueue($item1, ...);
  22.     # Signal that there is no more work to be sent
  23.     $q->end();
  24.     # Join up with the thread when it finishes
  25.     $thr->join();
  27.     ...
  29.     # Count of items in the queue
  30.     my $left = $q->pending();
  32.     # Non-blocking dequeue
  33.     if (defined(my $item = $q->dequeue_nb())) {
  34.         # Work on $item
  35.     }
  37.     # Blocking dequeue with 5-second timeout
  38.     if (defined(my $item = $q->dequeue_timed(5))) {
  39.         # Work on $item
  40.     }
  42.     # Get the second item in the queue without dequeuing anything
  43.     my $item = $q->peek(1);
  45.     # Insert two items into the queue just behind the head
  46.     $q->insert(1, $item1, $item2);
  48.     # Extract the last two items on the queue
  49.     my ($item1, $item2) = $q->extract(-2, 2);

DESCRIPTION

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
  • 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 blessings 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:

  1.     my @ary = qw/foo bar baz/;
  2.     $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:

  1.     my @ary :shared = qw/foo bar baz/;
  2.     $q->enqueue(\@ary);
  4.     my $obj = &shared({});
  5.     $$obj{'foo'} = 'bar';
  6.     $$obj{'qux'} = 99;
  7.     bless($obj, 'My::Class');
  8.     $q->enqueue($obj);

See LIMITATIONS for caveats related to passing objects via queues.

QUEUE CREATION

  • ->new()

    Creates a new empty queue.

  • ->new(LIST)

    Creates a new queue pre-populated with the provided list of items.

BASIC METHODS

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)

    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)

    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)

    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.

  • ->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.

ADVANCED METHODS

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:

  1.     {
  2.         lock($q);   # Keep other threads from changing the queue's contents
  3.         my $item = $q->peek();
  4.         if ($item ...) {
  5.             ...
  6.         }
  7.     }
  8.     # Queue is now unlocked
  • ->peek()
  • ->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:

    1.     $q->enqueue(1, 2, 3, 4);
    2.     $q->insert(1, qw/foo bar/);
    3.     # 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:

    1.     $q->enqueue(1, 2, 3, 4);
    2.     $q->insert(-2, qw/foo bar/);
    3.     # 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, 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:

    1.     $q->enqueue(1, 2, 3, 4);
    2.     my $item  = $q->extract(2)     # Returns 3
    3.                                    # Queue now contains:  1, 2, 4
    4.     my @items = $q->extract(1, 3)  # Returns (2, 4)
    5.                                    # 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.

    1.     $q->enqueue('foo');
    2.     my $nada = $q->extract(3)      # Returns undef
    3.     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):

    1.     $q->enqueue(qw/foo bar baz/);
    2.     my @nada = $q->extract(-6, 2);   # Returns ()         - (3+(-6)+2) <= 0
    3.     my @some = $q->extract(-6, 4);   # Returns (foo)      - (3+(-6)+4) > 0
    4.                                      # Queue now contains:  bar, baz
    5.     my @rest = $q->extract(-3, 4);   # Returns (bar, baz) - (2+(-3)+4) > 0

NOTES

Queues created by Thread::Queue can be used in both threaded and non-threaded applications.

LIMITATIONS

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.

SEE ALSO

Thread::Queue Discussion Forum on CPAN: http://www.cpanforum.com/dist/Thread-Queue

threads, threads::shared

Sample code in the examples directory of this distribution on CPAN.

MAINTAINER

Jerry D. Hedden, <jdhedden AT cpan DOT org>

LICENSE

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.