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Thread::Queue(3pm)                    Perl Programmers Reference Guide                    Thread::Queue(3pm)



NAME
       Thread::Queue - Thread-safe queues

VERSION
       This document describes Thread::Queue version 2.12

SYNOPSIS
           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 {
                                       while (my $item = $q->dequeue()) {
                                           # Do work on $item
                                       }
                                    })->detach();

           # Send work to the thread
           $q->enqueue($item1, ...);


           # Count of items in the queue
           my $left = $q->pending();

           # Non-blocking dequeue
           if (defined(my $item = $q->dequeue_nb())) {
               # Work on $item
           }

           # 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);

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 "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.

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.

       ->pending()
           Returns the number of items still 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:

           {
               lock($q);   # Keep other threads from changing the queue's contents
               my $item = $q->peek();
               if ($item ...) {
                   ...
               }
           }
           # 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:

               $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, 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

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
       <http://www.cpanforum.com/dist/Thread-Queue>

       threads, threads::shared

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.



perl v5.16.2                                     2012-10-11                               Thread::Queue(3pm)