manpagez: man pages & more
man Thread::Queue(3)
Home | html | info | man
Thread::Queue(3pm)     Perl Programmers Reference Guide     Thread::Queue(3pm)




NAME

       Thread::Queue - Thread-safe queues


VERSION

       This document describes Thread::Queue version 3.12


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 {
                   # 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);


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.

       ->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.

       ->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.


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 on MetaCPAN: <https://metacpan.org/release/Thread-Queue>

       Code repository for CPAN distribution:
       <https://github.com/Dual-Life/Thread-Queue>

       threads(3), threads::shared(3)

       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.



perl v5.26.1                      2017-07-18                Thread::Queue(3pm)

perl 5.26.1 - Generated Tue Nov 7 05:56:48 CST 2017
© manpagez.com 2000-2017
Individual documents may contain additional copyright information.