Thread::Queue(3p) Perl Programmers Reference Guide Thread::Queue(3p) #

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

NNAAMMEE #

 Thread::Queue - Thread-safe queues

VVEERRSSIIOONN #

 This document describes Thread::Queue version 3.14

SSYYNNOOPPSSIISS #

     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);

DDEESSCCRRIIPPTTIIOONN #

 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.

QQUUEEUUEE CCRREEAATTIIOONN #

 ->nneeww(())
     Creates a new empty queue.

 ->new(LIST)
     Creates a new queue pre-populated with the provided list of items.

BBAASSIICC MMEETTHHOODDSS #

 The following methods deal with queues on a FIFO basis.

 ->enqueue(LIST)
     Adds a list of items onto the end of the queue.

 ->ddeeqquueeuuee(())
 ->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).

 ->ddeeqquueeuuee__nnbb(())
 ->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 _e_p_o_c_h seconds the same as would be used with
     ccoonndd__ttiimmeeddwwaaiitt(()).  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".

 ->ppeennddiinngg(())
     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.

 ->eenndd(())
     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.

AADDVVAANNCCEEDD MMEETTHHOODDSS #

 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

 ->ppeeeekk(())
 ->peek(INDEX)
     Returns an item from the queue without dequeuing anything.  Defaults
     to 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.

 ->eexxttrraacctt(())
 ->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

NNOOTTEESS #

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

LLIIMMIITTAATTIIOONNSS #

 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.

SSEEEE AALLSSOO #

 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 _e_x_a_m_p_l_e_s directory of this distribution on CPAN.

MMAAIINNTTAAIINNEERR #

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

LLIICCEENNSSEE #

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

perl v5.36.3 2021-03-02 Thread::Queue(3p)