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2759882379
Support for (nullable) was added to G-I at the same time as nullable return values. Previous versions of G-I will not mark return values as nullable, even when an (allow-none) annotation is present, so it is not necessary to add (allow-none) annotations for compatibility with older versions of G-I. https://bugzilla.gnome.org/show_bug.cgi?id=730957
423 lines
11 KiB
C
423 lines
11 KiB
C
/* GStreamer
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* Copyright (C) 2009 Edward Hervey <bilboed@bilboed.com>
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* 2011 Wim Taymans <wim.taymans@gmail.com>
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*
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* gstatomicqueue.c:
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*
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* This library is free software; you can redistribute it and/or
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* modify it under the terms of the GNU Library General Public
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* License as published by the Free Software Foundation; either
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* version 2 of the License, or (at your option) any later version.
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*
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* This library is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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* Library General Public License for more details.
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*
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* You should have received a copy of the GNU Library General Public
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* License along with this library; if not, write to the
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* Free Software Foundation, Inc., 51 Franklin St, Fifth Floor,
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* Boston, MA 02110-1301, USA.
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*/
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#include "gst_private.h"
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#include <string.h>
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#include <gst/gst.h>
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#include "gstatomicqueue.h"
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#include "glib-compat-private.h"
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/**
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* SECTION:gstatomicqueue
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* @title: GstAtomicQueue
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* @short_description: An atomic queue implementation
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*
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* The #GstAtomicQueue object implements a queue that can be used from multiple
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* threads without performing any blocking operations.
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*/
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G_DEFINE_BOXED_TYPE (GstAtomicQueue, gst_atomic_queue,
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(GBoxedCopyFunc) gst_atomic_queue_ref,
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(GBoxedFreeFunc) gst_atomic_queue_unref);
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/* By default the queue uses 2 * sizeof(gpointer) * clp2 (max_items) of
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* memory. clp2(x) is the next power of two >= than x.
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*
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* The queue can operate in low memory mode, in which it consumes almost
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* half the memory at the expense of extra overhead in the readers. This
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* is disabled by default because even without LOW_MEM mode, the memory
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* consumption is still lower than a plain GList.
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*/
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#undef LOW_MEM
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typedef struct _GstAQueueMem GstAQueueMem;
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struct _GstAQueueMem
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{
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gint size;
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gpointer *array;
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volatile gint head;
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volatile gint tail_write;
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volatile gint tail_read;
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GstAQueueMem *next;
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GstAQueueMem *free;
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};
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static guint
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clp2 (guint n)
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{
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guint res = 1;
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while (res < n)
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res <<= 1;
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return res;
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}
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static GstAQueueMem *
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new_queue_mem (guint size, gint pos)
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{
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GstAQueueMem *mem;
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mem = g_new (GstAQueueMem, 1);
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/* we keep the size as a mask for performance */
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mem->size = clp2 (MAX (size, 16)) - 1;
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mem->array = g_new0 (gpointer, mem->size + 1);
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mem->head = pos;
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mem->tail_write = pos;
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mem->tail_read = pos;
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mem->next = NULL;
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mem->free = NULL;
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return mem;
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}
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static void
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free_queue_mem (GstAQueueMem * mem)
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{
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g_free (mem->array);
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g_free (mem);
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}
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struct _GstAtomicQueue
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{
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volatile gint refcount;
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#ifdef LOW_MEM
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gint num_readers;
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#endif
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GstAQueueMem *head_mem;
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GstAQueueMem *tail_mem;
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GstAQueueMem *free_list;
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};
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static void
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add_to_free_list (GstAtomicQueue * queue, GstAQueueMem * mem)
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{
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do {
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mem->free = g_atomic_pointer_get (&queue->free_list);
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} while (!g_atomic_pointer_compare_and_exchange (&queue->free_list,
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mem->free, mem));
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}
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static void
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clear_free_list (GstAtomicQueue * queue)
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{
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GstAQueueMem *free_list;
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/* take the free list and replace with NULL */
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do {
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free_list = g_atomic_pointer_get (&queue->free_list);
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if (free_list == NULL)
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return;
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} while (!g_atomic_pointer_compare_and_exchange (&queue->free_list, free_list,
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NULL));
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while (free_list) {
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GstAQueueMem *next = free_list->free;
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free_queue_mem (free_list);
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free_list = next;
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}
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}
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/**
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* gst_atomic_queue_new:
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* @initial_size: initial queue size
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*
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* Create a new atomic queue instance. @initial_size will be rounded up to the
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* nearest power of 2 and used as the initial size of the queue.
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*
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* Returns: a new #GstAtomicQueue
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*/
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GstAtomicQueue *
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gst_atomic_queue_new (guint initial_size)
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{
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GstAtomicQueue *queue;
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queue = g_new (GstAtomicQueue, 1);
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queue->refcount = 1;
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#ifdef LOW_MEM
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queue->num_readers = 0;
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#endif
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queue->head_mem = queue->tail_mem = new_queue_mem (initial_size, 0);
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queue->free_list = NULL;
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return queue;
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}
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/**
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* gst_atomic_queue_ref:
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* @queue: a #GstAtomicQueue
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*
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* Increase the refcount of @queue.
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*/
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void
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gst_atomic_queue_ref (GstAtomicQueue * queue)
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{
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g_return_if_fail (queue != NULL);
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g_atomic_int_inc (&queue->refcount);
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}
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static void
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gst_atomic_queue_free (GstAtomicQueue * queue)
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{
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free_queue_mem (queue->head_mem);
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if (queue->head_mem != queue->tail_mem)
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free_queue_mem (queue->tail_mem);
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clear_free_list (queue);
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g_free (queue);
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}
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/**
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* gst_atomic_queue_unref:
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* @queue: a #GstAtomicQueue
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*
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* Unref @queue and free the memory when the refcount reaches 0.
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*/
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void
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gst_atomic_queue_unref (GstAtomicQueue * queue)
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{
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g_return_if_fail (queue != NULL);
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if (g_atomic_int_dec_and_test (&queue->refcount))
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gst_atomic_queue_free (queue);
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}
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/**
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* gst_atomic_queue_peek:
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* @queue: a #GstAtomicQueue
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*
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* Peek the head element of the queue without removing it from the queue.
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*
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* Returns: (transfer none) (nullable): the head element of @queue or
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* %NULL when the queue is empty.
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*/
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gpointer
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gst_atomic_queue_peek (GstAtomicQueue * queue)
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{
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GstAQueueMem *head_mem;
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gint head, tail, size;
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g_return_val_if_fail (queue != NULL, NULL);
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while (TRUE) {
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GstAQueueMem *next;
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head_mem = g_atomic_pointer_get (&queue->head_mem);
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head = g_atomic_int_get (&head_mem->head);
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tail = g_atomic_int_get (&head_mem->tail_read);
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size = head_mem->size;
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/* when we are not empty, we can continue */
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if (G_LIKELY (head != tail))
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break;
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/* else array empty, try to take next */
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next = g_atomic_pointer_get (&head_mem->next);
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if (next == NULL)
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return NULL;
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/* now we try to move the next array as the head memory. If we fail to do that,
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* some other reader managed to do it first and we retry */
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if (!g_atomic_pointer_compare_and_exchange (&queue->head_mem, head_mem,
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next))
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continue;
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/* when we managed to swing the head pointer the old head is now
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* useless and we add it to the freelist. We can't free the memory yet
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* because we first need to make sure no reader is accessing it anymore. */
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add_to_free_list (queue, head_mem);
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}
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return head_mem->array[head & size];
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}
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/**
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* gst_atomic_queue_pop:
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* @queue: a #GstAtomicQueue
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*
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* Get the head element of the queue.
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*
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* Returns: (transfer full): the head element of @queue or %NULL when
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* the queue is empty.
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*/
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gpointer
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gst_atomic_queue_pop (GstAtomicQueue * queue)
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{
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gpointer ret;
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GstAQueueMem *head_mem;
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gint head, tail, size;
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g_return_val_if_fail (queue != NULL, NULL);
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#ifdef LOW_MEM
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g_atomic_int_inc (&queue->num_readers);
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#endif
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do {
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while (TRUE) {
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GstAQueueMem *next;
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head_mem = g_atomic_pointer_get (&queue->head_mem);
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head = g_atomic_int_get (&head_mem->head);
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tail = g_atomic_int_get (&head_mem->tail_read);
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size = head_mem->size;
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/* when we are not empty, we can continue */
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if G_LIKELY
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(head != tail)
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break;
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/* else array empty, try to take next */
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next = g_atomic_pointer_get (&head_mem->next);
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if (next == NULL)
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return NULL;
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/* now we try to move the next array as the head memory. If we fail to do that,
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* some other reader managed to do it first and we retry */
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if G_UNLIKELY
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(!g_atomic_pointer_compare_and_exchange (&queue->head_mem, head_mem,
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next))
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continue;
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/* when we managed to swing the head pointer the old head is now
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* useless and we add it to the freelist. We can't free the memory yet
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* because we first need to make sure no reader is accessing it anymore. */
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add_to_free_list (queue, head_mem);
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}
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ret = head_mem->array[head & size];
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} while G_UNLIKELY
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(!g_atomic_int_compare_and_exchange (&head_mem->head, head, head + 1));
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#ifdef LOW_MEM
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/* decrement number of readers, when we reach 0 readers we can be sure that
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* none is accessing the memory in the free list and we can try to clean up */
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if (g_atomic_int_dec_and_test (&queue->num_readers))
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clear_free_list (queue);
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#endif
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return ret;
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}
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/**
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* gst_atomic_queue_push:
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* @queue: a #GstAtomicQueue
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* @data: the data
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*
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* Append @data to the tail of the queue.
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*/
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void
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gst_atomic_queue_push (GstAtomicQueue * queue, gpointer data)
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{
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GstAQueueMem *tail_mem;
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gint head, tail, size;
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g_return_if_fail (queue != NULL);
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do {
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while (TRUE) {
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GstAQueueMem *mem;
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tail_mem = g_atomic_pointer_get (&queue->tail_mem);
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head = g_atomic_int_get (&tail_mem->head);
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tail = g_atomic_int_get (&tail_mem->tail_write);
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size = tail_mem->size;
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/* we're not full, continue */
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if G_LIKELY
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(tail - head <= size)
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break;
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/* else we need to grow the array, we store a mask so we have to add 1 */
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mem = new_queue_mem ((size << 1) + 1, tail);
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/* try to make our new array visible to other writers */
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if G_UNLIKELY
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(!g_atomic_pointer_compare_and_exchange (&queue->tail_mem, tail_mem,
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mem)) {
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/* we tried to swap the new writer array but something changed. This is
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* because some other writer beat us to it, we free our memory and try
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* again */
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free_queue_mem (mem);
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continue;
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}
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/* make sure that readers can find our new array as well. The one who
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* manages to swap the pointer is the only one who can set the next
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* pointer to the new array */
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g_atomic_pointer_set (&tail_mem->next, mem);
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}
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} while G_UNLIKELY
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(!g_atomic_int_compare_and_exchange (&tail_mem->tail_write, tail, tail + 1));
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tail_mem->array[tail & size] = data;
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/* now wait until all writers have completed their write before we move the
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* tail_read to this new item. It is possible that other writers are still
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* updating the previous array slots and we don't want to reveal their changes
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* before they are done. FIXME, it would be nice if we didn't have to busy
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* wait here. */
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while G_UNLIKELY
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(!g_atomic_int_compare_and_exchange (&tail_mem->tail_read, tail, tail + 1));
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}
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/**
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* gst_atomic_queue_length:
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* @queue: a #GstAtomicQueue
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*
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* Get the amount of items in the queue.
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*
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* Returns: the number of elements in the queue.
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*/
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guint
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gst_atomic_queue_length (GstAtomicQueue * queue)
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{
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GstAQueueMem *head_mem, *tail_mem;
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gint head, tail;
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g_return_val_if_fail (queue != NULL, 0);
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#ifdef LOW_MEM
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g_atomic_int_inc (&queue->num_readers);
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#endif
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head_mem = g_atomic_pointer_get (&queue->head_mem);
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head = g_atomic_int_get (&head_mem->head);
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tail_mem = g_atomic_pointer_get (&queue->tail_mem);
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tail = g_atomic_int_get (&tail_mem->tail_read);
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#ifdef LOW_MEM
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if (g_atomic_int_dec_and_test (&queue->num_readers))
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clear_free_list (queue);
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#endif
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return tail - head;
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}
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