gstclock.c 40.8 KB
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/* GStreamer
 * Copyright (C) 1999,2000 Erik Walthinsen <omega@cse.ogi.edu>
 *                    2000 Wim Taymans <wtay@chello.be>
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 *                    2004 Wim Taymans <wim@fluendo.com>
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 *
 * gstclock.c: Clock subsystem for maintaining time sync
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 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Library General Public License for more details.
 *
 * You should have received a copy of the GNU Library General Public
 * License along with this library; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 02111-1307, USA.
 */
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/**
 * SECTION:gstclock
 * @short_description: Abstract class for global clocks
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 * @see_also: #GstSystemClock, #GstPipeline
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 *
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 * GStreamer uses a global clock to synchronize the plugins in a pipeline.
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 * Different clock implementations are possible by implementing this abstract
 * base class.
 *
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 * The #GstClock returns a monotonically increasing time with the method
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 * gst_clock_get_time(). Its accuracy and base time depend on the specific
 * clock implementation but time is always expressed in nanoseconds. Since the
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 * baseline of the clock is undefined, the clock time returned is not
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 * meaningful in itself, what matters are the deltas between two clock times.
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 * The time returned by a clock is called the absolute time.
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 *
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 * The pipeline uses the clock to calculate the stream time. Usually all
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 * renderers synchronize to the global clock using the buffer timestamps, the
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 * newsegment events and the element's base time, see #GstPipeline.
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 *
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 * A clock implementation can support periodic and single shot clock
 * notifications both synchronous and asynchronous.
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 *
 * One first needs to create a #GstClockID for the periodic or single shot
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 * notification using gst_clock_new_single_shot_id() or
 * gst_clock_new_periodic_id().
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 *
 * To perform a blocking wait for the specific time of the #GstClockID use the
 * gst_clock_id_wait(). To receive a callback when the specific time is reached
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 * in the clock use gst_clock_id_wait_async(). Both these calls can be
 * interrupted with the gst_clock_id_unschedule() call. If the blocking wait is
 * unscheduled a return value of GST_CLOCK_UNSCHEDULED is returned.
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 *
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 * Periodic callbacks scheduled async will be repeadedly called automatically
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 * until it is unscheduled. To schedule a sync periodic callback,
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 * gst_clock_id_wait() should be called repeadedly.
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 *
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 * The async callbacks can happen from any thread, either provided by the core
 * or from a streaming thread. The application should be prepared for this.
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 *
 * A #GstClockID that has been unscheduled cannot be used again for any wait
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 * operation, a new #GstClockID should be created and the old unscheduled one
 * should be destroyed wirth gst_clock_id_unref().
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 *
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 * It is possible to perform a blocking wait on the same #GstClockID from
 * multiple threads. However, registering the same #GstClockID for multiple
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 * async notifications is not possible, the callback will only be called for
 * the thread registering the entry last.
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 *
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 * None of the wait operations unref the #GstClockID, the owner is responsible
 * for unreffing the ids itself. This holds for both periodic and single shot
 * notifications. The reason being that the owner of the #GstClockID has to
 * keep a handle to the #GstClockID to unblock the wait on FLUSHING events or
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 * state changes and if the entry would be unreffed automatically, the handle 
 * might become invalid without any notification.
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 *
 * These clock operations do not operate on the stream time, so the callbacks
 * will also occur when not in PLAYING state as if the clock just keeps on
 * running. Some clocks however do not progress when the element that provided
 * the clock is not PLAYING.
 *
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 * When a clock has the GST_CLOCK_FLAG_CAN_SET_MASTER flag set, it can be
 * slaved to another #GstClock with the gst_clock_set_master(). The clock will
 * then automatically be synchronized to this master clock by repeadedly
 * sampling the master clock and the slave clock and recalibrating the slave
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 * clock with gst_clock_set_calibration(). This feature is mostly useful for
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 * plugins that have an internal clock but must operate with another clock
 * selected by the #GstPipeline.  They can track the offset and rate difference
 * of their internal clock relative to the master clock by using the
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 * gst_clock_get_calibration() function. 
 *
 * The master/slave synchronisation can be tuned with the "timeout", "window-size"
 * and "window-threshold" properties. The "timeout" property defines the interval
 * to sample the master clock and run the calibration functions. 
 * "window-size" defines the number of samples to use when calibrating and
 * "window-threshold" defines the minimum number of samples before the 
 * calibration is performed.
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 *
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 * Last reviewed on 2006-08-11 (0.10.10)
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 */
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#include "gst_private.h"
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#include <time.h>
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#include "gstclock.h"
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#include "gstinfo.h"
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#include "gstutils.h"
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#ifndef GST_DISABLE_TRACE
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/* #define GST_WITH_ALLOC_TRACE */
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#include "gsttrace.h"
static GstAllocTrace *_gst_clock_entry_trace;
#endif
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#if GLIB_CHECK_VERSION (2, 10, 0)
#define ALLOC_ENTRY()     g_slice_new (GstClockEntry)
#define FREE_ENTRY(entry) g_slice_free (GstClockEntry, entry)
#else
#define ALLOC_ENTRY()     g_new (GstClockEntry, 1)
#define FREE_ENTRY(entry) g_free (entry)
#endif

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/* #define DEBUGGING_ENABLED */

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#define DEFAULT_STATS                   FALSE
#define DEFAULT_WINDOW_SIZE             32
#define DEFAULT_WINDOW_THRESHOLD        4
#define DEFAULT_TIMEOUT                 GST_SECOND / 10
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enum
{
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  PROP_0,
  PROP_STATS,
  PROP_WINDOW_SIZE,
  PROP_WINDOW_THRESHOLD,
  PROP_TIMEOUT
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};

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static void gst_clock_class_init (GstClockClass * klass);
static void gst_clock_init (GstClock * clock);
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static void gst_clock_dispose (GObject * object);
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static void gst_clock_finalize (GObject * object);
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static void gst_clock_set_property (GObject * object, guint prop_id,
    const GValue * value, GParamSpec * pspec);
static void gst_clock_get_property (GObject * object, guint prop_id,
    GValue * value, GParamSpec * pspec);
static void gst_clock_update_stats (GstClock * clock);
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static GstObjectClass *parent_class = NULL;
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/* static guint gst_clock_signals[LAST_SIGNAL] = { 0 }; */

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static GstClockID
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gst_clock_entry_new (GstClock * clock, GstClockTime time,
    GstClockTime interval, GstClockEntryType type)
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{
  GstClockEntry *entry;
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  entry = ALLOC_ENTRY ();
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#ifndef GST_DISABLE_TRACE
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  gst_alloc_trace_new (_gst_clock_entry_trace, entry);
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#endif
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  GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock,
      "created entry %p, time %" GST_TIME_FORMAT, entry, GST_TIME_ARGS (time));
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  gst_atomic_int_set (&entry->refcount, 1);
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  entry->clock = clock;
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  entry->type = type;
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  entry->time = time;
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  entry->interval = interval;
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  entry->status = GST_CLOCK_BUSY;
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  entry->func = NULL;
  entry->user_data = NULL;
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  return (GstClockID) entry;
}
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/**
 * gst_clock_id_ref:
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 * @id: The #GstClockID to ref
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 *
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 * Increase the refcount of given @id.
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 *
 * Returns: The same #GstClockID with increased refcount.
 *
 * MT safe.
 */
GstClockID
gst_clock_id_ref (GstClockID id)
{
  g_return_val_if_fail (id != NULL, NULL);

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  g_atomic_int_inc (&((GstClockEntry *) id)->refcount);
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  return id;
}

static void
_gst_clock_id_free (GstClockID id)
{
  g_return_if_fail (id != NULL);

  GST_CAT_DEBUG (GST_CAT_CLOCK, "freed entry %p", id);

#ifndef GST_DISABLE_TRACE
  gst_alloc_trace_free (_gst_clock_entry_trace, id);
#endif
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  FREE_ENTRY (id);
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}

/**
 * gst_clock_id_unref:
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 * @id: The #GstClockID to unref
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 *
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 * Unref given @id. When the refcount reaches 0 the
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 * #GstClockID will be freed.
 *
 * MT safe.
 */
void
gst_clock_id_unref (GstClockID id)
{
  gint zero;

  g_return_if_fail (id != NULL);

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  zero = g_atomic_int_dec_and_test (&((GstClockEntry *) id)->refcount);
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  /* if we ended up with the refcount at zero, free the id */
  if (zero) {
    _gst_clock_id_free (id);
  }
}

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/**
 * gst_clock_new_single_shot_id
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 * @clock: The #GstClockID to get a single shot notification from
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 * @time: the requested time
 *
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 * Get a #GstClockID from @clock to trigger a single shot
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 * notification at the requested time. The single shot id should be
 * unreffed after usage.
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 *
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 * Returns: A #GstClockID that can be used to request the time notification.
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 *
 * MT safe.
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 */
GstClockID
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gst_clock_new_single_shot_id (GstClock * clock, GstClockTime time)
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{
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  g_return_val_if_fail (GST_IS_CLOCK (clock), NULL);

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  return gst_clock_entry_new (clock,
      time, GST_CLOCK_TIME_NONE, GST_CLOCK_ENTRY_SINGLE);
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}

/**
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 * gst_clock_new_periodic_id
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 * @clock: The #GstClockID to get a periodic notification id from
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 * @start_time: the requested start time
 * @interval: the requested interval
 *
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 * Get an ID from @clock to trigger a periodic notification.
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 * The periodeic notifications will be start at time start_time and
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 * will then be fired with the given interval. @id should be unreffed
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 * after usage.
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 *
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 * Returns: A #GstClockID that can be used to request the time notification.
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 *
 * MT safe.
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 */
GstClockID
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gst_clock_new_periodic_id (GstClock * clock, GstClockTime start_time,
    GstClockTime interval)
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{
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  g_return_val_if_fail (GST_IS_CLOCK (clock), NULL);
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  g_return_val_if_fail (GST_CLOCK_TIME_IS_VALID (start_time), NULL);
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  g_return_val_if_fail (interval != 0, NULL);
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  g_return_val_if_fail (GST_CLOCK_TIME_IS_VALID (interval), NULL);
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  return gst_clock_entry_new (clock,
      start_time, interval, GST_CLOCK_ENTRY_PERIODIC);
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}

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/**
 * gst_clock_id_compare_func
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 * @id1: A #GstClockID
 * @id2: A #GstClockID to compare with
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 *
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 * Compares the two #GstClockID instances. This function can be used
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 * as a GCompareFunc when sorting ids.
 *
 * Returns: negative value if a < b; zero if a = b; positive value if a > b
 *
 * MT safe.
 */
gint
gst_clock_id_compare_func (gconstpointer id1, gconstpointer id2)
{
  GstClockEntry *entry1, *entry2;

  entry1 = (GstClockEntry *) id1;
  entry2 = (GstClockEntry *) id2;

  if (GST_CLOCK_ENTRY_TIME (entry1) > GST_CLOCK_ENTRY_TIME (entry2)) {
    return 1;
  }
  if (GST_CLOCK_ENTRY_TIME (entry1) < GST_CLOCK_ENTRY_TIME (entry2)) {
    return -1;
  }
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  return 0;
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}

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/**
 * gst_clock_id_get_time
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 * @id: The #GstClockID to query
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 *
 * Get the time of the clock ID
 *
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 * Returns: the time of the given clock id.
 *
 * MT safe.
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 */
GstClockTime
gst_clock_id_get_time (GstClockID id)
{
  g_return_val_if_fail (id != NULL, GST_CLOCK_TIME_NONE);

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  return GST_CLOCK_ENTRY_TIME ((GstClockEntry *) id);
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}

/**
 * gst_clock_id_wait
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 * @id: The #GstClockID to wait on
 * @jitter: A pointer that will contain the jitter, can be NULL.
 *
 * Perform a blocking wait on @id. 
 * @id should have been created with gst_clock_new_single_shot_id()
 * or gst_clock_new_periodic_id() and should not have been unscheduled
 * with a call to gst_clock_id_unschedule(). 
 *
 * If the @jitter argument is not NULL and this function returns #GST_CLOCK_OK
 * or #GST_CLOCK_EARLY, it will contain the difference
 * against the clock and the time of @id when this method was
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 * called. 
 * Positive values indicate how late @id was relative to the clock
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 * (in which case this function will return #GST_CLOCK_EARLY). 
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 * Negative values indicate how much time was spent waiting on the clock 
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 * before this function returned.
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 *
 * Returns: the result of the blocking wait. #GST_CLOCK_EARLY will be returned
 * if the current clock time is past the time of @id, #GST_CLOCK_OK if 
 * @id was scheduled in time. #GST_CLOCK_UNSCHEDULED if @id was 
 * unscheduled with gst_clock_id_unschedule().
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 *
 * MT safe.
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 */
GstClockReturn
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gst_clock_id_wait (GstClockID id, GstClockTimeDiff * jitter)
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{
  GstClockEntry *entry;
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  GstClock *clock;
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  GstClockReturn res;
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  GstClockTime requested;
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  GstClockClass *cclass;
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  g_return_val_if_fail (id != NULL, GST_CLOCK_ERROR);
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  entry = (GstClockEntry *) id;
  requested = GST_CLOCK_ENTRY_TIME (entry);
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  clock = GST_CLOCK_ENTRY_CLOCK (entry);

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  /* can't sync on invalid times */
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  if (G_UNLIKELY (!GST_CLOCK_TIME_IS_VALID (requested)))
    goto invalid_time;

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  /* a previously unscheduled entry cannot be scheduled again */
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  if (G_UNLIKELY (entry->status == GST_CLOCK_UNSCHEDULED))
    goto unscheduled;
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  cclass = GST_CLOCK_GET_CLASS (clock);
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  GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock, "waiting on clock entry %p", id);

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  /* if we have a wait_jitter function, use that */
  if (G_LIKELY (cclass->wait_jitter)) {
    res = cclass->wait_jitter (clock, entry, jitter);
  } else {
    /* check if we have a simple _wait function otherwise. The function without
     * the jitter arg is less optimal as we need to do an additional _get_time()
     * which is not atomic with the _wait() and a typical _wait() function does
     * yet another _get_time() anyway. */
    if (G_UNLIKELY (cclass->wait == NULL))
      goto not_supported;

    if (jitter) {
      GstClockTime now = gst_clock_get_time (clock);

      /* jitter is the diff against the clock when this entry is scheduled. Negative
       * values mean that the entry was in time, a positive value means that the
       * entry was too late. */
      *jitter = GST_CLOCK_DIFF (requested, now);
    }
    res = cclass->wait (clock, entry);
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  }

  GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock,
      "done waiting entry %p, res: %d", id, res);

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  if (entry->type == GST_CLOCK_ENTRY_PERIODIC)
    entry->time = requested + entry->interval;
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  if (clock->stats)
    gst_clock_update_stats (clock);

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  return res;
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  /* ERRORS */
invalid_time:
  {
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    GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock,
        "invalid time requested, returning _BADTIME");
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    return GST_CLOCK_BADTIME;
  }
unscheduled:
  {
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    GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock,
        "entry was unscheduled return _UNSCHEDULED");
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    return GST_CLOCK_UNSCHEDULED;
  }
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not_supported:
  {
    GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock, "clock wait is not supported");
    return GST_CLOCK_UNSUPPORTED;
  }
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}

/**
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 * gst_clock_id_wait_async:
 * @id: a #GstClockID to wait on
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 * @func: The callback function
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 * @user_data: User data passed in the calback
 *
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 * Register a callback on the given #GstClockID @id with the given
 * function and user_data. When passing a #GstClockID with an invalid
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 * time to this function, the callback will be called immediatly
 * with  a time set to GST_CLOCK_TIME_NONE. The callback will
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 * be called when the time of @id has been reached.
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 *
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 * Returns: the result of the non blocking wait.
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 *
 * MT safe.
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 */
GstClockReturn
gst_clock_id_wait_async (GstClockID id,
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    GstClockCallback func, gpointer user_data)
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{
  GstClockEntry *entry;
  GstClock *clock;
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  GstClockReturn res;
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  GstClockClass *cclass;
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  GstClockTime requested;
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  g_return_val_if_fail (id != NULL, GST_CLOCK_ERROR);
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  g_return_val_if_fail (func != NULL, GST_CLOCK_ERROR);
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  entry = (GstClockEntry *) id;
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  requested = GST_CLOCK_ENTRY_TIME (entry);
  clock = GST_CLOCK_ENTRY_CLOCK (entry);
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  /* can't sync on invalid times */
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  if (G_UNLIKELY (!GST_CLOCK_TIME_IS_VALID (requested)))
    goto invalid_time;

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  /* a previously unscheduled entry cannot be scheduled again */
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  if (G_UNLIKELY (entry->status == GST_CLOCK_UNSCHEDULED))
    goto unscheduled;
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  cclass = GST_CLOCK_GET_CLASS (clock);

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  if (G_UNLIKELY (cclass->wait_async == NULL))
    goto not_supported;

  entry->func = func;
  entry->user_data = user_data;

  res = cclass->wait_async (clock, entry);
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  return res;

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  /* ERRORS */
invalid_time:
  {
    (func) (clock, GST_CLOCK_TIME_NONE, id, user_data);
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    GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock,
        "invalid time requested, returning _BADTIME");
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    return GST_CLOCK_BADTIME;
  }
unscheduled:
  {
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    GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock,
        "entry was unscheduled return _UNSCHEDULED");
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    return GST_CLOCK_UNSCHEDULED;
  }
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not_supported:
  {
    GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock, "clock wait is not supported");
    return GST_CLOCK_UNSUPPORTED;
  }
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}

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/**
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 * gst_clock_id_unschedule:
 * @id: The id to unschedule
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 *
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 * Cancel an outstanding request with @id. This can either
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 * be an outstanding async notification or a pending sync notification.
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 * After this call, @id cannot be used anymore to receive sync or
 * async notifications, you need to create a new #GstClockID.
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 *
 * MT safe.
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 */
void
gst_clock_id_unschedule (GstClockID id)
{
  GstClockEntry *entry;
  GstClock *clock;
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  GstClockClass *cclass;
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  g_return_if_fail (id != NULL);

  entry = (GstClockEntry *) id;
  clock = entry->clock;
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  cclass = GST_CLOCK_GET_CLASS (clock);

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  if (G_LIKELY (cclass->unschedule))
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    cclass->unschedule (clock, entry);
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}

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/**
 * GstClock abstract base class implementation
 */
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GType
gst_clock_get_type (void)
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{
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  static GType clock_type = 0;

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  if (G_UNLIKELY (clock_type == 0)) {
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    static const GTypeInfo clock_info = {
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      sizeof (GstClockClass),
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      NULL,
      NULL,
      (GClassInitFunc) gst_clock_class_init,
      NULL,
      NULL,
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      sizeof (GstClock),
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      0,
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      (GInstanceInitFunc) gst_clock_init,
      NULL
    };
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    clock_type = g_type_register_static (GST_TYPE_OBJECT, "GstClock",
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        &clock_info, G_TYPE_FLAG_ABSTRACT);
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  }
  return clock_type;
}
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static void
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gst_clock_class_init (GstClockClass * klass)
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{
  GObjectClass *gobject_class;
  GstObjectClass *gstobject_class;
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  gobject_class = G_OBJECT_CLASS (klass);
  gstobject_class = GST_OBJECT_CLASS (klass);
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  parent_class = g_type_class_peek_parent (klass);
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  if (!g_thread_supported ())
    g_thread_init (NULL);

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#ifndef GST_DISABLE_TRACE
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  _gst_clock_entry_trace =
      gst_alloc_trace_register (GST_CLOCK_ENTRY_TRACE_NAME);
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#endif
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  gobject_class->dispose = GST_DEBUG_FUNCPTR (gst_clock_dispose);
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  gobject_class->finalize = GST_DEBUG_FUNCPTR (gst_clock_finalize);
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  gobject_class->set_property = GST_DEBUG_FUNCPTR (gst_clock_set_property);
  gobject_class->get_property = GST_DEBUG_FUNCPTR (gst_clock_get_property);

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  g_object_class_install_property (gobject_class, PROP_STATS,
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      g_param_spec_boolean ("stats", "Stats",
          "Enable clock stats (unimplemented)", DEFAULT_STATS,
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          G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
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  g_object_class_install_property (gobject_class, PROP_WINDOW_SIZE,
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      g_param_spec_int ("window-size", "Window size",
          "The size of the window used to calculate rate and offset", 2, 1024,
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          DEFAULT_WINDOW_SIZE, G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
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  g_object_class_install_property (gobject_class, PROP_WINDOW_THRESHOLD,
      g_param_spec_int ("window-threshold", "Window threshold",
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          "The threshold to start calculating rate and offset", 2, 1024,
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          DEFAULT_WINDOW_THRESHOLD,
          G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
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  g_object_class_install_property (gobject_class, PROP_TIMEOUT,
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      g_param_spec_uint64 ("timeout", "Timeout",
          "The amount of time, in nanoseconds, to sample master and slave clocks",
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          0, G_MAXUINT64, DEFAULT_TIMEOUT,
          G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
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}

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static void
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gst_clock_init (GstClock * clock)
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{
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  clock->last_time = 0;
  clock->entries = NULL;
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  clock->entries_changed = g_cond_new ();
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  clock->stats = FALSE;
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  clock->internal_calibration = 0;
  clock->external_calibration = 0;
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  clock->rate_numerator = 1;
  clock->rate_denominator = 1;

  clock->slave_lock = g_mutex_new ();
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  clock->window_size = DEFAULT_WINDOW_SIZE;
  clock->window_threshold = DEFAULT_WINDOW_THRESHOLD;
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  clock->filling = TRUE;
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  clock->time_index = 0;
  clock->timeout = DEFAULT_TIMEOUT;
  clock->times = g_new0 (GstClockTime, 4 * clock->window_size);
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}

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static void
gst_clock_dispose (GObject * object)
{
  GstClock *clock = GST_CLOCK (object);
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  GstClock **master_p;
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  GST_OBJECT_LOCK (clock);
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  master_p = &clock->master;
  gst_object_replace ((GstObject **) master_p, NULL);
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  GST_OBJECT_UNLOCK (clock);

  G_OBJECT_CLASS (parent_class)->dispose (object);
}

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static void
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gst_clock_finalize (GObject * object)
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{
  GstClock *clock = GST_CLOCK (object);

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  GST_CLOCK_SLAVE_LOCK (clock);
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  if (clock->clockid) {
    gst_clock_id_unschedule (clock->clockid);
    gst_clock_id_unref (clock->clockid);
    clock->clockid = NULL;
  }
  g_free (clock->times);
  clock->times = NULL;
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  GST_CLOCK_SLAVE_UNLOCK (clock);
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  g_cond_free (clock->entries_changed);
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  g_mutex_free (clock->slave_lock);

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  G_OBJECT_CLASS (parent_class)->finalize (object);
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}

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/**
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 * gst_clock_set_resolution
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 * @clock: a #GstClock
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 * @resolution: The resolution to set
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 *
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 * Set the accuracy of the clock. Some clocks have the possibility to operate
 * with different accuracy at the expense of more resource usage. There is
 * normally no need to change the default resolution of a clock. The resolution
 * of a clock can only be changed if the clock has the
 * #GST_CLOCK_FLAG_CAN_SET_RESOLUTION flag set.
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 *
 * Returns: the new resolution of the clock.
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 */
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GstClockTime
gst_clock_set_resolution (GstClock * clock, GstClockTime resolution)
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{
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  GstClockClass *cclass;

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  g_return_val_if_fail (GST_IS_CLOCK (clock), 0);
  g_return_val_if_fail (resolution != 0, 0);
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  cclass = GST_CLOCK_GET_CLASS (clock);

  if (cclass->change_resolution)
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    clock->resolution =
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        cclass->change_resolution (clock, clock->resolution, resolution);
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  return clock->resolution;
}
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/**
 * gst_clock_get_resolution
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 * @clock: a #GstClock
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 *
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 * Get the accuracy of the clock. The accuracy of the clock is the granularity
 * of the values returned by gst_clock_get_time().
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 *
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 * Returns: the resolution of the clock in units of #GstClockTime.
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 *
 * MT safe.
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 */
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GstClockTime
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gst_clock_get_resolution (GstClock * clock)
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{
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  GstClockClass *cclass;

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  g_return_val_if_fail (GST_IS_CLOCK (clock), 0);
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  cclass = GST_CLOCK_GET_CLASS (clock);

  if (cclass->get_resolution)
    return cclass->get_resolution (clock);
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  return 1;
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}

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/**
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 * gst_clock_adjust_unlocked
 * @clock: a #GstClock to use
 * @internal: a clock time
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 *
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 * Converts the given @internal clock time to the external time, adjusting for the
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 * rate and reference time set with gst_clock_set_calibration() and making sure
 * that the returned time is increasing. This function should be called with the
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 * clock's OBJECT_LOCK held and is mainly used by clock subclasses.
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 *
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 * This function is te reverse of gst_clock_unadjust_unlocked().
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 *
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 * Returns: the converted time of the clock.
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 */
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GstClockTime
gst_clock_adjust_unlocked (GstClock * clock, GstClockTime internal)
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{
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  GstClockTime ret, cinternal, cexternal, cnum, cdenom;

  /* get calibration values for readability */
  cinternal = clock->internal_calibration;
  cexternal = clock->external_calibration;
  cnum = clock->rate_numerator;
  cdenom = clock->rate_denominator;

  /* avoid divide by 0 */
  if (cdenom == 0)
    cnum = cdenom = 1;

  /* The formula is (internal - cinternal) * cnum / cdenom + cexternal
   *
   * Since we do math on unsigned 64-bit ints we have to special case for
   * interal < cinternal to get the sign right. this case is not very common,
   * though.
   */
  if (G_LIKELY (internal >= cinternal)) {
    ret = gst_util_uint64_scale (internal - cinternal, cnum, cdenom);
    ret += cexternal;
  } else {
    ret = gst_util_uint64_scale (cinternal - internal, cnum, cdenom);
    /* clamp to 0 */
    if (cexternal > ret)
      ret = cexternal - ret;
    else
      ret = 0;
  }
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  /* make sure the time is increasing */
  clock->last_time = MAX (ret, clock->last_time);

  return clock->last_time;
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}

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/**
 * gst_clock_unadjust_unlocked
 * @clock: a #GstClock to use
 * @external: an external clock time
 *
 * Converts the given @external clock time to the internal time of @clock,
 * using the rate and reference time set with gst_clock_set_calibration().
 * This function should be called with the clock's OBJECT_LOCK held and
 * is mainly used by clock subclasses.
 *
 * This function is te reverse of gst_clock_adjust_unlocked().
 *
 * Returns: the internal time of the clock corresponding to @external.
 *
 * Since: 0.10.13
 */
GstClockTime
gst_clock_unadjust_unlocked (GstClock * clock, GstClockTime external)
{
  GstClockTime ret, cinternal, cexternal, cnum, cdenom;

  /* get calibration values for readability */
  cinternal = clock->internal_calibration;
  cexternal = clock->external_calibration;
  cnum = clock->rate_numerator;
  cdenom = clock->rate_denominator;

  /* avoid divide by 0 */
  if (cnum == 0)
    cnum = cdenom = 1;

  /* The formula is (external - cexternal) * cdenom / cnum + cinternal */
  if (external >= cexternal) {
    ret = gst_util_uint64_scale (external - cexternal, cdenom, cnum);
    ret += cinternal;
  } else {
    ret = gst_util_uint64_scale (cexternal - external, cdenom, cnum);
    if (cinternal > ret)
      ret = cinternal - ret;
    else
      ret = 0;
  }
  return ret;
}

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/**
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 * gst_clock_get_internal_time
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 * @clock: a #GstClock to query
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 *
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 * Gets the current internal time of the given clock. The time is returned
 * unadjusted for the offset and the rate.
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 *
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 * Returns: the internal time of the clock. Or GST_CLOCK_TIME_NONE when
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 * giving wrong input.
 *
 * MT safe.
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 */
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GstClockTime
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gst_clock_get_internal_time (GstClock * clock)
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{
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  GstClockTime ret;
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  GstClockClass *cclass;
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  g_return_val_if_fail (GST_IS_CLOCK (clock), GST_CLOCK_TIME_NONE);
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  cclass = GST_CLOCK_GET_CLASS (clock);
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  if (G_UNLIKELY (cclass->get_internal_time == NULL))
    goto not_supported;

  ret = cclass->get_internal_time (clock);

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  GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock, "internal time %" GST_TIME_FORMAT,
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      GST_TIME_ARGS (ret));
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  return ret;
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  /* ERRORS */
not_supported:
  {
    GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock,
        "internal time not supported, return 0");
    return G_GINT64_CONSTANT (0);
  }
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}

/**
 * gst_clock_get_time
 * @clock: a #GstClock to query
 *
 * Gets the current time of the given clock. The time is always
 * monotonically increasing and adjusted according to the current
 * offset and rate.
 *
 * Returns: the time of the clock. Or GST_CLOCK_TIME_NONE when
 * giving wrong input.
 *
 * MT safe.
 */
GstClockTime
gst_clock_get_time (GstClock * clock)
{
  GstClockTime ret;

  g_return_val_if_fail (GST_IS_CLOCK (clock), GST_CLOCK_TIME_NONE);

  ret = gst_clock_get_internal_time (clock);

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  GST_OBJECT_LOCK (clock);
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  /* this will scale for rate and offset */
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  ret = gst_clock_adjust_unlocked (clock, ret);
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  GST_OBJECT_UNLOCK (clock);
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  GST_CAT_DEBUG_OBJECT (GST_CAT_CLOCK, clock, "adjusted time %" GST_TIME_FORMAT,
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      GST_TIME_ARGS (ret));
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  return ret;
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}

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/**
 * gst_clock_set_calibration
 * @clock: a #GstClock to calibrate
 * @internal: a reference internal time
 * @external: a reference external time
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 * @rate_num: the numerator of the rate of the clock relative to its
 *            internal time 
 * @rate_denom: the denominator of the rate of the clock
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 *
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 * Adjusts the rate and time of @clock. A rate of 1/1 is the normal speed of
 * the clock. Values bigger than 1/1 make the clock go faster.
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 *
 * @internal and @external are calibration parameters that arrange that
 * gst_clock_get_time() should have been @external at internal time @internal.
 * This internal time should not be in the future; that is, it should be less
 * than the value of gst_clock_get_internal_time() when this function is called.
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