Controlled shutdown of live sources in applications

Applications that record from a live source may want to stop recording in a controlled way, so that the recording is stopped, but the data already in the pipeline is processed to the end (remember that many live sources would go on recording forever otherwise). For that to happen the application needs to make the source stop recording and send an EOS event down the pipeline. The application would then wait for an EOS message posted on the pipeline's bus to know when all data has been processed and the pipeline can safely be stopped.

An application may send an EOS event to a source element to make it perform the EOS logic (send EOS event downstream or post a GST_MESSAGE_SEGMENT_DONE on the bus). This can typically be done with the gst_element_send_event() function on the element or its parent bin.

After the EOS has been sent to the element, the application should wait for an EOS message to be posted on the pipeline's bus. Once this EOS message is received, it may safely shut down the entire pipeline.

Last reviewed on 2007-12-19 (0.10.16)

struct GstBaseSrc

struct GstBaseSrc;

The opaque GstBaseSrc data structure.

struct GstBaseSrcClass

struct GstBaseSrcClass {
  GstElementClass parent_class;

  /* virtual methods for subclasses */

  /* get caps from subclass */
  GstCaps*      (*get_caps)     (GstBaseSrc *src, GstCaps *filter);
  /* decide on caps */
  gboolean      (*negotiate)    (GstBaseSrc *src);
  /* called if, in negotiation, caps need fixating */
  GstCaps *     (*fixate)       (GstBaseSrc *src, GstCaps *caps);
  /* notify the subclass of new caps */
  gboolean      (*set_caps)     (GstBaseSrc *src, GstCaps *caps);

  /* setup allocation query */
  gboolean      (*decide_allocation)   (GstBaseSrc *src, GstQuery *query);

  /* start and stop processing, ideal for opening/closing the resource */
  gboolean      (*start)        (GstBaseSrc *src);
  gboolean      (*stop)         (GstBaseSrc *src);

  /* given a buffer, return start and stop time when it should be pushed
   * out. The base class will sync on the clock using these times. */
  void          (*get_times)    (GstBaseSrc *src, GstBuffer *buffer,
                                 GstClockTime *start, GstClockTime *end);

  /* get the total size of the resource in bytes */
  gboolean      (*get_size)     (GstBaseSrc *src, guint64 *size);

  /* check if the resource is seekable */
  gboolean      (*is_seekable)  (GstBaseSrc *src);

  /* Prepare the segment on which to perform do_seek(), converting to the
   * current basesrc format. */
  gboolean      (*prepare_seek_segment) (GstBaseSrc *src, GstEvent *seek,
                                         GstSegment *segment);
  /* notify subclasses of a seek */
  gboolean      (*do_seek)      (GstBaseSrc *src, GstSegment *segment);

  /* unlock any pending access to the resource. subclasses should unlock
   * any function ASAP. */
  gboolean      (*unlock)       (GstBaseSrc *src);
  /* Clear any pending unlock request, as we succeeded in unlocking */
  gboolean      (*unlock_stop)  (GstBaseSrc *src);

  /* notify subclasses of a query */
  gboolean      (*query)        (GstBaseSrc *src, GstQuery *query);

  /* notify subclasses of an event */
  gboolean      (*event)        (GstBaseSrc *src, GstEvent *event);

  /* ask the subclass to create a buffer with offset and size, the default
   * implementation will call alloc and fill. */
  GstFlowReturn (*create)       (GstBaseSrc *src, guint64 offset, guint size,
                                 GstBuffer **buf);
  /* ask the subclass to allocate an output buffer. The default implementation
   * will use the negotiated allocator. */
  GstFlowReturn (*alloc)        (GstBaseSrc *src, guint64 offset, guint size,
                                 GstBuffer **buf);
  /* ask the subclass to fill the buffer with data from offset and size */
  GstFlowReturn (*fill)         (GstBaseSrc *src, guint64 offset, guint size,
                                 GstBuffer *buf);
};

Subclasses can override any of the available virtual methods or not, as needed. At the minimum, the create method should be overridden to produce buffers.

enum GstBaseSrcFlags

typedef enum {
  GST_BASE_SRC_FLAG_STARTING     = (GST_ELEMENT_FLAG_LAST << 0),
  GST_BASE_SRC_FLAG_STARTED      = (GST_ELEMENT_FLAG_LAST << 1),
  /* padding */
  GST_BASE_SRC_FLAG_LAST         = (GST_ELEMENT_FLAG_LAST << 6)
} GstBaseSrcFlags;

The GstElement flags that a basesrc element may have.

gst_base_src_wait_playing ()

GstFlowReturn       gst_base_src_wait_playing           (GstBaseSrc *src);

If the GstBaseSrcClass.create() method performs its own synchronisation against the clock it must unblock when going from PLAYING to the PAUSED state and call this method before continuing to produce the remaining data.

This function will block until a state change to PLAYING happens (in which case this function returns GST_FLOW_OK) or the processing must be stopped due to a state change to READY or a FLUSH event (in which case this function returns GST_FLOW_FLUSHING).

gst_base_src_is_live ()

gboolean            gst_base_src_is_live                (GstBaseSrc *src);

Check if an element is in live mode.

gst_base_src_set_live ()

void                gst_base_src_set_live               (GstBaseSrc *src,
                                                         gboolean live);

If the element listens to a live source, live should be set to TRUE.

A live source will not produce data in the PAUSED state and will therefore not be able to participate in the PREROLL phase of a pipeline. To signal this fact to the application and the pipeline, the state change return value of the live source will be GST_STATE_CHANGE_NO_PREROLL.

gst_base_src_set_format ()

void                gst_base_src_set_format             (GstBaseSrc *src,
                                                         GstFormat format);

Sets the default format of the source. This will be the format used for sending NEW_SEGMENT events and for performing seeks.

If a format of GST_FORMAT_BYTES is set, the element will be able to operate in pull mode if the GstBaseSrcClass.is_seekable() returns TRUE.

This function must only be called in states < GST_STATE_PAUSED.

gst_base_src_query_latency ()

gboolean            gst_base_src_query_latency          (GstBaseSrc *src,
                                                         gboolean *live,
                                                         GstClockTime *min_latency,
                                                         GstClockTime *max_latency);

Query the source for the latency parameters. live will be TRUE when src is configured as a live source. min_latency will be set to the difference between the running time and the timestamp of the first buffer. max_latency is always the undefined value of -1.

This function is mostly used by subclasses.

gst_base_src_get_blocksize ()

guint               gst_base_src_get_blocksize          (GstBaseSrc *src);

Get the number of bytes that src will push out with each buffer.

gst_base_src_set_blocksize ()

void                gst_base_src_set_blocksize          (GstBaseSrc *src,
                                                         guint blocksize);

Set the number of bytes that src will push out with each buffer. When blocksize is set to -1, a default length will be used.

gst_base_src_get_do_timestamp ()

gboolean            gst_base_src_get_do_timestamp       (GstBaseSrc *src);

Query if src timestamps outgoing buffers based on the current running_time.

gst_base_src_set_do_timestamp ()

void                gst_base_src_set_do_timestamp       (GstBaseSrc *src,
                                                         gboolean timestamp);

Configure src to automatically timestamp outgoing buffers based on the current running_time of the pipeline. This property is mostly useful for live sources.

gst_base_src_set_dynamic_size ()

void                gst_base_src_set_dynamic_size       (GstBaseSrc *src,
                                                         gboolean dynamic);

If not dynamic, size is only updated when needed, such as when trying to read past current tracked size. Otherwise, size is checked for upon each read.

gst_base_src_new_seamless_segment ()

gboolean            gst_base_src_new_seamless_segment   (GstBaseSrc *src,
                                                         gint64 start,
                                                         gint64 stop,
                                                         gint64 position);

Prepare a new seamless segment for emission downstream. This function must only be called by derived sub-classes, and only from the create() function, as the stream-lock needs to be held.

The format for the new segment will be the current format of the source, as configured with gst_base_src_set_format()

gst_base_src_set_caps ()

gboolean            gst_base_src_set_caps               (GstBaseSrc *src,
                                                         GstCaps *caps);

Set new caps on the basesrc source pad.

GST_BASE_SRC_PAD()

#define GST_BASE_SRC_PAD(obj)                 (GST_BASE_SRC_CAST (obj)->srcpad)

Gives the pointer to the GstPad object of the element.

The "blocksize" property

  "blocksize"                guint                 : Read / Write

Size in bytes to read per buffer (-1 = default).

Default value: 4096

The "do-timestamp" property

  "do-timestamp"             gboolean              : Read / Write

Apply current stream time to buffers.

Default value: FALSE

The "num-buffers" property

  "num-buffers"              gint                  : Read / Write

Number of buffers to output before sending EOS (-1 = unlimited).

Allowed values: >= G_MAXULONG

Default value: -1

The "typefind" property

  "typefind"                 gboolean              : Read / Write

Run typefind before negotiating.

Default value: FALSE