GstBaseSrc

GstBaseSrc — Base class for getrange based source elements

Synopsis


#include <gst/base/gstbasesrc.h>

                    GstBaseSrc;
                    GstBaseSrcClass;
enum                GstBaseSrcFlags;
GstFlowReturn       gst_base_src_wait_playing           (GstBaseSrc *src);
gboolean            gst_base_src_is_live                (GstBaseSrc *src);
void                gst_base_src_set_live               (GstBaseSrc *src,
                                                         gboolean live);
void                gst_base_src_set_format             (GstBaseSrc *src,
                                                         GstFormat format);
gboolean            gst_base_src_query_latency          (GstBaseSrc *src,
                                                         gboolean *live,
                                                         GstClockTime *min_latency,
                                                         GstClockTime *max_latency);
gulong              gst_base_src_get_blocksize          (GstBaseSrc *src);
void                gst_base_src_set_blocksize          (GstBaseSrc *src,
                                                         gulong blocksize);
gboolean            gst_base_src_get_do_timestamp       (GstBaseSrc *src);
void                gst_base_src_set_do_timestamp       (GstBaseSrc *src,
                                                         gboolean timestamp);
#define             GST_BASE_SRC_PAD                    (obj)

Object Hierarchy

  GObject
   +----GstObject
         +----GstElement
               +----GstBaseSrc
                     +----GstPushSrc

Properties

  "blocksize"                gulong                : Read / Write
  "do-timestamp"             gboolean              : Read / Write
  "num-buffers"              gint                  : Read / Write
  "typefind"                 gboolean              : Read / Write

Description

This is a generice base class for source elements. The following types of sources are supported:

  • random access sources like files

  • seekable sources

  • live sources

The source can be configured to operate in any GstFormat with the gst_base_src_set_format() method. The currently set format determines the format of the internal GstSegment and any GST_EVENT_NEWSEGMENT events. The default format for GstBaseSrc is GST_FORMAT_BYTES.

GstBaseSrc always supports push mode scheduling. If the following conditions are met, it also supports pull mode scheduling:

  • The format is set to GST_FORMAT_BYTES (default).

  • "is_seekable" returns TRUE.

Since 0.10.9, any GstBaseSrc can enable pull based scheduling at any time by overriding "check_get_range" so that it returns TRUE.

If all the conditions are met for operating in pull mode, GstBaseSrc is automatically seekable in push mode as well. The following conditions must be met to make the element seekable in push mode when the format is not ""

  • "is_seekable" returns TRUE.

  • "query" can convert all supported seek formats to the internal format as set with gst_base_src_set_format().

  • "do_seek" is implemented, performs the seek and returns TRUE.

When the element does not meet the requirements to operate in pull mode, the offset and length in the "create" method should be ignored. It is recommended to subclass GstPushSrc instead, in this situation. If the element can operate in pull mode but only with specific offsets and lengths, it is allowed to generate an error when the wrong values are passed to the "create" function.

GstBaseSrc has support for live sources. Live sources are sources that when paused discard data, such as audio or video capture devices. A typical live source also produces data at a fixed rate and thus provides a clock to publish this rate. Use gst_base_src_set_live() to activate the live source mode.

A live source does not produce data in the PAUSED state. This means that the "create" method will not be called in PAUSED but only in PLAYING. To signal the pipeline that the element will not produce data, the return value from the READY to PAUSED state will be GST_STATE_CHANGE_NO_PREROLL.

A typical live source will timestamp the buffers it creates with the current running time of the pipeline. This is one reason why a live source can only produce data in the PLAYING state, when the clock is actually distributed and running.

Live sources that synchronize and block on the clock (an audio source, for example) can since 0.10.12 use gst_base_src_wait_playing() when the ::create function was interrupted by a state change to PAUSED.

The "get_times" method can be used to implement pseudo-live sources. It only makes sense to implement the ::get_times function if the source is a live source. The ::get_times function should return timestamps starting from 0, as if it were a non-live source. The base class will make sure that the timestamps are transformed into the current running_time. The base source will then wait for the calculated running_time before pushing out the buffer.

For live sources, the base class will by default report a latency of 0. For pseudo live sources, the base class will by default measure the difference between the first buffer timestamp and the start time of get_times and will report this value as the latency. Subclasses should override the query function when this behaviour is not acceptable.

There is only support in GstBaseSrc for exactly one source pad, which should be named "src". A source implementation (subclass of GstBaseSrc) should install a pad template in its class_init function, like so: 

static void
my_element_class_init (GstMyElementClass *klass)
{
  GstElementClass *gstelement_class = GST_ELEMENT_CLASS (klass);
  // srctemplate should be a GstStaticPadTemplate with direction
  // GST_PAD_SRC and name "src"
  gst_element_class_add_pad_template (gstelement_class,
      gst_static_pad_template_get (&srctemplate));
  // see GstElementDetails
  gst_element_class_set_details (gstelement_class, &details);
}

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.

Since GStreamer 0.10.16 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.

The old behaviour for controlled shutdown introduced since GStreamer 0.10.3 is still available but deprecated as it is dangerous and less flexible.

Last reviewed on 2007-12-19 (0.10.16)

Details

GstBaseSrc

typedef struct _GstBaseSrc GstBaseSrc;

The opaque GstBaseSrc data structure.

GstBaseSrcClass

typedef struct {
  GstElementClass parent_class;

  /* virtual methods for subclasses */

  /* get caps from subclass */
  GstCaps*      (*get_caps)     (GstBaseSrc *src);
  /* notify the subclass of new caps */
  gboolean      (*set_caps)     (GstBaseSrc *src, GstCaps *caps);

  /* decide on caps */
  gboolean      (*negotiate)    (GstBaseSrc *src);

  /* generate and send a newsegment (UNUSED) */
  gboolean      (*newsegment)   (GstBaseSrc *src);

  /* 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);
  /* unlock any pending access to the resource. subclasses should unlock
   * any function ASAP. */
  gboolean      (*unlock)       (GstBaseSrc *src);

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

  /* ask the subclass to create a buffer with offset and size */
  GstFlowReturn (*create)       (GstBaseSrc *src, guint64 offset, guint size,
		                 GstBuffer **buf);

  /* additions that change padding... */
  /* notify subclasses of a seek */
  gboolean      (*do_seek)      (GstBaseSrc *src, GstSegment *segment);
  /* notify subclasses of a query */
  gboolean      (*query)        (GstBaseSrc *src, GstQuery *query);

  /* check whether the source would support pull-based operation if
   * it were to be opened now. This vfunc is optional, but should be
   * implemented if possible to avoid unnecessary start/stop cycles.
   * The default implementation will open and close the resource to
   * find out whether get_range is supported and that is usually
   * undesirable. */
  gboolean      (*check_get_range) (GstBaseSrc *src);

  /* called if, in negotiation, caps need fixating */
  void		(*fixate)	(GstBaseSrc *src, GstCaps *caps);

  /* Clear any pending unlock request, as we succeeded in unlocking */
  gboolean      (*unlock_stop)  (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); 
} GstBaseSrcClass;

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.

GstElementClass parent_class; Element parent class
get_caps () Called to get the caps to report
set_caps () Notify subclass of changed output caps
negotiate () Negotiated the caps with the peer.
newsegment () Generate and send a new_segment event (UNUSED)
start () Start processing. Subclasses should open resources and prepare to produce data.
stop () Stop processing. Subclasses should use this to close resources.
get_times () Given a buffer, return the start and stop time when it should be pushed out. The base class will sync on the clock using these times.
get_size () Return the total size of the resource, in the configured format.
is_seekable () Check if the source can seek
unlock () Unlock any pending access to the resource. Subclasses should unblock any blocked function ASAP
event () Override this to implement custom event handling.
create () Ask the subclass to create a buffer with offset and size.
do_seek () Perform seeking on the resource to the indicated segment.
query () Handle a requested query.
check_get_range () Check whether the source would support pull-based operation if it were to be opened now. This vfunc is optional, but should be implemented if possible to avoid unnecessary start/stop cycles. The default implementation will open and close the resource to find out whether get_range is supported, and that is usually undesirable.
fixate () Called during negotiation if caps need fixating. Implement instead of setting a fixate function on the source pad.
unlock_stop () Clear the previous unlock request. Subclasses should clear any state they set during unlock(), such as clearing command queues.
prepare_seek_segment () Prepare the GstSegment that will be passed to the do_seek vmethod for executing a seek request. Sub-classes should override this if they support seeking in formats other than the configured native format. By default, it tries to convert the seek arguments to the configured native format and prepare a segment in that format. Since: 0.10.13

enum GstBaseSrcFlags

typedef enum {
  GST_BASE_SRC_STARTED           = (GST_ELEMENT_FLAG_LAST << 0),
  /* padding */
  GST_BASE_SRC_FLAG_LAST         = (GST_ELEMENT_FLAG_LAST << 2)
} GstBaseSrcFlags;

The GstElement flags that a basesrc element may have.

GST_BASE_SRC_STARTED has source been started
GST_BASE_SRC_FLAG_LAST offset to define more flags

gst_base_src_wait_playing ()

GstFlowReturn       gst_base_src_wait_playing           (GstBaseSrc *src);

If the "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_WRONG_STATE).

src : the src
Returns : GST_FLOW_OK if src is PLAYING and processing can continue. Any other return value should be returned from the create vmethod.

Since 0.10.12

gst_base_src_is_live ()

gboolean            gst_base_src_is_live                (GstBaseSrc *src);

Check if an element is in live mode.

src : base source instance
Returns : TRUE if 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.

src : base source instance
live : new live-mode

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 "is_seekable" returns TRUE.

src : base source instance
format : the format to use

Since 0.10.1

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.

src : the source
live : if the source is live
min_latency : the min latency of the source
max_latency : the max latency of the source
Returns : TRUE if the query succeeded.

Since 0.10.13

gst_base_src_get_blocksize ()

gulong              gst_base_src_get_blocksize          (GstBaseSrc *src);

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

src : the source
Returns : the number of bytes pushed with each buffer.

Since 0.10.22

gst_base_src_set_blocksize ()

void                gst_base_src_set_blocksize          (GstBaseSrc *src,
                                                         gulong 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.

src : the source
blocksize : the new blocksize in bytes

Since 0.10.22

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.

src : the source
Returns : TRUE if the base class will automatically timestamp outgoing buffers.

Since 0.10.15

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.

src : the source
timestamp : enable or disable timestamping

Since 0.10.15

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.

obj : base source instance

Property Details

The "blocksize" property

  "blocksize"                gulong                : Read / Write

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

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: >= -1

Default value: -1

The "typefind" property

  "typefind"                 gboolean              : Read / Write

Run typefind before negotiating.

Default value: FALSE

See Also

GstPushSrc, GstBaseTransform, GstBaseSink
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