GstBaseSrc — Base class for getrange based source elements


#include <gst/base/gstbasesrc.h>

struct              GstBaseSrc;
struct              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);
void                gst_base_src_set_dynamic_size       (GstBaseSrc *src,
                                                         gboolean dynamic);
gboolean            gst_base_src_new_seamless_segment   (GstBaseSrc *src,
                                                         gint64 start,
                                                         gint64 stop,
                                                         gint64 position);
#define             GST_BASE_SRC_PAD                    (obj)

Object Hierarchy



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


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:

Since 0.10.9, any GstBaseSrc can enable pull based scheduling at any time by overriding GstBaseSrcClass.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 GST_FORMAT_BYTES:

When the element does not meet the requirements to operate in pull mode, the offset and length in the GstBaseSrcClass.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 GstBaseSrcClass.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 GstBaseSrcClass.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 GstBaseSrcClass.create() function was interrupted by a state change to PAUSED.

The GstBaseSrcClass.get_times() method can be used to implement pseudo-live sources. It only makes sense to implement the GstBaseSrcClass.get_times() function if the source is a live source. The GstBaseSrcClass.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 <GTKDOCLINK HREF="GstStaticPadTemplate">GstStaticPadTemplate</GTKDOCLINK> with direction
  gst_element_class_add_pad_template (gstelement_class,
      gst_static_pad_template_get (&srctemplate));
  // see <GTKDOCLINK HREF="GstElementDetails">GstElementDetails</GTKDOCLINK>
  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)


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);
  /* 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);

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. In particular, any create() function in progress should be unblocked and should return GST_FLOW_WRONG_STATE. Any future create() function call should also return GST_FLOW_WRONG_STATE until the unlock_stop() function has been called.

event ()

Override this to implement custom event handling.

create ()

Ask the subclass to create a buffer with offset and size. When the subclass returns GST_FLOW_OK, it MUST return a buffer of the requested size unless fewer bytes are available because an EOS condition is near. No buffer should be returned when the return value is different from GST_FLOW_OK. A return value of GST_FLOW_UNEXPECTED signifies that the end of stream is reached.

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 {
  /* padding */
} GstBaseSrcFlags;

The GstElement flags that a basesrc element may have.


has source been started


offset to define more flags

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_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 GstBaseSrcClass.is_seekable() returns TRUE.

This function must only be called in states < GST_STATE_PAUSED.

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. [out][allow-none]

min_latency :

the min latency of the source. [out][allow-none]

max_latency :

the max latency of the source. [out][allow-none]

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_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.

src :

base source instance

dynamic :

new dynamic size mode

Since 0.10.36

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()

src :

The source

start :

The new start value for the segment

stop :

Stop value for the new segment

position :

The position value for the new segent

Returns :

TRUE if preparation of the seamless segment succeeded.

Since 0.10.26


#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: >= G_MAXULONG

Default value: -1

The "typefind" property

  "typefind"                 gboolean              : Read / Write

Run typefind before negotiating.

Default value: FALSE

See Also

GstPushSrc, GstBaseTransform, GstBaseSink