gstreamer/docs/design/part-query.txt

Query
-----

Purpose
~~~~~~~

Queries are used to get information about the stream. 
A query is started on a specific pad and travels up or downstream.


Requirements
~~~~~~~~~~~~

  - multiple return values, grouped together when they make sense.
  - one pad function to perform the query
  - extensible queries.

Implementation
~~~~~~~~~~~~~~

  - GstQuery extends GstMiniObject and contains a GstStructure (see GstMessage)
  - some standard query types are defined below
  - methods to create and parse the results in the GstQuery.
  - define pad method:

     gboolean                (*GstPadQueryFunction)          (GstPad    *pad,
                                                              GstObject *parent,
                                                              GstQuery  *query);

    pad returns result in query structure and TRUE as result or FALSE when
    query is not supported.

Query types
~~~~~~~~~~~

 - GST_QUERY_POSITION:
  
     get info on current position of the stream in stream_time.

 - GST_QUERY_DURATION:

     get info on the total duration of the stream.
 
 - GST_QUERY_LATENCY:

     get amount of latency introduced in the pipeline. (See part-latency.txt)

 - GST_QUERY_RATE:

     get the current playback rate of the pipeline

 - GST_QUERY_SEEKING:
   
     get info on how seeking can be done 
       - getrange, with/without offset/size
       - ranges where seeking is efficient (for caching network sources)
       - flags describing seeking behaviour (forward, backward, segments,
	             play backwards, ...)

 - GST_QUERY_SEGMENT:

     get info about the currently configured playback segment.

 - GST_QUERY_CONVERT:

     convert format/value to another format/value pair.

 - GST_QUERY_FORMATS:

     return list of supported formats that can be used for GST_QUERY_CONVERT.

 - GST_QUERY_BUFFERING:

     query available media for efficient seeking (See part-buffering.txt)

 - GST_QUERY_CUSTOM:

     a custom query, the name of the query defines the properties of the query.

 - GST_QUERY_URI:

     query the uri of the source or sink element

 - GST_QUERY_ALLOCATION:

     the buffer allocation properties (See part-bufferpool.txt)

 - GST_QUERY_SCHEDULING:

     the scheduling properties (See part-scheduling.txt)

 - GST_QUERY_ACCEPT_CAPS:

     check if caps are supported (See part-negotiation.txt)

 - GST_QUERY_CAPS:

     get the possible caps (See part-negotiation.txt)