Hacker Guide/Demux
Contents
Description
The modules of 'demux' capability are designed to handle the different "file" formats, like AVI or MKV. They usually come after the access module in the modules chain, and create the different tracks, that are sent to the decoders.
The demuxer modules are working on the input stream that is provided by the core input to them. It splits the input stream into the different tracks (called elementary streams).
Technically, the demuxers are pulling data from the stream; data is not pushed by the access to the demux.
Examples of demuxers are WMV/ASF, Ogg or Mkv.
Write an demuxer module
To write an demuxer module, start by reading the introduction to module writing.
Then, you should specify your module of being of demux type:
set_capability( "demux", 60 ) set_category( CAT_INPUT ) set_subcategory( SUBCAT_INPUT_DEMUX )
Functions to implement
After implementing Open() and Close() functions, you will need to implement a few majors features that will be implemented by your functions.
As you can see include/vlc_demux.h, you should define:
- Demux, as in pf_demux
- Control, as in pf_control
After implementing those functions, you should assign them to the corresponding pf_ function.
Demux
Prototype:
int (*pf_demux) ( demux_t * );
Demux function is quite easy, it pulls data from s* the pointer to the stream_t.
Return:
It should return 0 for EOF, something positive when success and something negative when fail to demux.
Control
Prototype:
int (*pf_control)( access_t *, int i_query, va_list args);
Control function is quite easy, the input core will query the module using this function with:
- A pointer to the module structure.
- An i_query parameter that can be of several type. We will cover afterward the most important on.
- A list of arguments, that depends on the i_query type.
Return:
If the query has succeeded, it should return VLC_SUCCESS. Else it should return VLC_EGENERIC (fail).
Control Query types
Useful primitives and types
When implementing a demux, you will need to work on a stream (stream_t), but also to create and control tracks (ES) (es_out_id_t).
stream_t * is the input (multiplexed) byte stream. es_out_id_t * are the elementary streams, to hand to the decoder.
Tracks / ES manipulation
Tracks in VLC are named ES, as Elementary Streams
You will probably need
- es_out_Add
- es_out_Send
- es_out_Control
See include/vlc_es_out.h and include/vlc_es.h
Stream manipulation
Stream manipulation can be done with a few functions
- stream_Read
- stream_Peek
- stream_Control
- stream_ReadLine
- stream_Block
See include/vlc_stream.h for more information
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