Hacker Guide/Demux

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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.



 int (*pf_demux)  ( demux_t * ); 

Demux function is quite easy, it pulls data from s* the pointer to the stream_t.


It should return 0 for EOF, something positive when success and something negative when fail to demux.



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.


If the query has succeeded, it should return VLC_SUCCESS. Else it should return VLC_EGENERIC (fail).

Control Query types

See include/vlc_demux.h

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

This page is part of official VLC media player Documentation (User GuideStreaming HowToHacker GuideModules)
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