# VLC Overview

## LibVLC

LibVLC is the core part of VLC. It is a library providing an interface for programs such as VLC to a lot of functionalities such as stream access, audio and video output, plugin handling, a thread system. All the LibVLC source files are located in the `src/` directory and its subdirectories:

- `interface/`: contains code for user interaction such as key presses and device ejection.

- `playlist/`: manages playlist interaction such as stop, play, next, or random playback.

- `input/`: opens an input module, reads packets, parses them and passes reconstituted elementary streams to the decoder(s).

- `video_output/`: initializes the video display, gets all pictures and subpictures (ie. subtitles) from the decoder(s), optionally converts them to another format (such as YUV to RGB), and displays them.

- `audio_output/`: initializes the audio mixer, ie. finds the right playing frequency, and then resamples audio frames received from the decoder(s).

- `stream_output/`: TODO

- `misc/`: miscellaneous utilities used in other parts of libvlc, such as the thread system, the message queue, CPU detection, the object lookup system, or platform-specific code.

Data flow between modules

## VLC

VLC is a simple program written around LibVLC. It is very small, but is a fully featured multimedia player thanks to LibVLC's support for dynamic modules.

## Modules

Modules are located in the `modules/` subdirectory and are loaded at runtime. Every module may offer different features that will best suit a particular file or a particular environment. Besides, most portability works result in the writing of an audio_output/video_output/interface module to support a new platform (eg. BeOS or MacOS X).

Plugin modules are loaded and unloaded dynamically by functions in `src/misc/modules.c` and `include/modules*.h`. The API for writing modules will be discussed in a following chapter.

Modules can also be built directly into the application which uses LibVLC, for instance on an operating system that does not have support for dynamically loadable code. Modules statically built into the application are called builtins.

## Threads

### Thread management

VLC is heavily multi-threaded. We chose against a single-thread approach because decoder preemptibility and scheduling would be a mastermind (for instance decoders and outputs have to be separated, otherwise it cannot be warrantied that a frame will be played at the exact presentation time), and we currently have no plan to support a single-threaded client. Multi-process decoders usually imply more overhead (problems of shared memory) and communication between processes is harder.

Our threading structure is modeled on pthreads. However, for portability reasons, we don't call `pthread_*` functions directly, but use a similar wrapper, made of `vlc_thread_create`, `vlc_thread_exit`, `vlc_thread_join`, `vlc_mutex_init`, `vlc_mutex_lock`, `vlc_mutex_unlock`, `vlc_mutex_destroy`, `vlc_cond_init`, `vlc_cond_signal`, `vlc_cond_broadcast`, `vlc_cond_wait`, `vlc_cond_destroy`, and structures vlc_thread_t, vlc_mutex_t, and vlc_cond_t.

### Synchronization

Another key feature of VLC is that decoding and playing are asynchronous: decoding is done by a decoder thread, playing is done by audio_output or video_output thread. The design goal is to ensure that an audio or video frame is played exactly at the right time, without blocking any of the decoder threads. This leads to a complex communication structure between the interface, the input, the decoders and the outputs.

Having several input and video_output threads reading multiple files at the same time is permitted, despite the fact that the current interface doesn't allow any way to do it [this is subject to change in the near future]. Anyway the client has been written from the ground up with this in mind. This also implies that a non-reentrant library (including in particular liba52) cannot be used without using a global lock.

Presentation Time Stamps located in the system layer of the stream are passed to the decoders, and all resulting samples are dated accordingly. The output layers are supposed to play them at the right time. Dates are converted to microseconds ; an absolute date is the number of microseconds since Epoch (Jan 1st, 1970). The mtime_t type is a signed 64-bit integer.

The current date can be retrieved with `mdate()`. The execution of a thread can be suspended until a certain `date` via `mwait` `( mtime_t date )`. You can sleep for a fixed number of microseconds with `msleep` `( mtime_t delay )`.

Please remember to wake up slightly before the presentation date, if some particular treatment needs to be done (e.g. a chroma transformation). For instance in `modules/codec/mpeg_video/synchro.c`, track of the average decoding times is kept to ensure pictures are not decoded too late.

## Code conventions

### Function naming

All functions are named accordingly : module name (in lower case) + _ + function name (in mixed case, without underscores). For instance : `intf_FooFunction`. Static functions don't need usage of the module name.

### Variable naming

Hungarian notations are used, that means we have the following prefixes :

- i_ for integers (sometimes l_ for long integers) ;

- b_ for booleans ;

- d_ for doubles (sometimes f_ for floats) ;

- pf_ for function pointers ;

- psz_ for a Pointer to a String terminated by a Zero (C-string) ;

- More generally, we add a p when the variable is a pointer to a type.

If one variable has no basic type (for instance a complex structure), don't put any prefix (except p_* if it's a pointer). After one prefix, put an explicit variable name in lower case. If several words are required, join them with an underscore (no mixed case). Examples :

- data_packet_t * `p_buffer;`

- char `psz_msg_date[42];`

- int `pi_es_refcount[MAX_ES];`

- void `(* pf_next_data_packet)( int * );`

### A few words about white spaces

First, never use tabs in the source (you're entitled to use them in the Makefile :-). Use `set expandtab` under vim or the equivalent under emacs. Indents are 4 spaces long.

Second, put spaces before and after operators, and inside brackets. For instance :

     for( i = 0; i < 12; i++, j += 42 );

Third, leave braces alone on their lines (GNU style). For instance :

    if( i_es == 42 )
    {
        p_buffer[0] = 0x12;
    }

We write C, so use C-style comments /* ... */.