.

[vlc] / doc / developer / interface.xml

<chapter> <title> VLC interface </title>

  <sect1> <title> A typical VLC run course </title>

    <para>
This section describes what happens when you launch the <application> 
vlc</application>
program. After the ELF dynamic loader blah blah blah, the main thread
becomes the interface thread and starts up in <filename>
src/interface/main.c </filename>. It passes through the following steps :
    </para>

    <orderedlist>
      <listitem> <para> CPU detection : which CPU are we running on, what are
      its capabilities (MMX, MMXEXT, 3DNow, AltiVec...) ? </para> </listitem>
      <listitem> <para> Message interface initialization ; </para> </listitem>
      <listitem> <para> Command line options parsing ; </para> </listitem>
      <listitem> <para> Playlist creation ; </para> </listitem>
      <listitem> <para> Module bank initialization ; </para> </listitem>
      <listitem> <para> Interface opening ; </para> </listitem>
      <listitem> <para> Signal handler installation : SIGHUP, SIGINT
      and SIGQUIT are caught to manage a clean quit (please note that the SDL
      library also catches SIGSEGV) ; </para> </listitem>
      <listitem> <para> Audio output thread spawning ; </para> </listitem>
      <listitem> <para> Video output thread spawning ; </para> </listitem>
      <listitem> <para> Main loop : events management ; </para> </listitem>
    </orderedlist>

    <para>
Following sections describe each of these steps in particular, and many more.
    </para>

  </sect1>

  <sect1> <title> The message interface </title>

    <para>
It is a know fact that <function> printf() </function> functions are
not necessarily thread-safe. As a result,
one thread interrupted in a <function> printf() </function> call, followed
by another calls to it, will leave the program in an undetermined state. So
an API must be set up to print messages without crashing.
    </para>

    <para>
This API is implemented in two ways. If <constant> INTF_MSG_QUEUE </constant>
is defined in <filename> config.h </filename>, every <function>
printf</function>-like (see below) call will queue the message into a chained list.
This list will be printed and flushed by the interface thread once upon
an event loop. If <constant> INTF_MSG_QUEUE </constant> is undefined,
the calling thread will acquire the print lock (which prevents two
print operations to occur at the same time) and print the message
directly (default behaviour).
    </para>

    <para>
Functions available to print messages are :
    </para>

    <itemizedlist>
      <listitem> <para> <function> intf_Msg </function> ( <parameter>
      char * psz_format, ... </parameter> ) :
      Print a message to <constant> stdout </constant>, plain and
      stupid (for instance "vlc 0.2.72 (Apr 16 2001)"). </para> </listitem>

      <listitem> <para> <function> intf_ErrMsg </function> ( <parameter>
      char * psz_format, ... </parameter> ) :
      Print an error message to <constant> stderr </constant>. </para>
      </listitem>

      <listitem> <para> <function> intf_WarnMsg </function> ( <parameter>
      int i_level, char * psz_format, ... </parameter> ) :
      Print a message to <constant> stderr </constant> if the warning
      level (determined by -v, -vv and -vvv) is low enough. </para>
      <note> <para> Please note
      that the lower the level, the less important the message is (dayou
      spik ingliche ?). </para> </note> </listitem>

      <listitem> <para> <function> intf_DbgMsg </function> ( <parameter>
      char * psz_format, ... </parameter> ) :
      This function is designed for optional checkpoint messages, such
      as "we are now entering function dvd_foo_thingy". It does nothing
      in non-trace mode. If the VLC is compiled with --enable-trace, the
      message is either written to the file <filename> vlc-trace.log </filename>
      (if TRACE_LOG is defined in config.h), or printed to <constant> stderr
      </constant> (otherwise). </para> </listitem>

      <listitem> <para> <function> intf_MsgImm, intf_ErrMsgImm, intf_WarnMsgImm,
      intf_DbgMsgImm </function> :
      Same as above, except that the message queue, in case <parameter>
      INTF_MSG_QUEUE </parameter> is defined,
      will be flushed before the function returns.
      </para> </listitem>

      <listitem> <para> <function> intf_WarnHexDump </function> ( <parameter>
      int i_level, void * p_data, int i_size </parameter> ) :
      Dumps <parameter> i_size </parameter> bytes from <parameter>
      p_data </parameter> in hexadecimal. <parameter> i_level </parameter>
      works like <function> intf_WarnMsg </function>. This is useful for
      debugging purposes. </para> </listitem>

      <listitem> <para> <function> intf_FlushMsg </function> () :
      Flush the message queue, if it is in use. </para> </listitem>
    </itemizedlist>

  </sect1>

  <sect1> <title> Command line options </title>

    <para>
VLC uses GNU getopt to parse command line options. getopt structures are
defined in <filename> src/interface/main.c </filename> in the "Command
line options constants" section. To add a new option This section needs
to be changed, along with
<function> GetConfiguration </function> and <function> Usage</function>.
    </para>

    <para>
Most configuration directives are exchanged via the environment array,
using <function> main_Put*Variable </function> and <function>
main_Get*Variable</function>. As a result, <command> 
./vlc --height 240 </command> is strictly equivalent to : <command>
vlc_height=240 ./vlc</command>. That way configuration variables are
available everywhere, including plugins.
    </para>

    <warning> <para>
Please note that for thread-safety issues, you should not use
<function> main_Put*Variable </function> once the second thread has
been spawned.
    </para> </warning>

  </sect1>

  <sect1> <title> Playlist management </title>

    <para>
The playlist is created on startup from files given in the command line.
An appropriate interface plugin can then add or remove files from it.
Functions to be used are described in <filename>
src/interface/intf_playlist.c</filename>.
<function> intf_PlaylistAdd </function> and <function>
intf_PlaylistDelete</function> are typically the most common used.
</para>

    <para>
The main interface loop <function> intf_Manage </function> is then
supposed to <emphasis> start and kill input threads </emphasis> when necessary.
    </para>

  </sect1>

  <sect1> <title> Module bank </title>

    <para>
On startup, VLC creates a bank of all available .so files (plugins) in
<filename>., ./lib, /usr/local/lib/videolan/vlc</filename> <constant>
(PLUGIN_PATH)</constant>, and built-in plugins. Every plugin is checked
with its capabilities, which are :
    </para>

    <itemizedlist>
      <listitem> <para> MODULE_CAPABILITY_INTF : An interface plugin ;
      </para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_ACCESS : A sam-ism, unused at
      present ;</para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_INPUT : An input plugin, for
      instance PS or DVD ;</para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_DECAPS : A sam-ism, unused at
      present ;</para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_ADEC : An audio decoder ;
      </para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_VDEC : A video decoder ;
      </para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_MOTION : A motion compensation
      module (for the video decoder) ;</para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_IDCT : An IDCT module (for
      the video decoder) ;</para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_AOUT : An audio output module ;
      </para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_VOUT : A video output module ;
      </para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_YUV : A YUV module (for the
      video output) ;</para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_AFX : An audio effects plugin
      (for the audio output ; unimplemented) ;</para> </listitem>
      <listitem> <para> MODULE_CAPABILITY_VFX : A video effects plugin
      (for the video output ; unimplemented) ;</para> </listitem>
    </itemizedlist>

    <para>
How to write a plugin is described in the latter sections. Other threads
can request a plugin descriptor with <function> module_Need </function> 
<parameter> ( module_bank_t * p_bank, int i_capabilities, void * p_data ).
p_data </parameter> is an optional parameter (reserved for future use) for the
<function> pf_probe() </function> function. The returned module_t
structure contains pointers to the functions of the plug-in. See
<filename>include/modules.h</filename> for more information.
    </para>

  </sect1>

  <sect1> <title> The interface main loop </title>

    <para>
The interface thread will first look for a suitable interface plugin.
Then it enters the main interface loop, with the plugin's <function>
pf_run </function> function. This function will do what's appropriate,
and every 100 ms will call (typically via a GUI timer callback)
<function>intf_Manage</function>.
    </para>

    <para>
<function>intf_Manage</function> cleans up the module bank by unloading
unnecessary modules, manages the playlist, and flushes waiting
messages (if the message queue is in use).
    </para>

  </sect1>

  <sect1> <title> How to write an interface plugin </title>

  <sect2> <title> API for the Module </title>
    <para>
Have a look the files in directories
<filename>modules/misc/control</filename>, 
<filename> modules/misc/dummy</filename>, 
<filename> modules/misc/access</filename>, 
or <filename> modules/gui</filename>. However the GUI interfaces are
not very easy to understand, since they are quite big. I suggest to
start digging into a non-graphical interface modules first. For example
<filename>modules/control/hotkeys.c</filename>.</para>

  <para>An interface module is made of 3 entry functions and a module
description:
</para>

    <itemizedlist>

      <listitem> <para> The module description is made of macros that
   declares the capabilities of the module (interface, in this case)
   with their priority, the module description as it will appear in
   the preferences of GUI modules that implement them, some
   configuration variables specific to the module, shortcuts,
   sub-modules, etc. </para></listitem>

     <listitem> <para> <function> Open </function> ( <parameter>
           vlc_object_t* p_object </parameter> ): This is called by
           VLC to initialize the module. </para></listitem>

     <listitem> <para> <function> Run </function> ( <parameter>
           vlc_object_t* p_object </parameter> ): really does the job
           of the interface module (waiting for user input and
           displaying info). It should check periodically that
           <constant>p_intf->b_die</constant> is
           not <constant>VLC_TRUE</constant>.
     </para></listitem>

      <listitem> <para> <function> Close ( <parameter> vcl_object_t *
   p_object </parameter> ) </function> function is called by VLC to
   uninitialize the module (basically, this consists in destroying
   whatever have been allocated
   by <function>Open</function>) </para></listitem>

     </itemizedlist>

<para>The above functions take a <parameter>vlc_object_t*</parameter>
as argument, but that may need to be cast into
a <parameter>intf_thread_t*</parameter> depending on your needs. This
structure is often needed as a parameter for exported VLC functions,
such as <function>msg_Err()</function>, <function>msg_Warn()</function>,
...</para>

<para>
Define <parameter>intf_sys_t</parameter> to contain any variable you
need (don't use static variables, they suck in a multi-threaded
application :-).</para>

<para>If additional capabilities (such as Open button,
playlist, menus, etc.) are needed, consult one of the GUI modules.
One of the simpler GUI modules to consult might be 
<filename>modules/gui/ncurses/ncurses.c</filename>. It is a quite
simple complete interface module with playlist interaction, and
progress bar, among other things.

    </para>
</sect2>

<sect2><title>Arranging for your Module to get Compiled</title>

<para>If you create a new directory for your module, add
a <filename>Modules.am</filename> file in it.  In this file, put
something like : <constant>SOURCES_yourmodule = myfile1.c
myfile2.c</constant></para>

<para>Then go to the main <filename>configure.ac</filename> file, and
add in the <constant>AC_CONFIG_FILES</constant> section (towards the
end of the file) a line similar to the others.</para>

<para>If you don't create a directory for your plugin (but instead
just put it in an existing directory), you only have to add the two
SOURCES_... lines to the existing <filename>Modules.am</filename>
file</para>

<para>This declares your module; it does not arrange for it to be
automatically compiled; automatic compilatoin is described further
below.</para>

<para>You do not write a <filename>Makefile</filename> for your
module. Instead this is done via the bootstrap and configuration
process. So now run: 
</para>

<!---don't know if <xmp> or <example> works. Until then... -->
<para><filename>./bootstrap</filename></para>
<para><filename>./configure</filename> <emphasis>configure-options</emphasis></para>
<para><filename>make</filename></para>

<para>To build the module manually, go to the 
directory it resides and type
<constant>make libyourmodule_plugin.so</constant> (or .dll, or
  whatever the file type for a shared library is on your Operating
  System.)</para>

<para>To <emphasis>automatically</emphasis> have your module get
built, you also set this in the <filename>configure.ac</filename>
file; add your module name to the <constant>default modules</constant>
section in one of the
<constant>AX_ADD_PLUGINS</constant> directives.</para>
</sect2>

  </sect1>

</chapter>