Hacker Guide/How To Write a Module
LibVLC is based on many independent modules, like most competing multimedia frameworks. Each module provides specific functionality.
This article focuses on adding a new module (a.k.a. plug-in) to VLC (or any other LibVLC application). You will need to read VLC Core and Modules and How VLC loads modules first, otherwise you will not be able to flesh out the content of your new module.
Contents
In-tree and out-of-tree modules
Most existing VLC modules are provided as source code within the main VLC source code repository (and also the source tarballs). They are compiled at the same time as the VLC core, and usually provided distributed together with VLC binary packages and installers. These modules are called in-tree modules.
However, it is also possible to write and compile VLC modules outside of VLC. That has some pros and cons over developing modules in-tree:
Pros
- Compilation is a lot faster (VLC and other modules are not included in the process).
- You can use your own version control system, or even none at all.
- The copyright license does not need to abide by the requirements of the VideoLAN association for inclusion in VLC.
- The source code does not need to be provided, reviewed and accepted by the VLC developers.
- The release schedule is independent of VLC releases. New versions of the module can be published at any time regardless of VLC release planning.
- Different programming languages can be used at least in theory. (The main VLC code base only uses C, C++ and Lua, and on MacOS Objective C.)
- The module can use software libraries that would be inadequate for VLC to depend on.
Cons
- The VLC developers will not review the code, which would be a good opportunity to improve the code quality.
- VLC translators will not take care of localization for the module(s) where applicable. VLC is translated in many tens of languages.
- The module(s) cannot be distributed through the videolan.org website and use the VideoLAN infrastructure such as the bug tracker and the build bots.
- The module(s) will only work with the particular VLC (major) version that it has been compiled for. For instance, a module compiled for VLC 1.1.x will not work with VLC 1.0.x or VLC 2.0.x.
- The module(s) will only work on the particular operating systems and architecture that it has been compiled for. For instance, a Windows 32-bit module will only work with Windows 32-bit versions of VLC. VLC supports many different combinations of operating systems and architectures.
Example stub module
Let's start with a small example module in the C language:
/**
* @file hello.c
* @brief Hello world interface VLC module example
*/
#ifdef HAVE_CONFIG_H
# include "config.h"
#endif
#include <stdlib.h>
/* VLC core API headers */
#include <vlc_common.h>
#include <vlc_plugin.h>
#include <vlc_interface.h>
/* Forward declarations */
static int Open(vlc_object_t *);
static void Close(vlc_object_t *);
/* Module descriptor */
vlc_module_begin()
set_shortname(N_("Hello"))
set_description(N_("Hello interface"))
set_capability("interface", 0)
set_callbacks(Open, Close)
set_category(CAT_INTERFACE)
add_string("hello-who", "world", "Target", "Whom to say hello to.", false)
vlc_module_end ()
/* Internal state for an instance of the module */
struct intf_sys_t
{
char *who;
};
/**
* Starts our example interface.
*/
static int Open(vlc_object_t *obj)
{
intf_thread_t *intf = (intf_thread_t *)obj;
/* Allocate internal state */
intf_sys_t *sys = malloc(sizeof (*sys));
if (unlikely(sys == NULL))
return VLC_ENOMEM;
intf->p_sys = sys;
/* Read settings */
char *who = var_InheritString(intf, "hello-who");
if (who == NULL)
{
msg_Err(intf, "Nobody to say hello to!");
goto error;
}
sys->who = who;
msg_Info(intf, "Hello %s!", who);
return VLC_SUCCESS;
error:
free(sys);
return VLC_EGENERIC;
}
/**
* Stops the interface.
*/
static void Close(vlc_object_t *obj)
{
intf_thread_t *intf = (intf_thread_t *)obj;
intf_sys_t *sys = intf->p_sys;
msg_Info(intf, "Good bye %s!");
/* Free internal state */
free(sys->who);
free(sys);
}
And now some explanations about the code...
Module Descriptor
A VLC media player module must include a description of itself, and the parameters it accepts.
The module descriptor begins with:
vlc_module_begin()
You should set some basic information about your module. This is for the dvdread module:
set_shortname(N_("DVD without menus"))
set_description(N_("DVDRead Input"))
set_category(CAT_INPUT)
set_subcategory(SUBCAT_INPUT_ACCESS)
Note the use of N_("") to create a string that needs to be translated by gettext.
Capability and score
Definition Example:
set_capability("interface", 0)
This defines a module of "interface" capability and a score of 0.
The capability determines the type of module we are dealing with. It could be an access, a demux, a decoder, an interface, etc. Now is the time to re-read how VLC loads modules.
- If VLC needs to load a specific name, it will load it by its name and VLC directly opens this module
- If VLC needs a type of module ("I need a decoder"), VLC will load all modules matching this capability in a decreasing score order until one modules's Open() function (see later) returns VLC_SUCCESS.
See the major types of capabilities of VLC.
Score should be an integer, and related to other scores in the same category. Score 0 is a special case.
Configuration categories and sub-categories
You should use one of the predefined categories for configuration. The configuration categories and sub-categories specify where the module will appear in the preferences UI dialog.
The configuration categories include:
- CAT_INTERFACE
- CAT_AUDIO
- CAT_VIDEO
- CAT_INPUT
- CAT_SOUT
- CAT_ADVANCED
- CAT_PLAYLIST
You should use one of predefined sub-categories as well. See include/vlc_configuration.h for definition of all configuration categories and sub-categories.
Configuration parameters
You may need options to configure the run-time behavior of your module. Defining new options is easy.
All option definitions take the following argument list:
add_integer(name, value, text, longtext, advanced)
- name is the string that identifies this parameter in the configuration. This name is used at the command prompt to set the configuration value.
- value is the default value for this parameter,
- text A short description of the parameter, use _("") to create a string that needs to be translated,
- longtext A complete description of the parameter, use _("") to create a string that needs to be translated,
- advanced Boolean, ADVanced Configuration. If TRUE, this parameter will only be displayed when using the --advanced flag.
You may add the following options/parameter types to your module:
- add_integer,
- add_string,
- add_float,
- add_bool,
- add_key,
- add_file,
- add_directory,
For complete definitions, see include/vlc_plugin.h
Callback
The activation and deactivation functions, detailed afterwards, must be defined in the descriptor. This is so that the VLC core knows how to instantiate and run the module.
The set_callbacks() macro allows you to define 2 parameters: the first parameter is the pf_activate callback, and the second one, pf_deactivate. The functions are most often called "Open" and "Close" respectively, though. VLC invokes the pf_activate callback if/when it needs a plugin instance providing the correct interface, as declared with the set_capability() macro.
Conversely, VLC invokes the pf_deactivate callback when the plugin is no longer needed - but only if the pf_activate callback returned VLC_SUCCESS (0) earlier.
Open(vlc_object_t *)
The most important function of a module is the opening: the usually-named Open() function.
static int Open ( vlc_object_t * );
The Open() function is called when the VLC core tries to open the module, and wants to load it.
During Open(), setup of structures, devices or I/O, checks should be done. A successful open should return VLC_SUCCESS. If the module cannot complete its initialization, it can return any other value, usually VLC_EGENERIC or VLC_ENOMEM.
The Open() function is expected to allocate private data (if any), and set up the private structure.
If the Opening fails, you may need to free any already allocated resources before returning. Otherwise, leaks will occur.
Close(vlc_object_t *)
The second most important function of a module is the closing: the usually-named Close() function.
static int Close ( vlc_object_t * );
The Close() function is called when the VLC core tries to close or unload an already-loaded module.
NB: If the Open() function failed, Close() will not get called.
During Close(), closing devices or I/O, and cleaning of structures should be done. Do not leak memory here!
The Close() function should deallocate private data.
In-tree module integration
Git
If you plan to submit your work to VLC upstream, be sure to look at the git introduction and check the send patches part.
Compiling your module
Modules.am
First, find the right subdirectory under modules/ to add your new code.
- If the module has only one source code file module, simply add it in the subdirectory (e.g. modules/control/hello.c).
- Larger modules should get a sub-subdirectory of their own (e.g. modules/control/hello/*).
Then you need to declare the module in the build system. For example, the file modules/control/Modules.am tells the build system which source files are needed for each control module. For the example above, we could add these lines:
libhello_plugin_la_SOURCES = hello.c libhello_plugin_la_CFLAGS = $(AM_CFLAGS) libhello_plugin_la_LIBADD = $(AM_LIBADD) libhello_plugin_la_DEPENDENCIES = # Always compile the hello module: libvlc_LTLIBRARIES += libhello_plugin.la
Note that indentation in Modules.am (if needed) uses tabulations (ASCII 0x09), not white spaces.
configure.ac
If the module depends on some new library, some architecture or some operating system characteristics, you may need to extend configure.ac to detect when and how to build the module. Refer to the configure.ac file and the GNU autoconf documentation for details.
Once this is done, you should only need to rebuild VLC:
make
(This will probably trigger a re-run of autoconf and automake, so it might take a while.)
Loading your module
VLC keeps a cache of available modules for performance reasons. It should be updated automatically. But you can use ./vlc --reset-plugins-cache to force a reset.
Then use
./vlc -vvv --color --list
to check that your plugin is seen by VLC media player.
You should also see it in the plugins dialog of the Qt interface (Linux and Windows).
Out-of-tree module
There is a dedicated article. Please read out of tree compilation.
Sub-modules
Sub-modules, declared in some module descriptors with
add_submodule()
work exactly the same way as modules. They are useful when different modules (usually but not necessarily of different capability) share common code. All sub-modules will be included in the same run-time library as the main module.
Module types
Depending on the module capability, you will need more information, about the necessary functions to implement.
We will detail those here:
Module load troubleshooting
Sometimes when building an in-tree module, stuff doesn't work due to build system problems and other inconsistencies.
You probably need to go to the root of your VLC source tree, and do something akin to the following. The examples here assume the bash shell.
Mild version
In some cases, automake dependencies break (for instance after some filenames have changed). This might then work:
find . -name .deps -exec rm -rf \{\} \;
./config.status
make
...but not always, so it may save some headaches to always use the "medium version" below.
Medium version (try this first)
This is a more radical but still safe rebuild procedure:
find . -name .deps -exec rm -rf \{\} \;
./bootstrap
./configure
make
Extreme version
If the none of the above helped, you can clean the source tree as a measure of last resort. Before you proceed, it is highly recommended that you check which files are going to be erased:
git clean -nxd
And then check what source code changes you would lose (if any):
git diff
You can extremely easily lose entire days of hard work with the following commands. The first command will permanently remove any files not tracked in git, including files that you might have created yourself. The second command will remove any uncommitted modification to existing files. Consider yourself warned.
!!!BEWARE: THIS MAY CAUSE UNRECOVERABLE DATA LOSS!!!
git clean -fxd git reset --hard HEAD ./bootstrap ./configure make
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