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Building a VLC module in Rust

a VLC plugin is a DLL linked to libvlccore, exporting the functions vlc_entry__VERSION, and optionally vlc_entry_copyright__VERSION and vlc_entry_license__VERSION.

The C implementation for a module begins with code like this:

static int  Open ( vlc_object_t * );
static void Close( vlc_object_t * );

vlc_module_begin ()
    set_description( N_("WAV demuxer") )
    set_category( CAT_INPUT )
    set_subcategory( SUBCAT_INPUT_DEMUX )
    set_capability( "demux", 142 )
    set_callbacks( Open, Close )
vlc_module_end ()

This is a serie of macro calls (defined in include/vlc_plugin.h) to generate this entry function:

int vlc_entry__MODULE_NAME (vlc_set_cb, void *); int vlc_entry__MODULE_NAME (vlc_set_cb vlc_set, void *opaque) { module_t *module; module_config_t *config = ((void*)0); if (vlc_set (opaque, ((void*)0), VLC_MODULE_CREATE, &module)) goto error; if (vlc_set (opaque, module, VLC_MODULE_NAME, ("modules/demux/wav.c"))) goto error;·
    if (vlc_set (opaque, module, VLC_MODULE_DESCRIPTION, (const char *)(N_("WAV demuxer")))) goto error;
    vlc_set (opaque, ((void*)0), VLC_CONFIG_CREATE, (0x06), &config); vlc_set (opaque, config, VLC_CONFIG_VALUE, (int64_t)(4));
    vlc_set (opaque, ((void*)0), VLC_CONFIG_CREATE, (0x07), &config); vlc_set (opaque, config, VLC_CONFIG_VALUE, (int64_t)(403));
    if (vlc_set (opaque, module, VLC_MODULE_CAPABILITY, (const char *)("demux")) || vlc_set (opaque, module, VLC_MODULE_SCORE, (int)(142))) goto error;
    if (vlc_set (opaque, module, VLC_MODULE_CB_OPEN, Open) || vlc_set (opaque, module, VLC_MODULE_CB_CLOSE, Close)) goto error;
(void) config; return 0; error: return -1; }

With better indentation:

int vlc_entry__MODULE_NAME (vlc_set_cb, void *);
int vlc_entry__MODULE_NAME (vlc_set_cb vlc_set, void *opaque) {
    module_t *module;
    module_config_t *config = ((void*)0);
    if (vlc_set (opaque, ((void*)0), VLC_MODULE_CREATE, &module))
        goto error;
    if (vlc_set (opaque, module, VLC_MODULE_NAME, ("modules/demux/wav.c")))
        goto error;
    if (vlc_set (opaque, module, VLC_MODULE_DESCRIPTION, (const char *)(N_("WAV demuxer"))))
        goto error;
    vlc_set (opaque, ((void*)0), VLC_CONFIG_CREATE, (0x06), &config);
    vlc_set (opaque, config, VLC_CONFIG_VALUE, (int64_t)(4));
    vlc_set (opaque, ((void*)0), VLC_CONFIG_CREATE, (0x07), &config);
    vlc_set (opaque, config, VLC_CONFIG_VALUE, (int64_t)(403));
    if (vlc_set (opaque, module, VLC_MODULE_CAPABILITY, (const char *)("demux")) || vlc_set (opaque, module, VLC_MODULE_SCORE, (int)(142)))
        goto error;
    if (vlc_set (opaque, module, VLC_MODULE_CB_OPEN, Open) || vlc_set (opaque, module, VLC_MODULE_CB_CLOSE, Close))
        goto error;
    (void) config;
    return 0;

  error:
    return -1;
}

To create the corresponding function in Rust, we need to export a function receiving a callback with the following definition:

EXTERN_SYMBOL typedef int (*vlc_set_cb) (void *, void *, int, ...);

Place the built plugin in VLC.app/Contents/MacOS/plugins/ (OSX only, of course). Otherwise, you can see where plugins are stored by looking in the output for "recursively browsing". In my case, this is in "~/vlc/src/.libs/vlc/plugins".

Don't forget to make it end with "_plugin.so" (or "_plugin.dylib" on OSX)! For example, if your file is named "libflv.so", name it "libflv_plugin.so".

To verify that the plugin is loaded, run:

./VLC.app/Contents/MacOS/VLC -vvv --list --reset-plugins-cache

You should see this on one line: inrustwetrust FLV demuxer written in Rust

To play a file with the new plugin, run:

./VLC.app/Contents/MacOS/VLC -vvv --reset-plugins-cache ./zeldaHQ.flv

This will first call the open function passed as callback.

Importing functions from libvlccore

copy VLC.app/Contents/MacOS/lib/libvlccore.8.dylib to target/debug/libvlccore.dylib and build the project. Update the libvlccore loading path like this:

install_name_tool -change "@loader_path/lib/libvlccore.8.dylib" "@loader_path/../lib/libvlccore.8.dylib" target/debug/librustdemux.dylib

Defining the FFI interface

Taking a bit of inspiration from https://github.com/sfackler/rust-openssl :

  • the raw structures and C API is (for now) in src/ffi.rs and the actual Rust API is in src/vlc.rs.
  • VLC uses an object-like construction in C, with everything inheriting from vlc_object_t through the VLC_COMMON_MEMBERS macro
  • write wrappers for the C functions to make them more manageable. Example: stream_Peek takes a mutable reference to a pointer to a byte array, an expected size, and returns a size (possibly negative in case of error). The Rust-like stream_Peek will take a size as argument, and return (if successful) an immutable slice
  • all new VLC objects and structures that we do not need to implement will be represented as opaque pointers. Example: pub type module_t = c_void;