Binding to external libraries


The Summator example in Custom modules in C++ is great for small, custom modules, but what if you want to use a larger, external library? Let’s look at an example using Festival, a speech synthesis (text-to-speech) library written in C++.

To bind to an external library, set up a module directory similar to the Summator example:


Next, you will create a header file with a simple TTS class:

/* tts.h */
#ifndef GODOT_TTS_H
#define GODOT_TTS_H

#include <reference.h>

class TTS : public Reference {
    GDCLASS(TTS, Reference);

    static void _bind_methods();

    bool say_text(String txt);


#endif // GODOT_TTS_H

And then you’ll add the cpp file.

/* tts.cpp */

#include "tts.h"
#include "festival/src/include/festival.h"

bool TTS::say_text(String txt) {

    //convert Godot String to Godot CharString to C string
    return festival_say_text(txt.ascii().get_data());

void TTS::_bind_methods() {

    ClassDB::bind_method(D_METHOD("say_text", "txt"), &TTS::say_text);

TTS::TTS() {
    festival_initialize(true, 210000); //not the best way to do it as this should only ever be called once.

Just as before, the new class needs to be registered somehow, so two more files need to be created:


With the following contents:

/* register_types.h */

void register_tts_types();
void unregister_tts_types();
/* yes, the word in the middle must be the same as the module folder name */
/* register_types.cpp */

#include "register_types.h"

#include "class_db.h"

#include "tts.h"

void register_tts_types() {

void unregister_tts_types() {
   //nothing to do here

Next, you need to create a SCsub file so the build system compiles this module:

# SCsub

env_tts = env
env_tts.add_source_files(env.modules_sources,"*.cpp") # Add all cpp files to the build

You’ll need to install the external library on your machine to get the .a library files. See the library’s official documentation for specific instructions on how to do this for your operation system. We’ve included the installation commands for Linux below, for reference.

sudo apt-get install festival festival-dev <-- Installs festival and speech_tools libraries
apt-cache search festvox-* <-- Displays list of voice packages
sudo apt-get install festvox-don festvox-rablpc16k festvox-kallpc16k festvox-kdlpc16k <-- Installs voices


Important: The voices that Festival uses (and any other potential external/3rd-party resource) all have varying licenses and terms of use; some (if not most) of them may be be problematic with Godot, even if the Festival Library itself is MIT License compatible. Please be sure to check the licenses and terms of use.

The external library will also need to be installed inside your module to make the source files accessible to the compiler, while also keeping the module code self-contained. The festival and speech_tools libraries can be installed from the modules/tts/ directory via git using the following commands:

git clone
git clone

If you don’t want the external repository source files committed to your repository, you can link to them instead by adding them as submodules (from within the modules/tts/ directory), as seen below:

git submodule add
git submodule add


Important: Please note that Git submodules are not used in the Godot repository. If you are developing a module to be merged into the main Godot repository, you should not use submodules. If your module doesn’t get merged in, you can always try to implement the external library as a GDNative C++ plugin.

To add include directories for the compiler to look at you can append it to the environment’s paths:

env_tts.Append(CPPPATH="speech_tools/include", "festival/src/include") # this is a path relative to /modules/tts/
# <-- Festival library documentation
env_tts.Append(LIBPATH=['libpath']) # this is a path relative to /modules/tts/ where your .a library files reside
# You should check with the documentation of the external library to see which library files should be included/linked
env_tts.Append(LIBS=['Festival', 'estools', 'estbase', 'eststring'])

If you want to add custom compiler flags when building your module, you need to clone env first, so it won’t add those flags to whole Godot build (which can cause errors). Example SCsub with custom flags:

# SCsub

env_tts = env
env_tts.Append(CXXFLAGS=['-O2', '-std=c++11'])

The final module should look like this:


Using the module

You can now use your newly created module from any script:

var t =
var script = "Hello world.  This is a test!"
var is_spoken = t.say_text(script)
print('is_spoken: ', is_spoken)

And the output will be is_spoken: True if the text is spoken.