Using a loading library to access the latest OpenGL functionality

The OpenGL application binary interface (ABI) is frozen to OpenGL Version 1.1 on Windows. Unfortunately for Windows developers, that means that it is not possible to link directly to functions that are provided in newer versions of OpenGL. Instead, one must gain access to the OpenGL functions by acquiring a function pointer at runtime. Getting access to the function pointers is not difficult, but requires somewhat tedious work, and has a tendency to clutter code. Additionally, Windows typically comes with a standard OpenGL gl.h file that also conforms to OpenGL 1.1.

The OpenGL wiki states that Microsoft has no plans to ever update the gl.h and opengl32.lib that come with their compilers. Thankfully, others have provided libraries that manage all of this for us by transparently providing the needed function pointers, while also exposing the needed functionality in header files. Such a library is called an OpenGL Loading Library (or OpenGL function loader), and there are several such libraries available. One of the oldest is OpenGL Extension Wrangler (GLEW). However, there are a few issues with GLEW. First, it provides one large header file that includes everything from all versions of OpenGL. It might be preferable to have a more streamlined header file that only includes functions that we might use. Second, GLEW is distributed as a library that needs to be compiled separately and linked into our project. I find it preferable to have a loader that can be included into a project simply by adding the source files and compiling them directly into our executable, avoiding the need to support another link-time dependency.

In this recipe, we'll use a loader generator named GLAD, available from https://github.com/Dav1dde/glad. This very flexible and efficient library can generate a header that includes only the needed functionality, and also generates just a few files (a source file and a few headers) that we can add directly into our project.

Getting ready

To use GLAD, you can either download and install it using pip (or from https://github.com/Dav1dde/glad ), or you can use the web service available here: http://glad.dav1d.de/. If you choose to install it, you'll need Python. The install is simple and described in detail on the GitHub page.

How to do it...

The first step is to generate the header and source files for the OpenGL version and profile of your choice. For this example, we'll generate files for an OpenGL 4.6 core profile. We can then copy the files into our project and compile them directly alongside our code:

1. To generate the header and source files, run the following command:

glad --generator=c --out-path=GL --profile=core --api=gl=4.6

2. The previous step will generate its output into a directory named GL. There will be two directories: GL/include and GL/src. You can move the GL directory into your project as is, or move the individual files into appropriate locations. Include GL/src/glad.c in your build, and put GL/include into your include path. Within your program code, include glad/glad.h whenever you need access to the OpenGL functions. Note that this fully replaces gl.h, so there is no need to include that. 
3. In order to initialize the function pointers, you need to make sure to call a function that does so. The needed function is gladLoadGL(). Somewhere just after the GL context is created (typically in an initialization function), and before any OpenGL functions are called, use the following code:

if(!gladLoadGL()) {
  std::cerr << "Unable to load OpenGL functions!" << std::endl;
  exit(EXIT_FAILURE);
}

That's all there is to it!

How it works...

The command in step 1 generates a few header files and a source file. The header provides prototypes for all of the selected OpenGL functions, redefines them as function pointers, and defines all of the OpenGL constants as well. The source file provides initialization code for the function pointers as well as some other utility functions. We can include the glad/glad.h header file wherever we need prototypes for OpenGL functions, so all function entry points are available at compile time. At runtime, the gladLoadGL() call will initialize all available function pointers.

Some function pointers may not be successfully initialized. This might happen if your driver does not support the requested OpenGL version. If that happens, calling the functions will fail.

The command-line arguments available to GLAD are fully documented on the GitHub site and are available via glad -h. One can select any OpenGL version, select core/compatibility profiles, include desired extensions, and/or create debug callbacks.

There's more...

GLAD provides a web service at http://glad.dav1d.de/ that makes it easy to generate the loader source and header files without installing GLAD. Simply visit the URL, select the desired configuration, and the loader files will be generated and downloaded.

See also

• The ingredients/scenerunner.h file in the example code
• GLEW, an older, popular loader and extension manager, available from http://glew.sourceforge.net