#ifdef __egl_h_ /* mobile or embedded devices and OpenGL ES */
#define glGetProcAddress(fn)    eglGetProcAddress(fn)
#elif defined(GLX_H) /* Linux X-windowing system interface */
#define glGetProcAddress(fn)    glXGetProcAddress((const GLubyte *)fn)
#elif defined(_WINGDI_) /* Windows GDI generic software interface */
#define glGetProcAddress(fn)    wglGetProcAddress((LPCSTR)fn)
#elif defined(_SDL_H) /* Simple DirectMedia Layer */
#define glGetProcAddress(fn)    SDL_GL_GetProcAddress(fn)
#else
#error Unimplemented on this OS, or you forgot to include context creation API for it?
#endif