Minimal example of a custom OpenGL function loader

opengl.c

#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>

#include <SDL2/SDL.h>
#include <SDL2/SDL_opengl.h>

#include "opengl.h"

#define OPENGL_DEFINE(func_name, func_type)  \
    func_type func_name = NULL;

#define OPENGL_FUNCTION OPENGL_DEFINE
OPENGL_FUNCTIONS
#undef OPENGL_FUNCTION

union bridge {
    void* object_ptr;
    void (*function_ptr)(void);
};

#define OPENGL_LOAD(func_name, func_type)                \
    func_name = (func_type)(union bridge){               \
        .object_ptr = SDL_GL_GetProcAddress(#func_name)  \
    }.function_ptr;

#define OPENGL_VALIDATE(func_name, func_type)                      \
    if (func_name == NULL) {                                       \
        fprintf(stderr, "failed to load func: %s\n", #func_name);  \
        return false;                                              \
    }

bool opengl_load_functions(void) {
    #define OPENGL_FUNCTION OPENGL_LOAD
    OPENGL_FUNCTIONS
    #undef OPENGL_FUNCTION

    #define OPENGL_FUNCTION OPENGL_VALIDATE
    OPENGL_FUNCTIONS
    #undef OPENGL_FUNCTION

    return true;
}

opengl.h

#ifndef DEMO_OPENGL_H_INCLUDED
#define DEMO_OPENGL_H_INCLUDED

#include <stdbool.h>
#include <SDL2/SDL_opengl.h>

#define OPENGL_FUNCTIONS                                    \
    OPENGL_FUNCTION(glCreateShader, PFNGLCREATESHADERPROC)  \
    OPENGL_FUNCTION(glDeleteShader, PFNGLDELETESHADERPROC)

#define OPENGL_DECLARE(func_name, func_type)  \
    extern func_type func_name;

#define OPENGL_FUNCTION OPENGL_DECLARE
OPENGL_FUNCTIONS
#undef OPENGL_FUNCTION

bool opengl_load_functions(void);

#endif