OpenGL 3.1 with SDL2 context fails to output anything on screen - c

Question

I am trying to get a simple demo of OpenGL working with SDL2. I am using MacOS Big Sur 11.3.1, my SDL version is 2.0.16, and my attempted OpenGL version is 3.1.
Everything seems to work fine at first, given that SDL is initialized successfully, the SDL window is not null, and the OpenGL context is not null either. But when trying to print out the vendor name, renderer name, and version name of OpenGL, all of the strings are null:
vendor = (null)
renderer = (null)
version = (null)
This also explains why nothing appears on the window. I have followed plenty of tutorials that go over using SDL2 with OpenGL, but none of them have worked for me.
I am compiling like this: clang -O3 -lSDL2 -lGL gl_sdl.c.
If anyone knows what is going on, please let me know; I am very confused. My code is below.
#include <SDL2/SDL.h>
#include <GL/gl.h>
enum {w = 800, h = 600};
#define FAIL(msg) {fprintf(stderr, "Could not " msg "\n"); return 1;}
// https://www.khronos.org/opengl/wiki/Tutorial1:_Creating_a_Cross_Platform_OpenGL_3.2_Context_in_SDL_(C_/_SDL)
// clang -O3 -lSDL2 -lGL gl_sdl.c && ./a.out
int main(void) {
if (SDL_Init(SDL_INIT_VIDEO) < 0) FAIL("initialize SDL");
SDL_GL_SetAttribute(SDL_GL_CONTEXT_PROFILE_MASK, SDL_GL_CONTEXT_PROFILE_CORE);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MAJOR_VERSION, 3);
SDL_GL_SetAttribute(SDL_GL_CONTEXT_MINOR_VERSION, 1);
// Turn on double buffering with a 24bit Z buffer. You may need to change this to 16 or 32 for your system
SDL_GL_SetAttribute(SDL_GL_DOUBLEBUFFER, 1);
SDL_GL_SetAttribute(SDL_GL_DEPTH_SIZE, 24);
SDL_Window* const window = SDL_CreateWindow("gl_sdl", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, w, h, SDL_WINDOW_OPENGL);
if (window == NULL) FAIL("create a window");
SDL_GLContext context = SDL_GL_CreateContext(window);
if (context == NULL) FAIL("create a context");
// This makes our buffer swap syncronized with the monitor's vertical refresh
SDL_GL_SetSwapInterval(1);
printf("vendor = %s\nrenderer = %s\nversion = %s\n", glGetString(GL_VENDOR), glGetString(GL_RENDERER), glGetString(GL_VERSION));
SDL_Event event;
while (1) {
while (SDL_PollEvent(&event)) {
if (event.type == SDL_QUIT) {
SDL_GL_DeleteContext(context);
SDL_DestroyWindow(window);
SDL_Quit();
return 0;
}
}
// Blue
glClearColor(0.0f, 0.0f, 1.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
SDL_GL_SwapWindow(window);
SDL_Delay(20);
}
}

Answer 1

MacOS uses a different library flag for OpenGL than Linux and Windows. Instead of -lGL, use -framework OpenGL. E.g.
clang -O3 gl_sdl.c -lSDL2 -framework OpenGL
On a side note, the program requests a context for OpenGL 3.1, but does not use an OpenGL Loading Library (or loads any functions manually). As is, this happens to cause no problems, only because no functions from OpenGL >1.1 are used.