tests/sdl2_gl_frames_swap.c

#include <SDL.h>
#include <emscripten.h>

SDL_Window* window;
SDL_Renderer* renderer;

void frame()
{
    static int x = 0;
    static int y = 0;
    SDL_Rect rect;
    rect.x = x;
    rect.y = y;
    rect.w = 100;
    rect.h = 100;

    SDL_SetRenderDrawColor(renderer, 0, 0, 0, 255);
    SDL_RenderClear(renderer);
    SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255);
    SDL_RenderFillRect(renderer, &rect);
    SDL_RenderPresent(renderer);
}

int main()
{
    SDL_Init(SDL_INIT_VIDEO);
    window = SDL_CreateWindow("test", 0, 0, 640, 480, SDL_WINDOW_OPENGL);
    renderer = SDL_CreateRenderer(window, 0, SDL_RENDERER_ACCELERATED);
    SDL_GL_SetSwapInterval(1);
    emscripten_set_main_loop(frame, -1, 1);
}