sdl2 + emscripten

index.c

// emcc -s USE_SDL=2 index.c -o index.html
// For images, see https://lyceum-allotments.github.io/2016/06/emscripten-and-sdl2-tutorial-part-4-look-owl/
// See also https://github.com/paked/sdl2-cmake-emscripten/blob/master/src/main.cpp

#include <emscripten.h>
#include <SDL2/SDL.h>
    
SDL_Surface *screen;
SDL_Window *window;
SDL_Renderer *renderer;
const int WINDOW_W = 640;
const int WINDOW_H = 480;
int mouseX = 0;
int mouseY = 0;
 
void render_loop() {
    SDL_SetRenderDrawColor(renderer, 100, 150, 100, 200);

    for (SDL_Event event; SDL_PollEvent(&event);) {
        if (event.type == SDL_MOUSEBUTTONDOWN) {
                //|| event.type == SDL_MOUSEBUTTONUP) {
            printf("%d %d\n", mouseX, mouseY);
            SDL_FillRect(screen, NULL, 0);
        }
        else if (event.type == SDL_MOUSEMOTION) {
            SDL_GetMouseState(&mouseX, &mouseY);
            SDL_RenderDrawLine(renderer, 0, 0, mouseX, mouseY);
        }
    }

    SDL_RenderPresent(renderer);
}
 
int main(int argc, char **argv) {
    if (SDL_Init(SDL_INIT_VIDEO) < 0) {
        fprintf(stderr, "SDL_Init failed: %s\n", SDL_GetError());
        return 1;
    }

    SDL_CreateWindowAndRenderer(WINDOW_W, WINDOW_H, 0, &window, &renderer);
    emscripten_set_main_loop(render_loop, 0, 0);
    return 0;
}