emscripten draw pixels and scale by SDL_RenderCopy

build.bat

emcc test_pixel2.c ^
 -O2 ^
 -Wall -Wextra -pedantic ^
 -s USE_SDL=2 ^
 -s USE_SDL_IMAGE=2 -s SDL2_IMAGE_FORMATS="[""png""]" ^
 -s USE_SDL_TTF=2 ^
 --preload-file assets ^
 -o test_pixel2.html

test_pixel2.c

#include <stdbool.h>
#include <stdint.h>

#include <SDL2/SDL.h>
#ifdef __EMSCRIPTEN__
#include <emscripten.h>
#endif

typedef struct {
  SDL_Window *window;
  SDL_Renderer *renderer;
} Context;

void Cleanup(Context *ctx) {
  if (ctx) {
    if (ctx->renderer)
      SDL_DestroyRenderer(ctx->renderer);
    if (ctx->window)
      SDL_DestroyWindow(ctx->window);
  }
}

uint32_t Init(Context *ctx) {
  if (SDL_Init(SDL_INIT_VIDEO) != 0) {
    fprintf(stderr, "Error: %s", SDL_GetError());
    return -1;
  }
  ctx->window = SDL_CreateWindow("sandbox2",
    SDL_WINDOWPOS_UNDEFINED,
    SDL_WINDOWPOS_UNDEFINED,
    640,480,
    SDL_WINDOW_SHOWN);
  if (ctx->window == NULL) {
    fprintf(stderr, "Error: %s", SDL_GetError());
    return -1;
  }

  ctx->renderer = SDL_CreateRenderer(ctx->window,
    -1, SDL_RENDERER_ACCELERATED | SDL_RENDERER_PRESENTVSYNC);
  if (ctx->renderer == NULL) {
    fprintf(stderr, "Error: %s", SDL_GetError());
    return -1;
  }

  SDL_SetRenderDrawColor(ctx->renderer, 255 ,200 ,200, 255);

  /*
   * Init Display
   */ 
  return 0;
}

SDL_Surface *screen;

void Loop(void *arg) {
  Context *ctx = (Context *)arg;

  //
  // Manipulate
  //
  // if arg2 is NULL, draw all surface.
  SDL_FillRect(screen, NULL, SDL_MapRGBA(screen->format, 0xcc, 0xcc, 0xcc, 0xff));
  SDL_LockSurface(screen);
  // ARGB
  *((int*)screen->pixels + 0) = 0xFFFF0000;
  *((int*)screen->pixels + 1) = 0xFF00FF00;
  *((int*)screen->pixels + 2) = 0xFF0000FF;
  *((int*)screen->pixels + 3) = 0xFFFFFFFF;
  *((int*)screen->pixels + 4) = 0x00000000;

  *((int*)screen->pixels + 95)= 0xFF000000; 
  SDL_UnlockSurface(screen);

  //
  // Draw
  //
  SDL_Texture *tex = SDL_CreateTextureFromSurface(ctx->renderer, screen);
  SDL_RenderClear(ctx->renderer);

  SDL_RenderCopy(ctx->renderer, tex, NULL, &(SDL_Rect){0, 0, 640*2, 480*2});

  SDL_RenderPresent(ctx->renderer);

  SDL_DestroyTexture(tex);
}

int main() {
  Context ctx = {0,0};
  Init(&ctx);
  screen = SDL_CreateRGBSurface(0, 64, 32, 32, 0, 0, 0, 0);
  printf("screen format:%d\n", screen->format->format);

  Loop(&ctx);
  emscripten_set_main_loop_arg(Loop, &ctx,-1, 1/*block*/);
  printf("don't run this message by block.");

  return 0;
}