Makefile

CC      ?= zig cc
CFLAGS  ?= -std=c11 -Wall -Wextra `sdl2-config --cflags`
LDFLAGS ?= `sdl2-config --libs`

TARGET = sdl-minimal

all: $(TARGET)

$(TARGET): sdl-minimal.c
	$(CC) $(CFLAGS) $< -o $@ $(LDFLAGS)

clean:
	rm -f $(TARGET)

sdl-minimal.c

#include <SDL2/SDL.h>
#include <SDL2/SDL_hints.h>
#include <SDL2/SDL_video.h>
#include <stdbool.h>

const int WIDTH = 96 * 3;
const int HEIGHT = 54 * 3;
const int radius = 20;
const int speed = 6;

int x = WIDTH / 2;
int y = HEIGHT / 2;

bool running = true;

SDL_Event e;

static void draw_circle(SDL_Renderer *r, int cx, int cy, int radius) {
  for (int y = -radius; y <= radius; y++) {
    for (int x = -radius; x <= radius; x++) {
      if (x * x + y * y <= radius * radius) {
        SDL_RenderDrawPoint(r, cx + x, cy + y);
      }
    }
  }
}

static void draw(SDL_Renderer *r) {
  int cx = x;
  int cy = y;

  SDL_SetRenderDrawColor(r, 0, 0, 0, 255);
  SDL_RenderClear(r);

  SDL_SetRenderDrawColor(r, 255, 100, 0, 255);

  SDL_RenderDrawLine(r, cx, cy, 0, 0);
  SDL_RenderDrawLine(r, cx, cy, 0, HEIGHT);
  SDL_RenderDrawLine(r, cx, cy, WIDTH, 0);
  SDL_RenderDrawLine(r, cx, cy, WIDTH, HEIGHT);

  SDL_SetRenderDrawColor(r, 255, 255, 255, 255);

  draw_circle(r, cx, cy, radius);
}

void update() {
  while (SDL_PollEvent(&e)) {
    if (e.type == SDL_QUIT) {
      running = false;
    } else if (e.type == SDL_KEYDOWN) {
      if (e.key.keysym.sym == SDLK_z || e.key.keysym.sym == SDLK_ESCAPE) {
        running = false;
      }
    }
  }

  const Uint8 *keys = SDL_GetKeyboardState(NULL);

  if (keys[SDL_SCANCODE_LEFT])
    x -= speed;
  if (keys[SDL_SCANCODE_RIGHT])
    x += speed;
  if (keys[SDL_SCANCODE_UP])
    y -= speed;
  if (keys[SDL_SCANCODE_DOWN])
    y += speed;
}

int main(void) {
  SDL_Init(SDL_INIT_VIDEO);

  SDL_Window *window = SDL_CreateWindow("SDL minimal", SDL_WINDOWPOS_CENTERED,
                                        SDL_WINDOWPOS_CENTERED, WIDTH, HEIGHT,
                                        SDL_WINDOW_FULLSCREEN);

  SDL_Renderer *renderer =
      SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);

  SDL_RenderSetLogicalSize(renderer, WIDTH, HEIGHT);
  SDL_SetHint(SDL_HINT_RENDER_SCALE_QUALITY, "nearest");

  while (running) {
    update();
    draw(renderer);

    SDL_RenderPresent(renderer);
    SDL_Delay(16);
  }

  SDL_DestroyRenderer(renderer);
  SDL_DestroyWindow(window);
  SDL_Quit();

  return 0;
}