C and SDL in Ubuntu not working as intended - c

Question

I am following a tutorial on Youtube, but it does not work as intended when I try it.
Here is the code:
#include <stdio.h>
#include <SDL2/SDL.h>
void start(SDL_Window* window, SDL_Renderer* renderer)
{
SDL_Init(SDL_INIT_EVERYTHING);
window = SDL_CreateWindow("Hello", 200, 200, 200, 200, 0);
renderer = SDL_CreateRenderer(window, -1, SDL_RENDERER_ACCELERATED);
SDL_SetRenderDrawColor(renderer, 0, 0, 255, 255);
SDL_RenderClear(renderer);
SDL_SetRenderDrawColor(renderer, 255, 255, 255, 255);
SDL_Rect rect = {220, 140, 200, 200};
SDL_RenderFillRect(renderer, &rect);
}
void stop(SDL_Window* window, SDL_Renderer* renderer)
{
SDL_DestroyWindow(window);
SDL_DestroyRenderer(renderer);
}
int main(int argc, char **argv)
{
SDL_Window* window;
SDL_Renderer* renderer;
start(window, renderer);
SDL_Delay(5000);
stop(window, renderer);
return 0;
}
It is supposed to create a white rectangle in the middle of the window and fill the rest with blue color, but that does not happen when I run it. Instead, it "captures a picture" of the program under it, for example the background. Why is this happening, and how can I solve it?

Answer 1

You have two issues to fix. First, you need to modify your start() function to take pointers-to-pointers, rather than just pointers, so that the window and renderer variables in main() can be modified... otherwise, window and renderer within start() will be valid, but the corresponding pointer variables in main() will not be changed and will remain uninitialized. So...
/* Use 'SDL_Window**' and 'SDL_Renderer**' instead of 'SDL_Window*' and
* 'SDL_Renderer*', and dereference once during use (eg. '*window'
* instead of 'window' */
void start(SDL_Window** window, SDL_Renderer** renderer)
{
SDL_Init(SDL_INIT_EVERYTHING);
*window = SDL_CreateWindow("Hello", 200, 200, 200, 200, 0);
*renderer = SDL_CreateRenderer(*window, -1, SDL_RENDERER_ACCELERATED);
SDL_SetRenderDrawColor(*renderer, 0, 0, 255, 255);
SDL_RenderClear(*renderer);
SDL_SetRenderDrawColor(*renderer, 255, 255, 255, 255);
SDL_Rect rect = {220, 140, 200, 200};
SDL_RenderFillRect(*renderer, &rect);
}
Then in main() use:
start(&window, &renderer); /* pass addresses, not values */
The second issue is that you need to update the screen after finishing the drawing operations. You can do this with SDL_RenderPresent() when you're
done drawing, such as in main() right after calling your start() function:
...
start(&window, &renderer);
SDL_RenderPresent(renderer); /* add this */
SDL_Delay(5000);
...
You may also want to change the values in your SDL_Rect so that it's not drawn outside the window.