How do I separate window creation and a draw function in C using SDL2?

Question

I am trying to separate drawing (drawing.c) to the screen from the creation of a window and renderer (initialize.c) . I want them both in different source-files. The drawing code runs and is executed, but the graphics are not updated.

If I put the code from the drawing file inside the create window and rendering file, it works. The color in the window is now red.

main.c - The main file

SDL_Window *window;
SDL_Renderer *renderer;

int main()
{
    initialize(window, renderer);
    draw(renderer);

    SDL_Dealy(5000);
    SDL_Quit();
}

initialize.c - Create window and renderer

void initialize(SDL_Window *window, SDL_Renderer *renderer)
{
    SDL_Init(SDL_INIT_VIDEO);
    window = SDL_CreateWindow("Title", 
        SDL_WINDOWPOS_UNDEFINED, 
        SDL_WINDOWPOS_UNDEFINED, 
        640, 480, 0);
    renderer = SDL_CreateRenderer(window, -1, 0);
}

draw.c - Draw graphics to window

void draw(SDL_Renderer *renderer)
{
    SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255);
    SDL_RenderClear(renderer);
    SDL_RenderPresent(renderer);
}

When I run the code the window display is black. It is supposed to be filled with red. I get no error messages when I run the code. The new color graphics are never rendered visible.

Answer 1

I can't see how it could have worked in single file unless actual code is different from presented in question. Look at your initialise function:

void initialize(SDL_Window *window, SDL_Renderer *renderer)
{
    SDL_Init(SDL_INIT_VIDEO);
    window = SDL_CreateWindow("Title", 
        SDL_WINDOWPOS_UNDEFINED, 
        SDL_WINDOWPOS_UNDEFINED, 
        640, 480, 0);
    renderer = SDL_CreateRenderer(window, -1, 0);
}

When you call it from main as initialize(window, renderer); , it gets values of global window and renderer . Then it assigns new window/renderer to its local variables, which gets lost when function returns - no global variable is modified after this. Then you call draw(renderer); with the same global not-assigned renderer (NULL) - no way it would work as you expect.

Basically function can't modify anything passed as its arguments because it gets copy of passed values. Correct version would look like this:

void initialize(SDL_Window **window, SDL_Renderer **renderer)
{
    SDL_Init(SDL_INIT_WINDOW);
    *window = SDL_CreateWindow("Title", SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED, 640, 480, 0);
    *renderer = SDL_CreateRenderer(window, -1, 0);
}

// ...
// in file with main func
SDL_Window *window;
SDL_Renderer *renderer;
void initialize(SDL_Window **window, SDL_Renderer **renderer);
void draw(SDL_Renderer *renderer);

int main()
{
    // ...
    initialize(&window, &renderer);
    draw(renderer);
    // ...
}

This is the same way how SDL_CreateWindowAndRenderer operates.

Another thing is that you can't expect draw-and-delay to be correctly displayed; it may work on your configuration of OS/window manager/settings/luck but not others. Refresh loop is essential.