Why does this elapsed time (frametime) calculation lockup my game - c++

Question

currently I am trying to implement a fixed-step game loop but somehow my code seems to lockup my game.
Uint32 SDL_GetTicks(void) : Returns an unsigned 32-bit value representing the number of milliseconds since the SDL library initialized.
This works:
1.cc)
Uint32 FPS = 60;
Uint32 MS_PER_SEC = 1000 / FPS;
Uint32 current_time, last_time, elapsed_time;
current_time = last_time = elapsed_time = 0;
while(Platform.Poll())
{
current_time = SDL_GetTicks(); // Get time of frame begin
// Clear Window
Renderer.Clear();
// Update Input
//...
// Draw
Renderer.Draw();
// Update Window
Renderer.Update();
last_time = SDL_GetTicks(); // Get time of frame end
elapsed_time = last_time - current_time; // calculate frametime
SDL_Delay(MS_PER_SEC - elapsed_time);
}
However this does not:
2.cc)
Uint32 FPS = 60;
Uint32 MS_PER_SEC = 1000 / FPS;
Uint32 current_time, last_time, elapsed_time;
current_time = elapsed_time = 0;
last_time = SDL_GetTicks();
// Poll for Input
while(Platform.Poll())
{
current_time = SDL_GetTicks();
elapsed_time = current_time - last_time;
// Clear Window
Renderer.Clear();
// Update Input
//...
// Draw
Renderer.Draw();
// Update Window
Renderer.Update();
last_time = current_time;
SDL_Delay(MS_PER_SEC - elapsed_time);
}
I expect the results of 1.cc and 2.cc to be the same, meaning that SDL_Delay(MS_PER_SEC - elapsed_time) does delay by a fixed time minus frametime (here 16 - frametime).
But 2.cc does lockup my game.
Is not the elapsed_time (frametime) calculation from 2.cc equivalent to 1.cc ?

Answer 1

Let's unroll the loop a little...
// First iteration
last_time = SDL_GetTicks();
current_time = SDL_GetTicks();
elapsed_time = current_time - last_time; // Probably zero
...
SDL_Delay(MS_PER_SEC - elapsed_time); // Probably a whole second
// Second iteration
current_time = SDL_GetTicks();
elapsed_time = current_time - last_time; // Probably slightly more than a whole second
...
SDL_Delay(MS_PER_SEC - elapsed_time); // Probably negative (4 billion milliseconds)

Answer 2

That function is based on RTC and have very low accuracy and and uses much time to use, depending on platform. 10-30 milliseocnds accuracy is an optimistic guess.
This might be related: SDL_GetTicks() accuracy below the millisecond level