Thread: Problem with simple timer function

I shuffled the operations around to try and get my function working and I'm really confused by my output. Heres the code:

Code:

#include "SDL/SDL.h"
#include <stdio.h>


void FPS_Cap(unsigned cap)
{ 
	static unsigned this_frame;
	static unsigned last_frame;
	static unsigned time_taken;
	
	cap = 1000 / cap;	
	time_taken = this_frame - last_frame; 
	
	printf("Last: %u This: %u Time: %u ", last_frame, this_frame, time_taken);
	if(time_taken >= cap) 
	{
		last_frame = this_frame;
		this_frame = SDL_GetTicks();
		printf("Delay: 0  ");
		return;
	}

	SDL_Delay(cap-time_taken);	
	printf("Delay: %u   ", cap-time_taken);

	last_frame = this_frame;
	this_frame = SDL_GetTicks(); 			
}


int main()
{
	unsigned then, now;
	int i;
	int a, b;
	
	printf("Using Frame Cap Function\n");
	for(i=0; i<10; i++)
	{
		then = SDL_GetTicks();

		for(a=0; a<1000; a++)
		for(b=0; b<1000; b++)
			;
		
		FPS_Cap(60);

		now = SDL_GetTicks() - then;
		printf("Total Time: %u\n", now);
	}

	return 0;
}

And heres my output:

Code:

Using Frame Cap Function
Last: 0 This: 0 Time: 0 Delay: 16   Total Time: 542
Last: 0 This: 2048506560 Time: 2048506560 Delay: 0  Total Time: 4
Last: 2048506560 This: 2048506564 Time: 4 Delay: 12   Total Time: 15
Last: 2048506564 This: 2048506579 Time: 15 Delay: 1   Total Time: 5
Last: 2048506579 This: 2048506584 Time: 5 Delay: 11   Total Time: 24
Last: 2048506584 This: 2048506608 Time: 24 Delay: 0  Total Time: 4
Last: 2048506608 This: 2048506612 Time: 4 Delay: 12   Total Time: 24
Last: 2048506612 This: 2048506636 Time: 24 Delay: 0  Total Time: 3
Last: 2048506636 This: 2048506639 Time: 3 Delay: 13   Total Time: 17
Last: 2048506639 This: 2048506656 Time: 17 Delay: 0  Total Time: 4

The for the first 2 loops the calculations are going to be wrong, but then it seems to be calculating the delay correctly. The total time I am getting in my main function seems totally random. Can anyone explain it?

This is annoying. I should have given up on it hours ago

Does this work? Test both with and without the random delay.

Code:

#include "SDL/SDL.h"
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define TESTCAP 60
#define DORANDOM 0 /* 1=>call rnd_delay(), 0=>don't */

void FPS_Cap(unsigned cap)
{
    static unsigned then;
    unsigned diff;

    cap = 1000 / cap; /* Convert fps to ms (truncated) */

    diff = then - SDL_GetTicks();

    if (diff < cap) SDL_Delay(cap - diff);

    then = SDL_GetTicks(); 
}

void rnd_delay()
{
    SDL_Delay(rand() % (1000 / TESTCAP));
}

int main()
{
    unsigned before, i;

    srand(time(0));

    for (i = 0; i < 10; i++)
    {
        before = SDL_GetTicks(); /* To time body of loop */

#if DORANDOM
        rnd_delay();
#endif
        FPS_Cap(TESTCAP);

        printf("%u\n", SDL_GetTicks() - before); /* How long did it take? */
    }
}