Thread: Clock Troubles

I appear to be having trouble with clock(). I need to get precise time measurements (milliseconds would be nice, but I understand that that is not universally possible) and I would like to remain operating system unspecific. I decided to use clock(), but it has not been working as advertised.

The output of the following program is simply '0'. Yet when I increase the size of the for loop, the value becomes positive -- but is always an even number of seconds. Apparently, clock() is acting as if it has a resolution of seconds or worse. What do I do to make this work?

Code:

  1 #include <stdio.h>
  2 #include <time.h>
  3 
  4 int main(int argc, char** argv)
  5 {
  6     clock_t a = clock();
  7     int i; for(i = 0; i < 10000; i++);
  8     clock_t b = clock();
  9     printf("%f\n",(double)(b-a));
 10     return 0;
 11 }

Originally Posted by _Nate_

I appear to be having trouble with clock(). I need to get precise time measurements (milliseconds would be nice, but I understand that that is not universally possible) and I would like to remain operating system unspecific. I decided to use clock(), but it has not been working as advertised.

The output of the following program is simply '0'. Yet when I increase the size of the for loop, the value becomes positive -- but is always an even number of seconds. Apparently, clock() is acting as if it has a resolution of seconds or worse. What do I do to make this work?

Code:

  1 #include <stdio.h>
  2 #include <time.h>
  3 
  4 int main(int argc, char** argv)
  5 {
  6     clock_t a = clock();
  7     int i; for(i = 0; i < 10000; i++);
  8     clock_t b = clock();
  9     printf("%f\n",(double)(b-a));
 10     return 0;
 11 }

You need to use the macro CLOCKS_PER_SEC

Why does dividing by CLOCKS_PER_TICK help?

Also, I too thought that the compiler might be optimizing the loop out -- but as I said, you jack the size of the loop high enough, and the program prints something like '3000' or '4000' -- but any less than that, and it's right back to '0' -- no intermediary values.

Its CLOCKS_PER_SEC, not CLOCKS_PER_TICK.

http://www.acm.uiuc.edu/webmonkeys/b....15.html#clock

Originally Posted by _Nate_

but is always an even number of seconds. Apparently, clock() is acting as if it has a resolution of seconds or worse.

clock() measures the ticks that have passed since program start. The interval of the ticks however depends on the resolution of the operating system's timer.
I don't know what the frequency is on Windows, but I guess it'll be between 250 and 500Hz.
500Hz would mean that a tick occurs every 2ms, 250Hz means a tick every 4ms. So it'll probably be enough for measuring ms, but it will not be too acurately though

So what would be the best way going about getting millisecond precision?

Originally Posted by _Nate_

So what would be the best way going about getting millisecond precision?

Just do something like this:

Code:

	ticks = clock(); // gets the current ticks count
	semiprimes = semiprimegen(parameters); // this function executes
	ticks = clock() - ticks; // get the difference of ticks (results in the time the function ran)

	// now print the time in seconds with two decimal points
	printf("Took %.2f seconds to generate semiprimes.\n", ticks / (double)CLOCKS_PER_SEC);

Hope it helps.

Err, I don't think you can actually make your own function that sleeps. Not in simple way. You would need to tell the OS to suspend your program. So you would still need a function for that.

Why make your own function for that? I can only think of two things:
1) Don't what to sleep the program for X seconds, but inform when you want it to sleep and wake-up. In this case there should be other functions like sleep_wait_for_signal.

2) You want it to work with less CPU-power. In this case you are looking for something else. There was a thread about that.

Why do you want to make your own sleep() if I may ask?

nahh, I was just curious.

Thanks!