elapsed time in C

Question

#include <time.h> 
time_t start,end; 
time (&start);
//code here
time (&end); 
double dif = difftime (end,start); 
printf ("Elasped time is %.2lf seconds.", dif );

I'm getting 0.000 for both start and end times. I'm not understanding the source of error.

Also is it better to use time(start) and time(end) or start=clock() and end=clock() for computing the elapsed time.

This works fine for me. Have you verified the values of start and end with a debugger? (Note: to test it I put a "sleep(1)" between the time(start) and time(end).) — Tom, May 30 '11 at 18:07
If you need time measurements at finer resolution, have you tried `clock()`? — sverre, May 30 '11 at 18:08
@sverre: `clock()` does not measure the absolute passage of time. It measures the quantity of time spent specifically on this one process. Important distinction. If you're profiling, you may want this; otherwise you probably don't. — Lightness Races in Orbit, May 30 '11 at 19:06

score 7 · Answer 1 · answered May 30 '11 at 18:05

7

On most (practically all?) systems, time() only has a granularity of one second, so any sub-second lengths of time can't be measured with it. If you're on Unix, try using gettimeofday instead.

answered May 30 '11 at 18:05

jwodder

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score 4 · Answer 2 · answered May 30 '11 at 18:14

4

If you do want to use clock() make sure you understand that it measures CPU time only. Also, to convert to seconds, you need to divide by CLOCKS_PER_SEC.

answered May 30 '11 at 18:14

mhyfritz

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score 2 · Answer 3 · answered May 30 '11 at 18:17

Short excerpts of code typically don't take long enough to run for profiling purposes. A common technique is to repeat the call many many (millions) times and then divide the resultant time delta with the iteration count. Pseudo-code:

count = 10,000,000

start = readCurrentTime()

loop count times:
    myCode()

end = readCurrentTime()

elapsedTotal = end - start
elapsedForOneIteration = elapsedTotal / count

If you want accuracy, you can discount the loop overhead. For example:

loop count times:
    myCode()
    myCode()
and measure elapsed1 (2 x count iterations + loop overhead)

loop count times:
    myCode()
and measure elapsed2 (count iterations + loop overhead)

actualElapsed = elapsed1 - elapsed2
(count iterations -- because rest of terms cancel out)

score 0 · Answer 4 · edited May 23 '17 at 12:32

0

time has (at best) second resolution. If your code runs in much less than that, you aren't likely to see a difference.

Use a profiler (such a gprof on *nix, Instruments on OS X; for Windows, see "Profiling C code on Windows when using Eclipse") to time your code.

edited May 23 '17 at 12:32

Community

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answered May 30 '11 at 18:08

outis

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DallaRosa · Answer 5 · 2011-05-30T18:24:10.453

0

The code you're using between the measurements is running too fast. Just tried your code printing numbers from 0 till 99,999 and I got

Elasped time is 1.00 seconds.

edited May 30 '11 at 18:24

answered May 30 '11 at 18:12

DallaRosa

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score 0 · Answer 6 · answered May 30 '11 at 19:06

0

Your code is taking less than a second to run.

answered May 30 '11 at 19:06

Lightness Races in Orbit

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elapsed time in C

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