elapsed time in C

#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开发者_如何学编程. 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.

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.

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.

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)

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.

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.

Your code is taking less than a second to run.