C - measuring computing time

Question

is there any simple way how to measure computing time in C? I tried time utility when executed, but I need to measure specific part of a program.

Thanks

Solution 1

You can use the clock function in <time.h> along with the macro CLOCKS_PER_SEC:

clock_t start = clock() ;
do_some_work() ;
clock_t end = clock() ;
double elapsed_time = (end-start)/(double)CLOCKS_PER_SEC ;

Now elapsed_time holds the time it took to call do_some_work, in fractional seconds.

Solution 2

You can try the profiler "gprof". More information here: http://www.cs.utah.edu/dept/old/texinfo/as/gprof.html

Solution 3

You can generally use the clock() function to get the start and end times of a single call to your function being tested. If, however, do_some_work() is particularly fast, it needs to be put in a loop and have the cost of the loop itself factored out, something like:

#define COUNT 10000

// Get cost of naked loop.

clock_t start_base = clock();
for (int i = count; i > 0; i--)
    ;
clock_t end_base = clock();

// Get cost of loop plus work.

clock_t start = clock();
for (int i = count; i > 0; i--)
    do_some_work() ;
clock_t end = clock();

// Calculate cost of single call.

double elapsed_time = end - start - (end_base - start_base);
elapsed_time = elapsed_time / CLOCKS_PER_SEC / COUNT;

This has at least two advantages:

• you'll get an average time which is more representative of the actual time it should take; and
• you'll get a more accurate answer in the case where the clock() function has a limited resolution.

Comments

[Kiran Kumar]

You mean you want to profile your code? If so, take a look at this question: stackoverflow.com/questions/1794816/…

[Jonathan Leffler]

The only problem here is that CLOCKS_PER_SEC, but the value is typically 60 or 100, so the timing is not very precise. You can do quite a lot in 10 or 17 ms if your processor has multiple cores and runs at 3 GHz.

[Rune Aamodt]

That's true, but I'm pretty sure this is the best you can get with pure ANSI C. On Windows, you can use timeGetTime or even QueryPerformanceCounter to do better.

[wallyk]

If do_some_work() is very quick, it might be necessary to put it inside a loop which repeats it many times, like 10,000,000 and divide the elapsed time by the same number to calculate the time for one unit of work.

[Mike Dunlavey]

He can try it, but he might not like it. See here.

[BiGYaN]

@Mike, I never knew about the recursion issues. Thanks for pointing this out.