clock() - execution time for c function

Question

I am trying to measure the execution time of a code block in C. I have something like this in my code:

clock_t begin, end;
double time_spent;
begin = clock();
ATL_dsymv(122,n,alfa,A,n,X,1,beta,Y,1);
end = clock();
time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
printf ("(%f seconds)",time_spent);

But it always returns: (0.000000 seconds). I tried the same thing on simpler code blocks like for's but it has the same result. What am I doing wrong? Thanks a lot.

Use the same code but execute the timed part several million times. — Kerrek SB, Apr 04 '13 at 22:38
About how long does the code take to run? What is the expected value, roughly? — David Schwartz, Apr 04 '13 at 22:38
the problem is that no matter what code block i have it only returns (0.000000 seconds) — biggdman, Apr 04 '13 at 22:40

score 4 · Accepted Answer · answered Apr 04 '13 at 22:56

clock usually has very poor resolution, on the order of 10 milliseconds. This is most likely your problem. If you're on a POSIX system, use clock_gettime with the CLOCK_PROCESS_CPUTIME_ID clock to get a high-resolution result. Other types of systems probably have system-specific ways to achieve the same.

clock() - execution time for c function

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