C++ counting execution time eror

Question

Hi, I'm trying to count the execution of function in my code (that takes over an hour), and I'm using clock(), but I'm getting some errors, since the time calculated is negative. I'm doing like this:

long double time;
clock_t start, t_end;

t_start = clock();
algorithm->execute();
t_end = clock();

time = ((long double) t_end - t_start) / CLOCKS_PER_SEC;

cout << time << endl;

I'm I doing something wrong? Thanks in advance.

Answer 1

Have you checked to make sure neither call to clock() is returning -1?

The clock() function returns the processor time since the program started, or - 1 if that information is unavailable.

Another approach of doing this is:

#include <time.h>

time_t start, end;
time(&start);
algorithm->execute();
time(&end);
double diff = difftime(end, start);

Answer 2

I'm not 100% sure, but do you cast only t_end to a long double ???

Shouldn't it be:

((long double)t_end - (long double)t_start)/CLOCKS_PER_SEC

or

((long double)(t_end - t_start))/CLOCKS_PER_SEC

???

Answer 3

CLOCKS_PER_SEC is 1000000 and clock() returns a signed 32-bit value so will go negative after about 36 minutes and wrap after about 72 minutes.

See http://www.guyrutenberg.com/2007/09/22/profiling-code-using-clock_gettime for details on how to measure long execution times.