struct timespec start,end;
clock_gettime (CLOCK_REALTIME, &start);
<add code here>
clock_gettime (CLOCK_REALTIME, &end);
float time= (end.tv_sec - start.tv_sec) + (end.tv_nsec - start.tv_nsec) / 1000000000;
This code snippet would give you the time taken to execute the added code in real time.
Dont forget to compile your code with a -lrt flag. Not including this flag could give you a ld error for not finding clock_gettime function.