How to measure the ACTUAL execution time of a C program under Linux?

Question

I know this question may have been commonly asked before, but it seems most of those questions are regarding the elapsed time (based on wall clock) of a piece of code. The elapsed time of a piece of code is unlikely equal to the actual execution time, as other processes may be executing during the elapsed time of the code of interest.

I used getrusage() to get the user time and system time of a process, and then calculate the actual execution time by (user time + system time). I am running my program on Ubuntu. Here are my questions:

1. How do I know the precision of getrusage()?
2. Are there other approaches that can provide higher precision than getrusage()?

Answer 1

You can check the real CPU time of a process on linux by utilizing the CPU Time functionality of the kernel:

 #include <time.h>   clock_t start, end;  double cpu_time_used;   start = clock();  ... /* Do the work. */  end = clock();  cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC; 

Source: http://www.gnu.org/s/hello/manual/libc/CPU-Time.html#CPU-Time

That way, you count the CPU ticks or the real amount of instructions worked upon by the CPU on the process, thus getting the real amount of work time.