count execution time in ubuntu(nanoseconds)

hi i asked few days back if it was possible to count the execution time using clock, well i got replies telling that its not possible

my code runs on linux, are there any specific functions that can be used only in linux in order count the execution time in nanoseconds? and then get the seconds?

for example if we have a running time in nanoseconds

33939

i want 33939*10^-9 = 0.000033939 seconds

i ve researched a little bit and i found that the clock_gettime can do the job

but i havent found any source code on how to do this if you could help me with a simple sample code or suggest me something else it would be appreciated


i have found another one for gettimeofday

Code:

---

#include <iostream>
#include <sys/time.h>
using namespace std;

int main(void)
{
timeval tim;
            gettimeofday(&tim, NULL);
            double t1=tim.tv_sec+(tim.tv_usec/1000000.0);

                    int i,j=0;
                    for(i=0;i<1000;i++){
                            j=j+1;
                  }
            gettimeofday(&tim, NULL);
            double t2=tim.tv_sec+(tim.tv_usec/1000000.0);
            cout<<t2-t1<<" seconds elapsed\n";


}

---

gives output

5.00679e-06 seconds elapsed


is this correct?? but it is not in nanoseconds

Quote:

---

Originally Posted by nik

i want 33939*10^-9 = 0.000033939 seconds

---

Quote:

---

Originally Posted by nik

5.00679e-06 seconds elapsed
is this correct?? but it is not in nanoseconds

---

I'm confused. Do you want the answer in seconds or nanoseconds? If it's in seconds and you want nanoseconds, just multiply by 1,000,000,000. I don't see what there is to be confused about.

If you're worried about execution time down to nanoseconds, you've got bigger problems!

I am guessing that timeval.tv_usec actually gives usec known as microseconds.
So to go to nanoseconds is impossible.

As a note, you shouldn't divide like that. You should save into two separate timevals, subtract the tv_sec and tv_usec respectively. Not of carriers on the tv_usec, so you should make a function to deal with this (probably already available).