n900 - maemo - timing

Question

I am attempting to save a file every second within +- 100ms (10% error). The problem I am having is that my timing measurement is saying that execution took 1150 ms, but in reality it appears to be 3 or 4 seconds.

What's going on?

If I issue the command, sleep(1), it appears to be very accurate. However, when I measure how long something took, it must be off by quite a bit.

I am using clock() to measure program execution. All of this stuff is within a while loop.

Walter

Answer 1

Your problem is that clock() reports you CPU time used by your process and it is usually different from the "real" time used.

For example following code:

#include <time.h>
#include <iostream>
#include <unistd.h>

using namespace std;

int main()
{
        clock_t scl = clock();
        sleep(1);
        cout << "CPU clock time " << clock()-scl << endl;
}

gives

time ./a.out 
CPU clock time 0

real    0m1.005s
user    0m0.000s
sys 0m0.004s