How can I make my own timer without standard library?

问题

Is there some specific number of iterations, that I could make using a for loop, so that it exactly takes 1 second for the loop to be executed completely? For example the following code took 0.125s on my machine to execute:

#include <iostream>
#include <cmath>

using namespace  std;

int main(){
    long long a=0;

    for (a=0;a<=pow(10,4);a++);
}

Though, a <= 8*pow(10,4) took 0.206 s. Compiler is GCC 4.9.2. IDE is codeblocks.

My PC's Specs: OS: Windows 8.1

回答1:

I am posting this answer to your question, as per the comments received.

It is not possible to make a timer because:

• The time that an iteration will take is unpredictable, this depends not only on the CPU used, but you need to take into account power management, the scheduler. (By tux3)

• one would have to use a real time OS to accomplish that. There's too much jitter in non realtime OSs. Windows could decide to schedule other processes for a while, or use the CPU for e.g. kernel networking, disk I/O etc. that preempts the timing. (By nos)

• One can't "make own timer" in a hosted environment just in standard C++. A timer is essentially a mechanism to communicate with the OS scheduler, and one needs platform-specific OS services for that. (By Kerrek SB)

• The compiler would optimize such a loop and will remove it through dead-code elimination (By πάντα ῥεῖ and Jongware).

来源：https://stackoverflow.com/questions/30828774/how-can-i-make-my-own-timer-without-standard-library