How does the C language calculate program run time?

The timing function in C/s + + is clock (), and the data type associated with it is clock_t. In MSDN, the check for the clock function is defined as follows:

clock_t clock (void);

This function returns the number of CPU clock ticks (clock ticks) from "Open this program process" to "call clock () function in program", called Wall Clock Time (Wal-clock) in MSDN. Where clock_t is the data type used to hold the time, in the Time.h file, we can find the definition of it:

#ifndef _clock_t_defined
typedef long CLOCK_T;
#define _clock_t_defined
#endif

It is clear that clock_t is a long shaping number. In the Time.h file, you also define a constant clocks_per_sec, which is used to indicate how many clock units are in a second, defined as follows:

#define CLOCKS_PER_SEC ((clock_t) 1000)

You can see that every 1 per thousand seconds (1 milliseconds), the value returned by the call to the clock () function increases by 1. As an example, you can use the formula clock ()/clocks_per_sec to calculate the run time of a process itself:

void Elapsed_time ()
{
printf ("Elapsed time:%u secs.\n", Clock ()/clocks_per_sec);
}

Of course, you can also use the clock function to calculate how long it takes for your machine to run a loop or handle other events:

#include "stdio.h"
#include "Stdlib.h"
#include "time.h"

int main (void)
{
Long i = 10000000L;
clock_t start, finish;
Double duration;
/* Measure the duration of an event */
printf ("Time to does%ld empty loops is", i);
start = Clock ();
while (i--);
finish = Clock ();
Duration = (double) (finish-start)/clocks_per_sec;
printf ("%f seconds\n", duration);
System ("pause");
}

On the author's machine, the results of the operation are as follows:

Time to does 10000000 empty loops is 0.03000 seconds

We can see that the clock timing unit is 1 milliseconds in length, and the accuracy of the timing is 1 milliseconds, so could we change the definition of the clocks_per_sec by making it larger, so that the timing is more accurate? By trying, you will find that this is not possible. In standard C/s + +, the smallest unit of timing is one millisecond.

Referenced from: http://blog.csdn.net/querdaizhi/article/details/6925156

How does the C language calculate program run time?