Function implementation of C ++ clock ()

In the C ++ programming language, there are many specific function applications that can help us implement many different specific functions and facilitate the actual development of programmers. For example, the C ++ clock () function mainly helps us implement the timing function. The related data type is clock_t. In MSDN, the clock function is defined as follows:

 
 

  
  
clock_t clock( void );  
 
 

This function returns the number of CPU clock timer units clock tick between "enabling this program process" and "calling the C ++ clock () function in the program, in MSDN, it is called the wall clock time wal-clock ). Clock_t is the data type used to save the time. In the time. h file, we can find its definition:

 
 

  
  
#ifndef _CLOCK_T_DEFINED   
  
  
typedef long clock_t;   
  
  
#define _CLOCK_T_DEFINED   
  
  
#endif  
 
 

Clock_t is a long integer. In the time. h file, a constant CLOCKS_PER_SEC is also defined to indicate the number of clock units in one second. Its definition is as follows:

 
 

  
  
#define CLOCKS_PER_SEC ((clock_t)1000) 
 
 

We can see that every 1‰ seconds, 1 millisecond), the value returned by calling the C ++ clock () function is increased by 1. For example, you can use the formula clock ()/CLOCKS_PER_SEC to calculate the running time of a process:

 
 

  
  
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 much time your machine spends running a loop or processing other events:

 
 

  
  
# Include "stdio. h"
  
  
# Include "stdlib. h"
  
  
# Include "time. h"
  
  
Int main (void)
  
  
{
  
  
Long I = 0000000l;
  
  
Clock_t start, finish;
  
  
Double duration;
  
  
/* Measure the duration of an event */
  
  
Printf ("Time to do % 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 running result is as follows:

 
 

  
  
Time to do 10000000 empty loops is 0.03000 seconds  
 
 

The preceding section describes the application of the C ++ clock () function. We can see that the length of the clock timing unit is 1 millisecond, and the timing accuracy is also 1 millisecond. Can we change the definition of CLOCKS_PER_SEC and define it as larger, so that the timing accuracy is higher? By trying, you will find that this is not the case. In standard C/C ++, the minimum unit of time is one millisecond.