# 标准库time.h函数clock()

**函数原型**

```c
clock_t clock(void);
```

**说明**

这个函数返回的是程序执行时的处理器时钟，其中 `clock_t` 其实是基本类型的别名，如果函数运行失败，则会返回-1值。在标准库time.h中有一个宏定义 `CLOCKS_PER_SEC` 这个就是CPU计算中定义的每秒的时钟脉冲频率，由硬件定义，当然在特定情况下可以重新定义，因为不同的CPU时钟脉冲频率都不同。

**实例**

```c
/* clock example: frequency of primes */
#include <stdio.h>      /* printf */
#include <time.h>       /* clock_t, clock, CLOCKS_PER_SEC */
#include <math.h>       /* sqrt */

int frequency_of_primes (int n) {
  int i,j;
  int freq=n-1;
  for (i=2; i<=n; ++i) for (j=sqrt(i);j>1;--j) if (i%j==0) {--freq; break;}
  return freq;
}

int main ()
{
  clock_t t;
  int f;
  t = clock();
  printf ("Calculating...\n");
  f = frequency_of_primes (99999);
  printf ("The number of primes lower than 100,000 is: %d\n",f);
  t = clock() - t;
  printf ("It took me %d clicks (%f seconds).\n",t,((float)t)/CLOCKS_PER_SEC);
  return 0;
}
```


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