程序计时:clock

    有时我们需要评价一个算法性能，那么算法的执行时间便是一个不可忽视的因素，而由于某种特殊的需求，我们需要算法的执行时间尽可能地精确，linux下就提供了这样的一个函数：clock_gettime。该函数提供的可是提供了纳秒的精确度。

函数的原型如下：

int clock_gettime(clockid_t clk_id, struct timespect *tp);

clockid_t clk_id用于指定计时时钟的类型，对于我们Programmr以下三种比较常用：

CLOCK_REALTIME, a system-wide realtime clock.

CLOCK_PROCESS_CPUTIME_ID, high-resolution timer provided by the CPU for each process.

CLOCK_THREAD_CPUTIME_ID, high-resolution timer provided by the CPU for each of the threads.

CLOCK_REALTIME, a system-wide realtime clock.

CLOCK_PROCESS_CPUTIME_ID, high-resolution timer provided by the CPU for each process.

CLOCK_THREAD_CPUTIME_ID, high-resolution timer provided by the CPU for each of the threads.

struct timespect *tp用来存储当前的时间，其结构如下：

1 struct timespec {
2     time_t tv_sec; /* seconds */
3     long tv_nsec; /* nanoseconds */
4 };

测试代码：

#include <stdio.h>
#include <time.h>

struct timespec diff(struct timespec start,struct timespec end)
{
    struct timespec temp;
    if((end.tv_nsec-start.tv_nsec)<0)
    {
        temp.tv_sec = end.tv_sec-start.tv_sec-1;
        temp.tv_nsec = 1000000000 + end.tv_nsec-start.tv_nsec;
    }
    else
    {
        temp.tv_sec = end.tv_sec-start.tv_sec;
        temp.tv_nsec = end.tv_nsec-start.tv_nsec;
    }
    return temp;
}

int main()
{
    struct timespec time1,time2,temp;
    int i;
    clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time1);
    for(i=0; i<10000;i++)
    {
        ;
    }
    clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time2);
    temp = diff(time1, time2);
    printf("%d:%d\n", temp.tv_sec, temp.tv_nsec);
    return 0;    
}


这里需要注意一点的就是，编译时需要链接lrt库，否则会提示找不到clock_gettime函数，
[root@localhost ~]# gcc timetest.c -o timetest -lrt