函数dup和dup2

#include<unistd.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<stdio.h>
#include<stdlib.h>

int main()
{
    int fd;
    int newfd;
    int flag;
    fd = open("./new.txt",O_RDWR|O_APPEND);     //向后追加      
    if(-1 == fd)
    {
        perror("open fail");
        exit(1);
    }

    newfd = dup(fd);        //复制文件描述符
    if(-1 == newfd)
    {
        perror("dup fail");
        exit(1);
    }

    flag = write(newfd,"I am append",11);       //向后面追加一句话
    if(-1 == flag)
    {
        perror("write fail");
        exit(1);
    }

    close(fd);      //dup复制的文件描述符由dup自己关闭
    return 0;
}