/*#include <unistd.h>int dup2(int oldfd, int newfd);作用：重定向文件描述符oldfd 指向 a.txt, newfd 指向b.txt,调用函数之后，newfd和b.txt  close，newfd指向a.txtoldfd必须是一个有效的文件描述符
*/
#include <unistd.h>
#include<stdio.h>
#include<sys/fcntl.h>
#include<sys/stat.h>
#include<fcntl.h>
#include<string.h>
int main() {int fd = open("1.txt", O_CREAT | O_RDWR, 0664);if(fd == -1) {perror("open");return -1;}int fd1 = open("2.txt", O_CREAT | O_RDWR, 0664);if(fd1 == -1) {perror("dup");return -1;}printf("fd : %d, fd1 : %d \n", fd, fd1);int fd2 = dup2(fd, fd1);if(fd2 == -1) {perror("dup2");return -1;}char * str = "hello,world";int len = write(fd1, str, strlen(str));if(len == -1) {perror("write");return -1;}printf("fd : %d, fd1 : %d, fd2 : %d \n", fd, fd1, fd2);close(fd);close(fd1);return 0;
}