dup2函数

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>

ssize_t readn(int fd, void * ptr, size_t n)
{
        size_t nleft;
        ssize_t nread;

        nleft = n;
        while(nleft > 0)
        {
                if((nread = read(fd, ptr, nleft)) < 0)
                {
                        if(nleft == n)
                                return -1;
                        else
                                break;
                }
                else if(nread == 0)
                {
                        break;
                }
                nleft -= nread;
                ptr += nread;
        }
        return(n - nleft);
}

int main(int argc, char * * argv)
{
        printf("**************************************************\n");
        int fd2;
        fd2 = open("dup2.2.txt", O_CREAT|O_RDWR, 0777);
        printf("fd2 = [%d]\n", fd2);
        if(fd2 < 0)
        {
                printf("open err!\n");
                exit(0);
        }

        write(fd2, "wangxiancai\n", strlen("wangxiancai\n"));
        lseek(fd2, SEEK_SET, 0); /*如果open是追加打开的，或者，write过了，这里必须用lseek*/
        dup2(fd2, STDIN_FILENO);

        char buf[128];
        memset(buf, 0, sizeof(buf));
        char ch;

        //scanf("%c", &ch);
        int ret = -1;
        ret = read(STDIN_FILENO, buf, 5);
        //ret = read(fd2, buf, 4);
        //ret = readn(fd2, buf, 4);
        printf("ret = [%d]\n", ret);
        printf("buf = [%s]\n", buf);
        //printf("ch = [%c]\n", ch);

        printf("**************************************************\n");
        int fd;
        fd = open("dup2.txt", O_CREAT|O_APPEND|O_RDWR, 0777);
        if(fd < 0)
        {
                printf("open err!\n");
                exit(0);
        }
        dup2(fd, STDOUT_FILENO);
        /*标准输出关闭，之后任何写到标准输出的东西，现在都写到“dup2.txt”文件中去*/
        printf("wangxiancai\n"); //这句，原本想写到标准输出的语句，写到“dup2.txt”文件中去了。



        return 0;
}