I/O重定向函数dup2

从标准输入拷贝一个文本文件到标准输出，使用dup2函数重定向到把一个磁盘文本文件拷贝到另一个磁盘文件的结尾处。

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

#define MAXBUF 20
#define DEF_MODE  S_IRUSR|S_IWUSR|S_IRGRP|S_IWGRP|S_IROTH|S_IWOTH
#define DEF_UMASK S_IWGRP|S_IWOTH

ssize_t rio_readn(int fd, void *usrbuf, size_t n);
ssize_t rio_writen(int fd, void *usrbuf, size_t n);

int main(int argc, char **argv)
{
    int n,fd1,fd2;
    char buf[MAXBUF];

    fd1=open("hello.txt",O_RDONLY,0);
    umask(DEF_UMASK);
    fd2 = open("bar.txt", O_WRONLY|O_APPEND|O_CREAT, DEF_MODE);
    dup2(fd1,0);
    close(fd1);
    dup2(fd2,1);
    close(fd2);
    while((n=rio_readn(STDIN_FILENO,buf,MAXBUF))!= 0)
       rio_writen(STDOUT_FILENO,buf,n);
    exit(0);   
}

ssize_t rio_readn(int fd, void *usrbuf, size_t n)
{
    size_t nleft = n;
    ssize_t nread;
    char *bufp = usrbuf;

    while (nleft > 0) {
        if ((nread = read(fd, bufp, nleft)) < 0) {
            if (errno == EINTR)    /* Interrupted by sig handler return */
                nread = 0;        /* and call read() again */
            else
                return -1;       /* errno set by read() */
        }
        else if (nread == 0)
            break;               /* EOF */
        nleft -= nread;
        bufp += nread;
    }
    return (n - nleft);         /* Return >= 0 */
}

ssize_t rio_writen(int fd, void *usrbuf, size_t n)
{

  size_t nleft = n;
  ssize_t nwritten;  
  char *bufp = usrbuf;
  while (nleft > 0){
    if ((nwritten = write(fd, bufp, nleft)) <= 0){
      if (errno == EINTR)  /* Interrupted by sig handler return */
        nwritten = 0;      /* and call write() again */
      else
        return  -1;        /* errno set by write() */
    }
    nleft -= nwritten;
    bufp += nwritten;
  }
  return n;
}