#include <stdio.h>   
#include <string.h>  
#include <unistd.h>  
#include <fcntl.h>  
#include <errno.h>   
#include <termios.h>
#include <stdlib.h>
int fd;/* File descriptor for the port */
int open_port(void)
  {
  fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY | O_NDELAY);
  if (fd == -1)
    {
   /*
    * Could not open the port.
    */

    perror("open_port: Unable to open /dev/ttyS0 - ");
     }
  else
    fcntl(fd, F_SETFL, 0);
  return (fd);
   }
main()
{
   int n;
   int open_port(void);
   /*write data to the serial*/
    n = write(fd, "abcdefgh\n", 12);
    if (n < 0)
  fputs("write serial port failed!\n", stderr);
    else
  printf("write serial port is:\n");
    /*read data for the serail*/
  fcntl(fd, F_SETFL, FNDELAY);
   close(fd);
}

以上程序执行完后都是得到
abcdefgh
wrwrite serial port is:
而且不管串口有没有连接开发板；
不知道是哪里的问题，求教了；