#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
#include <sys/ioctl.h>
#include <termios.h>

#define dev_uart "/dev/ttySAC3"
#define test "testwrite.txt"

int main(int argc , char *argv[])
{
  int fh, i, fd, cnt = 0, status;
  char c, bfr[1];
  struct termios original_t, new_t;
  
  printf("I/O test for UART driver \n");
  printf("Press Ctrl C to exit\n");
  fh = open(dev_uart, O_RDWR);
  c = (fh < 0) ? 'F' : 'T';
  //printf("%c\n", c );
  fd = fileno(fopen(test, "w+"));
  write(fd, &c, 1);

#ifndef __LINE_BUF__
  // Disable line buffering
  tcgetattr(fh, &original_t);
  bzero(&new_t, sizeof(new_t));
  memcpy(&new_t, &original_t, sizeof(new_t));
  tcflush(fh, TCIFLUSH);
  new_t.c_cflag = (B9600 | CS8 | CREAD | CLOCAL);
  new_t.c_cflag &= ~(PARENB | CSTOPB);
  new_t.c_iflag = IGNPAR;
  new_t.c_oflag = 0;
  //new_t.c_oflag &= ~OPOST;
  new_t.c_lflag = 0;
  //new_t.c_lflag &= ~(ICANON | ECHO);
  //new_t.c_lflag &= ~(ICANON | ECHO | ECHOE | ISIG);
  new_t.c_cc[VTIME] = 0;
  new_t.c_cc[VMIN] = 0;
  cfsetispeed(&new_t,B9600);
  cfsetospeed(&new_t,B9600);
  tcflush(fh, TCIFLUSH);
  status = tcsetattr(fh, TCSANOW, &new_t);
  if (status != 0) { printf("tcsetattr fd1"); return; }
  printf("tcsetattr /dev/ttySAC3 Success!\n");
  tcflush(fh, TCIFLUSH);

#endif
 
  while(1) {  

    memset(bfr, 0, sizeof(bfr) );
    printf("sizeof(bfr) : %d\n", sizeof(bfr));

    bfr[0]='A';

    cnt = 1;
    system("dmesg | grep tty");
	cnt = write(fh, &bfr, cnt);
	if(cnt < 0){
		printf("Write to UART failed!\n");
	}else{
		printf("Write to UART Success!\n");
	}
	system("dmesg | grep tty");
	system("busybox cat /dev/ttSAC3");
    write(fd, &bfr, 1);
    if(cnt > 0) {
      printf("write %d characters: %s\n", cnt, bfr);
    }
    tcdrain(fh);
	sleep(2);
  }
  close(fh);
  close(fd);

#ifndef __LINE_BUF__
  // Restore line buffering
  tcsetattr(fh, TCSANOW, &original_t);
  //tcsetattr(fileno(stdin), TCSANOW, &original_t);
#endif

  return 0;
}

參考資料:
推薦 --> [Linux RS-232 程式設計](http://shyuanliang.blogspot.com/2010/09/linux-rs-232.html)
[Linux下串口编程简单实例](https://blog.csdn.net/u011006622/article/details/73330792)
[留言觀念區](https://www.ubuntu-tw.org/modules/newbb/viewtopic.php?topic_id=96306&forum=12)