linux下串口编程（读写）

以下是我在linux终端下写的C代码，目的是想让其能接受到串口的数据，可是不知道为什么总是不能接受到，也没有报错信息，请大侠们看看，帮帮忙指点以下，谢谢！

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <stdio.h>


#define BAUDRATE B38400
#define MODEMDEVICE "/dev/ttyS0"
//#define _POSIX_SOURCE 1 /* POSIX compliant source */

#define FALSE 0
#define TRUE 1

volatile int STOP=FALSE;

main()
{
int fd,c, res;
struct termios oldtio,newtio;
char buf[255];

fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY );
if (fd <0) {perror(MODEMDEVICE); exit(-1); }

tcgetattr(fd,&oldtio);
bzero(&newtio, sizeof(newtio));


newtio.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD;

newtio.c_iflag = IGNPAR | ICRNL;


newtio.c_oflag = 0;

newtio.c_lflag = ICANON;

newtio.c_cc[VINTR] = 0; /* Ctrl-c */
newtio.c_cc[VQUIT] = 0; /* Ctrl-\ */
newtio.c_cc[VERASE] = 0; /* del */
newtio.c_cc[VKILL] = 0; /* @ */
newtio.c_cc[VEOF] = 4; /* Ctrl-d */
newtio.c_cc[VTIME] = 0; /* inter-character timer unused */
newtio.c_cc[VMIN] = 1; /* blocking read until 1 character arrives */
newtio.c_cc[VSWTC] = 0; /* '\0' */
newtio.c_cc[VSTART] = 0; /* Ctrl-q */
newtio.c_cc[VSTOP] = 0; /* Ctrl-s */
newtio.c_cc[VSUSP] = 0; /* Ctrl-z */
newtio.c_cc[VEOL] = 0; /* '\0' */
newtio.c_cc[VREPRINT] = 0; /* Ctrl-r */
newtio.c_cc[VDISCARD] = 0; /* Ctrl-u */
newtio.c_cc[VWERASE] = 0; /* Ctrl-w */
newtio.c_cc[VLNEXT] = 0; /* Ctrl-v */
newtio.c_cc[VEOL2] = 0; /* '\0' */


tcflush(fd, TCIFLUSH);
tcsetattr(fd,TCSANOW,&newtio);


while (STOP==FALSE) {
res = read(fd,buf,255);
buf[res]=0;
printf(":%s:%d\n", buf, res);
if (buf[0]=='z') STOP=TRUE;
}

tcsetattr(fd,TCSANOW,&oldtio);
}