시리얼통신 질문입니다.

int main(void)
{
        fd_set readfds;
        int fd;
        struct termios options;
 
        if ( ( fd = open("/dev/ttyS1",O_RDWR | O_NOCTTY | O_NDELAY) ) < 0 ){
                printf("device file open error\n");
                return 1;
        }
 
        bzero(&options,sizeof(options));
        options.c_cflag = B19200 | CLOCAL | CREAD | CS8;
        options.c_iflag |= IGNPAR;
        options.c_oflag |= OPOST;
        options.c_oflag |= ONLCR;
        options.c_lflag = 0;
        options.c_cc[VTIME] = 0;
        options.c_cc[VMIN] = 0;
 
        tcflush(fd,TCIOFLUSH);
 
        if ( tcsetattr(fd, TCSANOW, &options) < 0 ){
                printf("tcsetattr  error\n");
                close(fd);
                return 1;
        }
 
        while(1){
                FD_ZERO(&readfds);
                FD_SET(fd,&readfds);
 
                if ( select(fd+1,&readfds,NULL,NULL,0) > 0 ){
                        printf("select() return\n");
                        if ( FD_ISSET(fd,&readfds) ){
                                fprintf(stdout,"port1 \n");
                                show_data(fd);
                        }
                 }
        }
        return 0;
}