自己写的串口通信，红色为主要问题部分，打开fd没问题，问题是write函数在写的同时会被自己的read函数读到，试了各种办法都不行
int speed_arr[] =
{ B115200,B38400, B19200, B9600, B4800, B2400, B1200, B300, B115200,B38400, B19200, B9600,B4800, B2400, B1200, B300, };
int name_arr[] =
{ 115200,38400, 19200, 9600, 4800, 2400, 1200, 300,115200, 38400, 19200, 9600, 4800, 2400,1200, 300, };
void *mywrite(void *sock)
{

    char buf[BUFSIZ];
    char ch;
    while(1)
    {
        bzero(buf,BUFSIZ);
        fgets(buf,BUFSIZ,stdin);
        pthread_mutex_lock(&mutex);
        if(write(fd,buf,strlen(buf)) < 0)
        {
            perror("write failed!");
            exit(1);
        }   
        //lseek(fd,strlen(buf),SEEK_CUR);
        pthread_mutex_unlock(&mutex);
        fflush(stdin);
//        while((ch = getchar()) !='\n');

    }
    close(fd);
    pthread_join(thread,NULL);
   
}
void create_thread(void)
{
    int ret;
    ret = pthread_create(&thread,NULL,mywrite,NULL);
    if(pthread_create(&tid,NULL,net_connect,NULL)!=0){
        perror("pthread_create net");
        exit(1);
    }
}

static int set_parity(int fd,int databits,int stopbits,int parity)
{
    struct termios options;
    if (tcgetattr (fd, &options) != 0)
    {
        perror ("SetupSerial 1");
        return (FALSE);
    }
    options.c_cflag &= ~CSIZE;
    switch (databits)
    {
        case 7:
            options.c_cflag |= CS7;
            break;
        case 8:
            options.c_cflag |= CS8;
            break;
        default:
            fprintf (stderr, "Unsupported data size\n");
            return (FALSE);
    }
    switch (parity)
    {
        case 'n':
        case 'N':
            options.c_cflag &= ~PARENB;   /* Clear parity enable */
            options.c_iflag &= ~INPCK;    /* Enable parity checking */
            break;
        case 'o':
        case 'O':
            options.c_cflag |= (PARODD | PARENB);
            options.c_iflag |= INPCK;     /* Disnable parity checking */
            break;
        case 'e':
        case 'E':
            options.c_cflag |= PARENB;      /* Enable parity */
            options.c_cflag &= ~PARODD;
            options.c_iflag |= INPCK;     /* Disnable parity checking */
            break;
        case 'S':
        case 's':            /*as no parity */
            options.c_cflag &= ~PARENB;
            options.c_cflag &= ~CSTOPB;
            break;
        default:
            fprintf (stderr, "Unsupported parity\n");
            return (FALSE);
    }

    switch (stopbits)
    {
        case 1:
            options.c_cflag &= ~CSTOPB;
            break;
        case 2:
            options.c_cflag |= CSTOPB;
            break;
        default:
            fprintf (stderr, "Unsupported stop bits\n");
            return (FALSE);
    }
    /* Set input parity option */
    if (parity != 'n')
        options.c_iflag |= INPCK;
    tcflush (fd, TCIFLUSH);
    options.c_cc[VTIME] = 150;
    options.c_cc[VMIN] = 0;     /* Update the options and do it NOW */
    if (tcsetattr (fd, TCSANOW, &options) != 0)
    {
        perror ("SetupSerial 3");
        return (FALSE);
    }
    return (TRUE);
}

void set_speed(int fd,int speed)
{
    int i;
    int status;
    struct termios opt;
    tcgetattr(fd,&opt);
    for(i=0;i<sizeof(speed_arr)/sizeof(int);i++)
    {
        if(speed == name_arr[i])
        {
            tcflush(fd,TCIOFLUSH);   
            cfsetispeed(&opt,speed_arr[i]);
            cfsetospeed(&opt,speed_arr[i]);
            status = tcsetattr(fd,TCSANOW,&opt);
            if(status != 0)
            {
                perror("tcsetattr fd failed!");
                return;
            }
            tcflush(fd,TCIOFLUSH);
        }
    }
}
int main(void)
{
    init_buf_link_list(&pbl);
    printf("This program updates last time at %s   %s\n",__TIME__,__DATE__);
    fd = open("/dev/ttyAT2",O_RDWR);
    if(fd < 0 )
    {
        perror("open ttyUSBx failed!");
        exit(1);
    }
    else
        printf("open %s successfully\n",ttyname(fd));


    create_thread();
    if(pthread_mutex_init(&mutex,NULL) < 0)
    {
        perror("pthread_mutex_init failed!");
        exit(1);
    }

    struct sigaction signal;

    signal.sa_handler = fun;
    sigemptyset(&signal.sa_mask);
    signal.sa_flags = 0;
    signal.sa_restorer = NULL;

    sigaction(SIGIO,&signal,NULL);

    fcntl(fd,F_SETOWN,getpid());
    fcntl(fd,F_SETFL,FASYNC);

    set_speed(fd,115200);

    if(set_parity(fd,8,1,'N')  == 0)
    {
        printf("Set parity error");
        exit(1);
    }

    char buf[255];
    while(1)
    {
            usleep(5000);
            if(wait_flag == 0){   
            memset(buf,0,sizeof(buf));
            if((ret = read(fd,buf,255)) > 0){
                printf("from pc_uart: %s",buf);
                insert_buf_link_list(pbl,buf,ret);
            }
            fflush(stdout);
             }
             wait_flag = flag;
    }
    close(fd);
    return 0;
}