写入和读取 Linux COM 端口

我想写入 com 端口并从中读取一定数量的字节。录制似乎没有问题，它不write返回错误。但是在阅读问题时：它read挂起。也就是说，港口什么都没有，但如果我以前做过，怎么可能write呢？或者它是否以其他方式工作？

int main(int argc, char* argv[])
{
    int fd = open("/dev/ttyS0", O_RDWR | O_NOCTTY /*| O_NDELAY*/);
    if (fd == -1)
    {
        std::cout << " Unable to open /dev/ttyS0" << std::endl;
        return 0;
    }

    // configure
    termios attr_old;
    termios attr;
    memset( &attr_old, 0, sizeof(termios) );
    memset( &attr, 0, sizeof(termios) );

    if ( tcgetattr(fd, &attr_old) )
    {
         std::cout << "Error: " << strerror(errno) << " from tcgetattr" << std::endl;
        return 0;
    }

    attr = attr_old;

    //Baud rate
    cfsetospeed(&attr, B9600);
    cfsetispeed(&attr, B9600);

    attr.c_cflag     &=  ~PARENB;            // Make 8n1
    attr.c_cflag     &=  ~CSTOPB;
    attr.c_cflag     &=  ~CSIZE;
    attr.c_cflag     |=  CS8;
    attr.c_cflag     |=  CREAD | CLOCAL;     // turn on READ & ignore ctrl lines
    attr.c_cflag     &=  ~CRTSCTS;           // no flow control

    attr.c_cc[VMIN]   =  1;                  // read doesn't block
    attr.c_cc[VTIME]  =  5;                  // 0.5 seconds read timeout


    // Make raw 
    cfmakeraw(&attr);

   // Flush Port, then applies attributes
    tcflush( fd, TCIFLUSH );

    if ( tcsetattr ( fd, TCSANOW, &attr ) != 0)
    {
        std::cout << "Error: " << strerror(errno) << " from tcsetattr" << std::endl;
        return 0;
    }

    //Write
    int nw = write(fd, "A\r", 2);
    if ( nw < 0)
    {
         std::cout << "Error: " << strerror(errno) << " from write" << std::endl;
         return 0;
    }

    //Read
    char buff[32];
    memset( &buff, '\0', sizeof(buff) );

    int nr = 0;
    int spot = 0;
    char ch = '\0';
    do 
    {
        nr = read(fd, &ch, 1);
        if (nr < 0 )
        {
              int er = errno;
              std::cout << "Error:  " << er << " " << strerror(er) << " from read" << std::endl;
              return 0;
        }
        sprintf(&buff[spot], "%c", ch);
        spot += nr;
    }
    while( ch != '\r' && nr > 0 );

    close(fd);

    return 0;
}