Unable to Read From Serial Device After Unplugging and Replugging Connector

Question

I'm have a Linux application that is supposed to read from serial device /dev/ttyS0. The serial device is opened in the following manner:

// Open the serial port
if((serial_device = open("/dev/ttyS0", O_RDWR | O_NOCTTY)) < 0){
    fprintf(stderr, "ERROR: Open\n");
    exit(EXIT_FAILURE);
}

// Get serial device attributes
if(tcgetattr(serial_device,&options)){
    fprintf(stderr, "ERROR: Terminal Get Attributes\n");
    exit(EXIT_FAILURE);
}

cfsetspeed(&options,speed);             // Set I/O baud rates
cfmakeraw(&options);                    // Set options to transceive raw data
options.c_cflag |= (CLOCAL | CREAD);    // Enable the receiver and set local mode
options.c_cflag &= ~CSTOPB;             // 1 stop bit
options.c_cflag &= ~CRTSCTS;            // Disable hardware flow control
options.c_cc[VMIN]  = 1;                // Minimum number of characters to read
options.c_cc[VTIME] = 10;               // One second timeout

// Set the new serial device attributes
if(tcsetattr(serial_device, TCSANOW, &options)){
    fprintf(stderr, "ERROR: Terminal Set Attributes\n");
    exit(EXIT_FAILURE);
}

I then use the select function to try and read from the serial device:

// Flush I/O Bffer
if(tcflush(serial_device,TCIOFLUSH)){
    fprintf(stderr, "ERROR: I/O Flush\n");
    exit(EXIT_FAILURE);
}
// Write message to board
if(write(serial_device,msg, strlen(msg)) != (int)strlen(msg)){
    fprintf(stderr, "ERROR: Write\n");
    exit(EXIT_FAILURE);
}


switch(select(serial_device+1, &set, NULL, NULL, &timeout)){
    // Error
    case -1:
        fprintf(stderr, "ERROR: Select\n");
        exit(EXIT_FAILURE);
    // Timeout
    case 0:
        success = false;
        break;
    // Input ready
    default:
        // Try to read a character
        switch(read(serial_device, &c, 1)){
            // Error (miss)
            case -1:
                success = false;
                break;
            // Got a character
            default:
                msg[i++] = c;
                break;
        }
        break;
    }
    // Set 200ms timeout
    this->timeout.tv_sec = 0;
    this->timeout.tv_usec = 200000;
}

I've tried reopening the port by determining if the read was not successful:

if(!success)
    close(serial_device);
    openPort(); // Same as above
}

However, the act of physically unplugging the serial connector will result in the application being unable to read anything further, and select will do nothing but time out. Plugging the connector back in while the application is running will not fix the issue, and select will continue to detect nothing.

The only way to successfully read from the serial port again is to restart the application. I'm wondering why this is, and how I can recover from the serial connector being unplugged at runtime.

It's hard to know if the problem is that your host stops receiving or the board stops transmitting. An RS-232 diagnostic dongle with LEDs showing activity can help in cases like this. I do notice that you're not checking for EOF (read returns 0). Perhaps that could be an issue? — Celada
I know that in raw mode EOF should never be returned, but I recommend programming defensively and handling that case (at least just to log an error) anyway. If, for whatever reason, an EOF is being delivered at some earlier point (e.g. immediately when the serial cable is disconnected) it could explain why no further data are received. — Celada
You set success = false; but never success = true;, so once an error occurs, you appear forever un-successful. — chux - Reinstate Monica

sawdust sawdust · Accepted Answer · 2013-06-27T01:34:49

The use of select() with just one file descriptor is unusual. It also adds a level of complexity.
Since the serial port is configured for non-canonical input, with proper selection of VMIN and VTIME, you might be able to accomplish the read of a char at a time with simpler code. E.G. try VMIN = 1 and VTIME = 10*timeout.tv_sec

However as you figured out, and if you are willing to handle (or want) a timeout rather than wait for at least one character to arrive, then VMIN = 0 will emulate your original code with the select().

VMIN = 0 and VTIME > 0
This is a pure timed read. If data are available in the input queue, it's transferred to the caller's buffer up to a maximum of nbytes, and returned immediately to the caller. Otherwise the driver blocks until data arrives, or when VTIME tenths expire from the start of the call. If the timer expires without data, zero is returned. A single byte is sufficient to satisfy this read call, but if more is available in the input queue, it's returned to the caller. Note that this is an overall timer, not an intercharacter one.

However (like the OP) I'm baffled as to why reconnecting the port connector should disrupt any reads or select monitoring, and have never encountered such an issue.

Unable to Read From Serial Device After Unplugging and Replugging Connector

1 Answers