[CSE]  Advanced Operating Systems 
COMP9242 2014/S2 
CRICOS Provider
Number: 00098G

PRINTER Printer-Friendly Version

Serial Library

The serial library provides a very simple device driver that uses UDP/IP to send data, without error correction or packet loss detection, to the AOS06 port (UDP 26706) on your development machine. The port number was chosen to be easy to remember; Imagine you are dialing 'AOS06', the port number is the series of digits you dialled.


struct serial * serial_init()

serial_init initalises the state of the serial driver and returns a handle that must be passed to other serial functions.


int serial_send(struct serial *serial, char *data, int len);

serial_send will write len bytes of data to the AOS06 port (UDP 26706) on your development machine. This function returns the number of bytes written, which may be less than len. This occurs if the serial driver's internal buffer fills faster than it can actually output data. In this case it is up to the calling code to handle the situation, either by retrying or returning an error to the user.

You can see this output by running netcat: nc -lup 26706 in a terminal window.


int serial_register_handler(struct serial *serial, void (*handler)((struct serial *) serial, char c));

To receive input from the netcat terminal you must register a handler function, which will be called by the serial driver when the network makes data available. The provided function simply takes the inputted character as its only argument. This function will be called during an interrupt context so it should be reasonably light-weight.

What's really happening here

Due to historical reasons we call this driver a serial driver, even though it has nothing to do with serial ports anymore. The serial_send() function, in the driver, takes a block of data, wraps it into a single UDP frame and sends it to You should be aware that LWIP is not particularly smart and it will not break up large buffers for you. Limit your buffer size to 1450 bytes or so.

netcat(nc) is a pretty cool program, once it has received data from a particular machine it will connect() to the machine and send all input data onto your service. However this comes with some pretty serious limitations, as the networking stack on your machine will happily do break up of IP packets. This is good and to be expected, but LWIP can't deal with the deluge and will just drop everything on the floor. If needed you will have to write your own bulk transfer protocol.

Last modified: 29 Jul 2014.