Receiving Data in UDP Mode

Top  Previous  Next

In the previous section we have already explained how to handle data reception when the socket is in the TCP mode. This section provides explanation for receiving data with UDP.

Using on_sock_data_arrival event

With UDP, you still (typically) process incoming data basing on the on_sock_data_arrival event. Two differences apply:

Each time the on_sock_data_arrival event handler is entered you only get to process a single UDP datagram waiting in the RX buffer. Unless you use a special method (see below), you won't be able to get the data from the next datagram, even if this datagram is already available in the RX buffer.
If, while within the on_sock_data_arrival event handler, you don't read out entire contents of the "current" datagram and exit the event handler, then the unread portion of the datagram is discarded.

Here is an example: supposing, two datagrams are waiting in the RX buffer and their contents are "ABC" and "123". The following code will then execute twice:

 

 

sub on_sock_data_arrival

dim s as string(2)

s = sock.getdata(255) ' will get 2 bytes as this is the capacity of s

end sub

 

 

Since s has a maximum capacity of 2 the first time s=sock.getdata(255) executes you will get "AB". When the event handler is exited the rest of the first UDP datagram will be discarded, so next time you will get "12"!

Using sock.nextpacket method

Using on_sock_data_arrival event is a preferred method of handling an incoming data stream. Still, in selected cases you may need to process RX data in a loop, without leaving the event handler.

The sock.nextpacket method exists for just such a case. The result of this method execution is equivalent to exiting/(re)entering the on_sock_data_arrival event handler: the unread portion of the previous UDP datagram is discarded and we move to the next UDP datagram (if any).

Here is a code example where we handle all incoming UDP data without exiting event handler:

 

 

dim s as string

 

...

 

l1:

while sock.rxlen = 0

 doevents 'good practice: let other events execute while we are waiting

wend

 

sock.nextpacket 'Now we know that we do have data, 'move to' the next UDP datagram. This is like entering the on_sock_data_arrival.

s = sock.getdata(255) 'get data

goto l1

 

...