Question

I am writing two python scripts to communicate over UDP using python sockets. Here’s the related part of code

s = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)
s.bind((HOST, PORT))
s.setblocking(True) #I want it to be blocking
#(...)
(msg, addr) = sock.recvfrom(4)
#(...)
(msg2, addr2) = sock.recvfrom(2)

I want the receiving to be blocking and I don’t know the size of the whole message before I read the first 4-byte part. The above code becomes blocked on the sock.recvrfom(2) part, whereas modified, with one sock.recvfrom instead of two works alright:

(msg, addr) = sock.recvfrom(6) #works ok, but isn't enough for my needs

Any idea how I can conveniently read the incoming data in two parts or why the code doesn’t work as expected?

Answer 1

socket.recvfrom(size) will (for UDP sockets) read one packet, up to size bytes. The excess data is discarded. If you want to receive the whole packet, you have to pass a larger bufsize, then process the packet in bits (instead of trying to receive it in bits.)

If you want a more convenient, less fickle interface to network I/O, consider Twisted.