python: invoking socket.recvfrom() twice

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.re开发者_C百科cvrfom(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?

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.

Read from UDP socket dequeues the whole datagram.

UDP is a message-based protocol. recvfrom will read the entire message that was originally sent, but if the buffer isn't big enough, it will throw an exception:

socket.error: [Errno 10040] A message sent on a datagram socket was larger than the internal message buffer or some other network limit, or the buffer used to receive a datagram into was smaller than the datagram itself

So I am not sure why you would hang on the 2nd recvfrom if a 6-byte message was originally sent. You should throw an exception on the first recvfrom. Perhaps post an actual working, minimal example of the client and the server program.