python：调用 socket.recvfrom() 两次

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 .

Answer 2

Read from UDP socket dequeues the whole datagram .

Answer 3

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.