struct.pack() Python unknown fields length to send through a socket

Question

I am developing a TCP client with Python. My problem comes when I want to send the structure to the server, I use the method struct.pack() to send and struct.unpack() to receive. I have my own protocol over TCP, with some headers which I already know the length of them, but of course, I don't know the length of the body, How could I pack all the fields without knowing the length of the body? Here is an example:

Request to the server:

pack('!BBBBHHII', P2P_h.version, P2P_h.ttl, P2P_h.msgType, P2P_h.reserved, P2P_h.sendPort, P2P_h.payloadLength, P2P_h.ipSrc, P2P_h.messageId)

Response from the server:

queryHit = unpack ("!BBBBHHII", queryResponse)

In those examples there are just the headers, I mean, the fields that I already know their length, how could I add the body field?

What's the type of the body?

loopbackbee
– loopbackbee

2013-10-22 08:49:42 +00:00
Commented Oct 22, 2013 at 8:49 — loopbackbee
– loopbackbee, Commented Oct 22, 2013 at 8:49

loopbackbee · Accepted Answer · 2013-10-30 10:02:39Z

1

Python socket's send and `recv operate on strings. By using pack, you're already transforming other kinds of types (integers, booleans...) into strings. Assuming your body is just raw binary data - a string, that is - you just need to concatenate the string you already have with it. Also, since the body is of variable length, you probably want to also send the body size in the message, so that you know what size to read in recv:

data=pack('!BBBBHHII', P2P_h.version, P2P_h.ttl, P2P_h.msgType, P2P_h.reserved, P2P_h.sendPort, P2P_h.payloadLength, P2P_h.ipSrc, P2P_h.messageId) #your current header
data+= pack("!I",len(body)) #assumes len(body) fits into a integer
data+= body #assumes body is a string
socket.send(data)

To receive, you do the reverse:

data= socket.recv( struct.calcsize('!BBBBHHII') )
P2P_h.version, P2P_h.ttl, P2P_h.msgType, P2P_h.reserved, P2P_h.sendPort, P2P_h.payloadLength, P2P_h.ipSrc, P2P_h.messageId= unpack('!BBBBHHII', data)
data= socket.recv( struct.calcsize("!I") )
body_size= unpack("!I", data)
body= socket.recv( body_size )

I separated the body size from the other fields for clarity, but you can combine them.

edited Oct 30, 2013 at 10:02

answered Oct 22, 2013 at 8:51

loopbackbee

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2 Comments

Alvaro Garcia de la Villa Over a year ago

And how would be the unpack when I recive data from the server?

loopbackbee Over a year ago

@AlvaroGarciadelaVilla you just socket.recv with the received size. You can use struct.calcsize to calculate the size you should ask to receive. See code on edit

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