python struct module (python 2.4)

Question

I am trying to send a buffer over unix domain socket to an application , receive the same structure back with updated values and then send that buffer back to that application. I am able to send a newly packed data and receive the response back, but if i try to pack the received buffer and send it again over the socket, i am facing errors saying there is a mismatch in the size which is sent to the application which is listening over the socket and it closes the socket.

The below snip is what i am trying to achieve. Looks like the endianess / string conversion I am depending on to send the data back is not correct.

""" request struct
#structure i am sending over unix domain socket
struct prod_entry {
    unsigned int        Model;
    unsigned int        year;
    char                    prodname[64];
    }
"""


value = (1, 1992, "mustang")

What is that i am doing wrong here. I want to receive a buffer pack it and send it again.

prod_entry = struct.pack('I I 64s', *value)


def update(update_records):
    try:
        comm_sock = socket.socket(socket.AF_UNIX,socket.SOCK_STREAM)
    except socket.error:
        return 


    try:
        comm_sock.connect(PROD_UNIX_DOMAIN_SKT)
    except socket.gaierror:
        return 

    try:
        comm_sock.sendall(update_records)
    except socket.error:
        return  

    reply = comm_sock.recv(struct.calcsize('I I 64s '))
    out1 = struct.unpack('<I I 64s',reply)

    rebound = struct.pack('I I 64s', *out1)
    comm_sock.sendall(rebound)

    reply2 = comm_sock.recv(struct.calcsize('I I 64s '))
    out2 = struct.unpack('<I I 64s',reply2)

    comm_sock.close()

update(prod_entry)

I am getting ::struct.error: unpack str size does not match format

Answer 1

The recv bufsize argument only specifies a maximum size. recv may return a smaller chunk than the maximum specified size. You need to loop over recv in case it's not returning the full packed string.

You'll quickly find that you need a protocol for any network communication like this. It might be useful for you to familiarize yourself with (eg) the HTTP protocol, esp. the request methods .