Convert string data received from network packet to Structure

Question

I am a newbiew to python and ctypes. what I have is:-

C program:

struct query
{
  uint16_t req_no;
  uint32_t req_len;
  uint64_t req;
};

struct response
{
  uint16_t req_no;
  uint16_t status;
  uint32_t value_len;
  uint64_t value;
};

// functions for creating query and response packets using
// above structs respectively, returning char buffer.
char* create_query(//some args);
char* create_response(//some args);

I have Created a libquery.so for the above C code. My TCP Server is a C program. I am trying to create a TCP python client (my project needs it!) for the same.

I can successfully send query and receive data(using functions in libquery.so) from python client. But when i get response data, I want to convert it to "struct response" type. I have create a similar "Structure" class in python, but can't get anything out of it.

Please help.

Some code snippet of my Python code:-

// some ctypes imports 
lib = cdll.LoadLibrary('./libquery.so')

class Info1(Structure):
    _fields_ = [("req_no",c_int),
                ("status",c_int),
                ("value_len",c_int),
                ("value",c_int)]

header = Info1()

// Did some TCP connection code here and send data to server by calling
// create_query() method, data confirmed to be correct on server side...

# Receive response data
data = sock.recv(512)
header = str_to_class('Info1')

header.req_no    =  int(ord(data[0])) // Works; but I don't want to go this ways..
header.status    =  int(ord(data[1]))
header.value_len =  int(ord(data[2]))
header.value     =  int(ord(data[3]))

print above header values..

I tried using :-

 def str_to_class(Info1):
    return getattr(sys.modules[__name__], Info1)

But don't know how to make it work.

Anybody know how to make it work OR is there any other way??

Answer 1

Your 'Info1' does not match C 'struct response'. So I changed in following code. You can use ctypes.memmove .

from ctypes import *

class Info1(Structure):
    _fields_ = [("req_no", c_uint16),
                ("status", c_uint16),
                ("value_len", c_uint32),
                ("value", c_uint64)]

data = (
    '\x01\x00'
    '\x02\x00'
    '\x03\x00\x00\x00'
    '\x04\x00\x00\x00\x00\x00\x00\x00'
)
# Assumed data was received. I assumed both server, clients are little-endian.
# else, use socket.ntoh{s|l}, socket.hton{s|l} ....

header = Info1()
memmove(addressof(header), data, sizeof(header))
assert header.req_no == 1
assert header.status == 2
assert header.value_len == 3
assert header.value == 4

You can also use struct .

import struct
data = '....' # same as above
struct.unpack('HHLQ', data) == (1, 2, 3, 4) # '>HHLQ' if data is htonl/htons-ed in sencding part.