如何从 ctypes.Structure 取回原始缓冲区？

Question

msg is created using extra bytes ie more than sizeof(Message)

from ctypes import *

class Message(Structure):
    _fields_ = [("length",  c_ushort),
                ("version", c_ubyte),
                ("type",    c_ubyte),
                ("index",   c_int),
                ("flags",   c_int),
    ]

msg = Message.from_buffer(bytearray.fromhex('1000 05 06 01000000 deadbeef cccccccc'))

But, bytes(msg) just returns sizeof(Message) bytes.

>>> bytes(msg).hex()
'1000050601000000deadbeef'

How do I retrieve the original bytearray from msg ? Do I need to make a more elaborate Message class that overrides __bytes__ , from_buffer etc.?

Answer 1

It looks like "Message" is a header and the total size in length is the header size plus the variable data after the header. Define the header, then make a factory function that dynamically creates a structure with a header and an array of the appropriate size:

from ctypes import *

class Header(Structure):
    _fields_ = [("length", c_ushort),
                ("version", c_ubyte),
                ("type", c_ubyte),
                ("index", c_int),
                ("flags", c_int)]

def make_msg(hexstring):

    buf = bytearray.fromhex(hexstring)
    datalen = len(buf) - sizeof(Header)

    class Message(Structure):
        _fields_ = [("header", Header),
                    ("data", c_char * datalen)]

    return Message.from_buffer(buf)

msg = make_msg('1000 05 06 01000000 deadbeef cccccccc')
print(msg.header.length)
print(msg.data)

Output:

16
b'\xcc\xcc\xcc\xcc'