python: Convert bytearray to ctypes Struct

Question

I have an 'n' Byte bytearray. This is same as one of the defined ctypes.Sturcture. I want to typecast this bytearray to this sturcture. so that I can access each of this member. How can I do that?

class ABC(Structure):
    _fields_ = [("a", c_uint), ("b", c_ushort), ("c", c_ushort)]

class DEF(Structure):
    _fields_ = [("abc", ABC), ("i", I)]

b = bytearray(b'\x88\x08\xc0\xf9\x02\x85\x10\x00\xcc')

s = DEF()
print(s.abc.a)

How do I get the the correct value of the above print statement?

Answer 1

You can use from_buffer on the desired type (rather than the object instance):

from ctypes import Structure, c_uint, c_ushort, c_uint8


class ABC(Structure):
    _pack_ = 1
    _fields_ = [("a", c_uint), ("b", c_ushort), ("c", c_ushort)]


class DEF(Structure):
    _pack_ = 1
    _fields_ = [("abc", ABC), ("i", c_uint8)]


def main():
    b = bytearray(b'\x88\x08\xc0\xf9\x02\x85\x10\x00\xcc')

    # check if bytearray can be applied to structure.
    if len(b) < ctypes.sizeof(DEF):
        print("error: bytearray is too short for DEF.")
        return

    s = DEF.from_buffer(b)
    print("abc.a: {:#x}".format(s.abc.a))
    print("abc.b: {:#x}".format(s.abc.b))
    print("abc.c: {:#x}".format(s.abc.c))
    print("i: {:#x}".format(s.i))

if __name__ == '__main__':
    main()

Note that the structure must be packed accordingly, thus I used a _pack_ = 1 so the expected size of DEF structure is 9 bytes (4 + 2 + 2 + 1) rather than 12. I also used c_uint8 for the DEF.i field as this is probably what you meant (I, in your example, is not a type).

Output:

abc.a: 0xf9c00888
abc.b: 0x8502
abc.c: 0x10
i: 0xcc

If you want the value in big endian (rather than the default little endian), change the endianness of the structure by using ctypes.BigEndianStructure.