python ctypes pragma pack for byte aligned read

Question

I have a C++ application with below structure written to file. Now I need to unmarshal them using python, The basic problem here is how to reflect the pragma pack

Answer 1

You can change the packing in ctypes as described here

By default, Structure and Union fields are aligned in the same way the C compiler does it. It is possible to override this behavior be specifying a pack class attribute in the subclass definition. This must be set to a positive integer and specifies the maximum alignment for the fields. This is what #pragma pack(n) also does in MSVC.

For your example this would be:

from ctypes import *

class abc(Structure):
    _pack_ = 1
    _fields_ = [
        ('r1',c_ubyte),
        ('r2',c_ubyte),
        ('p1',c_ubyte),
        ('id',c_uint)]