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 option in python.

C++ Structure

#pragma pack(1)
struct abc  
{  
unsigned char r1;  
unsigned char r2;  
unsigned char p1;  
unsigned int id;  
};  
#pragma pack()

Now, the structure size is 7 not 8,this data is written into a data file. How do I retrieve this data using python.

Note :
1. I am using ctypes, and the above structure is a sample structure.

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ctypes uses the native byte order for Structures and Unions. To build structures with non-native byte order, you can use one of the BigEndianStructure, LittleEndianStructure, BigEndianUnion, and LittleEndianUnion base classes. These classes cannot contain pointer fields

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The above information from python docs, does not delve into details.

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)]