Convert byte array to Ctype struct values

Question

I just want to convert that raw byte array(unknown encoding) to Ctype values using python with following structure specified in C# documentation. But output is incorrect. I mean, LTP is shown negative,which is never the case. Please guide if i am missing some encoding.

I just want LTP field, i have tried using offset still value is not correct. Can i get bytposition directly converted to Ctype value or is there any other way?

Structure

BytePosition :1-2 Type:int8 Field:Exchange

BytePosition :2-6 Type:int32 Field:Instrument token

BytePosition :6-10 Type:int32 Field:Ltp

BytePosition :10-14 Type:int32 Field:Change

BytePosition :14-18 Type:int32 Field:Exchange Timestamp

BytePosition :18-22 Type:int32 Field:Volume

from ctypes import Structure,c_int32, c_byte

class CompactMarketData(Structure):
    _fields_ = [("Mode", c_byte),("Exchange", c_byte),("InstrumentToken", c_int32), ("LastTradedPrice", c_int32), ("Change", c_int32), ("ExchangeTimeStamp", c_int32), ("Volume", c_int32)]

def main():
    raw=b'\x02\x06\x00\x08;8\x00\x01n\x04\xff\xff\xff\x06_0\xc5\xea\x00\x00",_0\xc5\xea'
    b = bytearray(raw)
    s = CompactMarketData.from_buffer(b)

    print("Mode: "+format(s.Mode))
    print("Exchange: "+format(s.Exchange))
    print("InstrumentToken: "+format(s.InstrumentToken))
    print("LastTradedPrice: ",str(s.LastTradedPrice))
    print("Change: "+format(s.Change))
    print("ExchangeTimeStamp: "+format(s.ExchangeTimeStamp))
    print("Volume: "+format(s.Volume))

if __name__ == '__main__':
    main()

Output:

Exchange: 2

InstrumentToken: 16791611

LastTradedPrice: -64402

Change: 811534079

ExchangeTimeStamp: 60101

Volume: 811543586

Answer 1

Using the _pack_ = 1 (documentation) structure attribute satisfies the conditions:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
from ctypes import Structure, c_int32, c_byte


class CompactMarketData(Structure):
    _pack_ = 1
    _fields_ = [
        ("Mode", c_byte),
        ("Exchange", c_byte),
        ("InstrumentToken", c_int32),
        ("LastTradedPrice", c_int32),
        ("Change", c_int32),
        ("ExchangeTimeStamp", c_int32),
        ("Volume", c_int32)
    ]

    def view_offsets(self):
        for field in self._fields_:
            name, field_type = field
            field_start = getattr(self.__class__, name).offset
            field_end = field_start + ctypes.sizeof(field_type)
            print(f"{name} - {field_start}:{field_end}")



def main():
    raw=b'\x02\x06\x00\x08;8\x00\x01n\x04\xff\xff\xff\x06_0\xc5\xea\x00\x00",_0\xc5\xea'
    b = bytearray(raw)
    s = CompactMarketData.from_buffer(b)

    s.view_offsets()

    print(f"\n{'-' * 79}")

    print(f"Struct Size: {ctypes.sizeof(CompactMarketData)}")
    print(f"buffer len: {len(raw)}")

    print(f"Mode: {s.Mode:#x}")
    print(f"Exchange: {s.Exchange:#x}")
    print(f"InstrumentToken: {s.InstrumentToken:#x}")
    print(f"LastTradedPrice: {s.LastTradedPrice:#x}")
    print(f"Change: {s.Change:#x}")
    print(f"ExchangeTimeStamp: {s.ExchangeTimeStamp:#x}")
    print(f"Volume: {s.Volume:#x}")


if __name__ == "__main__":
    main()

Ouput:

Mode - 0:1
Exchange - 1:2
InstrumentToken - 2:6
LastTradedPrice - 6:10
Change - 10:14
ExchangeTimeStamp - 14:18
Volume - 18:22

-------------------------------------------------------------------------------
Struct Size: 22
buffer len: 26
Mode: 0x2
Exchange: 0x6
InstrumentToken: 0x383b0800
LastTradedPrice: 0x46e0100
Change: 0x6ffffff
ExchangeTimeStamp: -0x153acfa1
Volume: 0x2c220000

Note: be wary that your bytes buffer is longer than the size of the Structure.