Hướng dẫn python struct bitfield

//c struct code with filed width:

    struct{
      unsigned int x:1;
      unsigned int y:25;
      unsigned int z:6;
    };

Now I want to rewrite it in python, pack it and send to network,

The package struct in python, can pack/unpack data types.

such as:

struct.pack('!I10s',1,'hello')

But I don't know how to deal with struct with filed width as the c struct example. Any one know?

asked Dec 21, 2016 at 6:08

3

I realize it's late, but the following could help newcomers.

You can combine ctypes with struct.pack_into/struct.unpack_from:

import struct
import ctypes


class CStruct(ctypes.BigEndianStructure):
    _fields_ = [
        ("x", ctypes.c_uint32, 1),  # 1 bit wide
        ("y", ctypes.c_uint32, 25), # 25 bits wide
        ("z", ctypes.c_uint32, 6)   # 6 bits wide
    ]


cs = CStruct()

# then you can pack to it:
struct.pack_into('!I', cs,
                 0, # offset
                 0x80000083)
print(f'x={cs.x} y={cs.y} z={cs.z}')
# x=1 y=2 z=3

# or unpack from it:
cs_value, = struct.unpack_from('!I', cs)
print(hex(cs_value))
# 0x80000083

answered Feb 14 at 21:02

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Performance

Parts of this package has been re-implemented in C for faster pack and unpack operations. There are two independent C implementations; bitstruct.c, which is part of this package, and the standalone package cbitstruct. These implementations are only available in CPython 3, and must be explicitly imported. By default the pure Python implementation is used.

To use bitstruct.c, do import bitstruct.c as bitstruct.

To use cbitstruct, do import cbitstruct as bitstruct.

bitstruct.c has a few limitations compared to the pure Python implementation:

• Integers and booleans must be 64 bits or less.
• Text and raw must be a multiple of 8 bits.
• Bit endianness and byte order are not yet supported.
• byteswap() can only swap 1, 2, 4 and 8 bytes.

See cbitstruct for its limitations.

Example usage

A basic example of packing and unpacking four integers using the format string 'u1u3u4s16':

>>> from bitstruct import *
>>> pack('u1u3u4s16', 1, 2, 3, -4)
b'\xa3\xff\xfc'
>>> unpack('u1u3u4s16', b'\xa3\xff\xfc')
(1, 2, 3, -4)
>>> calcsize('u1u3u4s16')
24

An example compiling the format string once, and use it to pack and unpack data:

>>> import bitstruct
>>> cf = bitstruct.compile('u1u3u4s16')
>>> cf.pack(1, 2, 3, -4)
b'\xa3\xff\xfc'
>>> cf.unpack(b'\xa3\xff\xfc')
(1, 2, 3, -4)

Use the pack into and unpack from functions to pack/unpack values at a bit offset into the data, in this example the bit offset is 5:

>>> from bitstruct import *
>>> data = bytearray(b'\x00\x00\x00\x00')
>>> pack_into('u1u3u4s16', data, 5, 1, 2, 3, -4)
>>> data
bytearray(b'\x05\x1f\xff\xe0')
>>> unpack_from('u1u3u4s16', data, 5)
(1, 2, 3, -4)

The unpacked values can be named by assigning them to variables or by wrapping the result in a named tuple:

>>> from bitstruct import *
>>> from collections import namedtuple
>>> MyName = namedtuple('myname', ['a', 'b', 'c', 'd'])
>>> unpacked = unpack('u1u3u4s16', b'\xa3\xff\xfc')
>>> myname = MyName(*unpacked)
>>> myname
myname(a=1, b=2, c=3, d=-4)
>>> myname.c
3

Use the pack_dict and unpack_dict functions to pack/unpack values in dictionaries:

>>> from bitstruct import *
>>> names = ['a', 'b', 'c', 'd']
>>> pack_dict('u1u3u4s16', names, {'a': 1, 'b': 2, 'c': 3, 'd': -4})
b'\xa3\xff\xfc'
>>> unpack_dict('u1u3u4s16', names, b'\xa3\xff\xfc')
{'a': 1, 'b': 2, 'c': 3, 'd': -4}

An example of packing and unpacking an unsigned integer, a signed integer, a float, a boolean, a byte string and a string:

>>> from bitstruct import *
>>> pack('u5s5f32b1r13t40', 1, -1, 3.75, True, b'\xff\xff', 'hello')
b'\x0f\xd0\x1c\x00\x00?\xffhello'
>>> unpack('u5s5f32b1r13t40', b'\x0f\xd0\x1c\x00\x00?\xffhello')
(1, -1, 3.75, True, b'\xff\xf8', 'hello')
>>> calcsize('u5s5f32b1r13t40')
96

The same format string and values as in the previous example, but using LSB (Least Significant Bit) first instead of the default MSB (Most Significant Bit) first:

>>> from bitstruct import *
>>> pack('<u5s5f32b1r13t40', 1, -1, 3.75, True, b'\xff\xff', 'hello')
b'\x87\xc0\x00\x03\x80\xbf\xff\xf666\xa6\x16'
>>> unpack('<u5s5f32b1r13t40', b'\x87\xc0\x00\x03\x80\xbf\xff\xf666\xa6\x16')
(1, -1, 3.75, True, b'\xff\xf8', 'hello')
>>> calcsize('<u5s5f32b1r13t40')
96

An example of unpacking values from a hexstring and a binary file:

>>> from bitstruct import *
>>> from binascii import unhexlify
>>> unpack('s17s13r24', unhexlify('0123456789abcdef'))
(582, -3751, b'\xe2j\xf3')
>>> with open("test.bin", "rb") as fin:
...     unpack('s17s13r24', fin.read(8))
...
...
(582, -3751, b'\xe2j\xf3')

Change endianness of the data with byteswap, and then unpack the values:

>>> from bitstruct import *
>>> packed = pack('u1u3u4s16', 1, 2, 3, 1)
>>> unpack('u1u3u4s16', byteswap('12', packed))
(1, 2, 3, 256)

A basic example of packing and unpacking four integers using the format string 'u1u3u4s16' using the C implementation:

>>> from bitstruct.c import *
>>> pack('u1u3u4s16', 1, 2, 3, -4)
b'\xa3\xff\xfc'
>>> unpack('u1u3u4s16', b'\xa3\xff\xfc')
(1, 2, 3, -4)

Contributing

1. Fork the repository.

2. Install prerequisites.

   pip install -r requirements.txt
   

3. Implement the new feature or bug fix.

4. Implement test case(s) to ensure that future changes do not break legacy.

5. Run the tests.

   make test
   

6. Create a pull request.