Problem with ctypes bitfield

ctypes_bitfield_problem.ipynb

{
 "cells": [
  {
   "cell_type": "markdown",
   "id": "a1afba99-d7b1-484c-97c4-8f145920ba96",
   "metadata": {},
   "source": [
    "# ctypes bitfield problem\n",
    "\n",
    "I have some binary data, produced with C/C++. When using the code given below,\n",
    "something seems to be a bit off. Probably I missed some flag or so.\n",
    "\n",
    "An example of the data is:\n",
    "\n",
    "```\n",
    "bytes([67, 165, 195, 254, 255, 18, 0, 191, 0, 64])\n",
    "```"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "a836ad4f-9ce1-4eee-9b0c-266aaa404883",
   "metadata": {},
   "source": [
    "# packing using C/C++"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 1,
   "id": "2189afd9-abab-4408-8149-623693ee0d08",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "Overwriting test.cpp\n"
     ]
    }
   ],
   "source": [
    "%%writefile test.cpp\n",
    "\n",
    "#include <iostream>\n",
    "#include <cstdint>\n",
    "\n",
    "#define MSG_LEN 10\n",
    "\n",
    "using namespace std;\n",
    "union  measurement_msg {\n",
    "        struct __attribute__((__packed__)) {\n",
    "            uint16_t ts : 16;\n",
    "            int32_t gyro_x : 20;\n",
    "            int32_t gyro_y : 20;\n",
    "            int32_t gyro_z : 20;\n",
    "        } values;\n",
    "        std::uint8_t raw[MSG_LEN];\n",
    "};\n",
    "\n",
    "int main()\n",
    "{\n",
    "    union  measurement_msg test;\n",
    "    \n",
    "    test.values.ts = 42307;\n",
    "    test.values.gyro_x = -317;\n",
    "    test.values.gyro_y = 303;\n",
    "    test.values.gyro_z = 191;\n",
    "    \n",
    "    for (auto &val : test.raw) {\n",
    "        cout << (int)val << \", \";\n",
    "    }\n",
    "    cout << endl;\n",
    "\n",
    "    return 0;\n",
    "}"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "de7120ef-5fa4-44d3-aa8f-77a7452f091e",
   "metadata": {},
   "source": [
    "WARNING: next file compiles and runs code on your computer"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 55,
   "id": "31124e46-b7bd-42dd-8f6f-b1cf5a903bca",
   "metadata": {},
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "67, 165, 195, 254, 255, 18, 0, 191, 0, 0, \n"
     ]
    }
   ],
   "source": [
    "!g++ test.cpp && ./a.out"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "e4532b3e-bf0f-43b0-a388-8bafd769e981",
   "metadata": {},
   "source": [
    "## ctypes unpacking\n",
    "\n",
    "using `67 165 195 254 255 18 0 191 0 0`"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 31,
   "id": "e6acb961-3c2f-4a60-89a7-0dd434285507",
   "metadata": {},
   "outputs": [],
   "source": [
    "from ctypes import *\n",
    "\n",
    "MSG_LEN = 10\n",
    "\n",
    "class Values(LittleEndianStructure):\n",
    "    _pack_ = 1\n",
    "    _fields_ = [\n",
    "        (\"ts\", c_uint16, 16), \n",
    "        (\"gyro_x\", c_int32, 20),\n",
    "        (\"gyro_y\", c_int32, 20),\n",
    "        (\"gyro_z\", c_int32, 20)\n",
    "    ]\n",
    "\n",
    "class Data(Union):\n",
    "    _fields_ = [(\"values\", Values), (\"raw\", c_ubyte * MSG_LEN)]"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 56,
   "id": "24a6ae71-56f3-481d-8c72-9275c8ac0589",
   "metadata": {},
   "outputs": [
    {
     "data": {
      "text/plain": [
       "(42307, -317, 48896, 0)"
      ]
     },
     "execution_count": 56,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "example = Data()\n",
    "\n",
    "#example.raw = bytes([67, 165, 195, 254, 255, 18, 0, 191, 0, 64])\n",
    "example.raw = (c_ubyte*10)(67, 165, 195, 254, 255, 18, 0, 191, 0, 0)\n",
    "example.values.ts, example.values.gyro_x, example.values.gyro_y, example.values.gyro_z"
   ]
  },
  {
   "cell_type": "markdown",
   "id": "c00856a0-1758-4a59-a2d2-ddb3259c5e21",
   "metadata": {},
   "source": [
    "result is off starting from `gyro_y` :("
   ]
  },
  {
   "cell_type": "code",
   "execution_count": null,
   "id": "af26670a-5aa6-4624-9485-86a2eb035133",
   "metadata": {},
   "outputs": [],
   "source": []
  }
 ],
 "metadata": {
  "kernelspec": {
   "display_name": "Python 3 (ipykernel)",
   "language": "python",
   "name": "python3"
  },
  "language_info": {
   "codemirror_mode": {
    "name": "ipython",
    "version": 3
   },
   "file_extension": ".py",
   "mimetype": "text/x-python",
   "name": "python",
   "nbconvert_exporter": "python",
   "pygments_lexer": "ipython3",
   "version": "3.10.2"
  }
 },
 "nbformat": 4,
 "nbformat_minor": 5
}