Python使用ctypes实现与C++互相调用的实践教程

#include <iostream>
#include <string>

// 回调类型：void callback(int, const char*)
using CallbackType = void(*)(int, const char*);

extern "C" {

// 导出函数：循环调用回调
void run_with_callback(CallbackType cb, int count) {
    if (!cb) {
        std::cerr << "[C++] callback is null" << std::endl;
        return;
    }

    std::cout << "[C++] run_with_callback start, count = " << count << std::endl;

    for (int i = 0; i < count; ++i) {
        std::cout << "[C++] before calling callback, i = " << i << std::endl;

        std::string message = "msg from C++ index = " + std::to_string(i);
        cb(i, message.c_str());  // 回调到 Python

        std::cout << "[C++] after calling callback, i = " << i << std::endl;
    }

    std::cout << "[C++] run_with_callback end" << std::endl;
}

} // extern "C"

cmake_minimum_required(VERSION 3.10)
project(pyc_callback_demo LANGUAGES CXX)

add_library(mycallback SHARED callback_lib.cpp)

set_target_properties(mycallback PROPERTIES
    OUTPUT_NAME "mycallback"
    CXX_STANDARD 17
    CXX_STANDARD_REQUIRED YES
)

mkdir -p build
cd build
cmake ..
cmake --build .

import ctypes
import pathlib
import os

# 当前文件所在目录
BASE_DIR = pathlib.Path(__file__).resolve().parent

LIB_PATH = BASE_DIR / "cpp_lib" / "build" / "libmycallback.so"

if not LIB_PATH.exists():
    raise FileNotFoundError(f"找不到动态库: {LIB_PATH}. 请先在 cpp_lib 目录下用 CMake 编译生成 libmycallback.so")

# 加载动态库
lib = ctypes.CDLL(str(LIB_PATH))

# 定义 C 端回调类型
# 第二个参数用 ctypes.c_char_p（C 端是 const char*）
CALLBACK_CTYPE = ctypes.CFUNCTYPE(None, ctypes.c_int, ctypes.c_char_p)

# 声明 C 函数的签名：void run_with_callback(CallbackType cb, int count)
lib.run_with_callback.argtypes = [CALLBACK_CTYPE, ctypes.c_int]
lib.run_with_callback.restype = None


# Python 侧的回调实现
@CALLBACK_CTYPE
def py_callback(i: int, msg: bytes) -> None:
    # msg 是 bytes，需要按合适编码解码，这里假设 UTF-8
    text = msg.decode("utf-8") if msg is not None else "<NULL>"
    print(f"[Python] in callback, i = {i}, msg = {text}")


def main() -> None:
    print("[Python] before calling C function")

    # 调用 C++ 库函数，并把 Python 回调传进去
    lib.run_with_callback(py_callback, 3)

    print("[Python] after calling C function")


if __name__ == "__main__":
    main()