如何在python中使用ctypes重载C库的弱声明函数?

时间:2019-03-26 12:42:47

标签: python c ctypes

我正在尝试建立一个测试环境来验证我正在使用的C库。该库嵌入在运行自定义linux的设备上。经过一番网上阅读后,我决定使用python和ctypes从python调用我的库函数。几乎我所有的功能都可以很好地工作,但是在使用回调函数时会卡住。

我的回调函数在库中定义为“弱”。我想知道是否可以使用python ctypes重载弱C函数吗?

以我的尝试为例:

libtest.c

#include <stdio.h>
#include <stdlib.h>
#include "libtest.h"

int multiply(int a, int b)
{
    custom_callback(a);
    return a*b;
}

int __attribute__((weak)) custom_callback(int a)
{
    printf("Callback not redefined, a = %d\n", a);
    return 0;
}

main.py

from ctypes import *

lib = cdll.LoadLibrary("libtest.so")

CALLBACKFUNC = CFUNCTYPE(c_int, c_int)

def py_callback_func(a):
    print("py redifinition", a)
    return 0


lib.custom_callback = CALLBACKFUNC(py_callback_func)

lib.multiply(c_int(5), c_int(10))

然后我用python3运行main.py

$ python3 main.py 
Callback not redefined, a = 5

我希望输出为py redifinition 5

我做错什么了吗?还是ctypes根本无法重新定义一个弱声明的C函数?

1 个答案:

答案 0 :(得分:2)

您正在感到困惑:符号在 link 时间处理,而您在 运行 时间。链接器可能无法了解您的 Python 回调函数。尽管它不是官方消息来源,但请查看[Wikipedia]: Weak symbol了解更多详细信息。
还有[Python 3]: ctypes - A foreign function library for Python
这是一个小演示。

libtest.c

#include <stdio.h>


int __attribute__((weak)) custom_callback(int a) {
    printf("Callback not redefined, a = %d\n", a);
    return 0;
}


int multiply(int a, int b) {
    custom_callback(a);
    return a * b;
}

callback.c

#include <stdio.h>


int custom_callback(int a) {
    printf("C callback redefinition, a = %d\n", a);
    return 0;
}

code.py

#!/usr/bin/env python3

import sys
import ctypes


DLL_NAME = "./libtest.so"
CALLBACKFUNCTYPE = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_int)


def py_callback_func(a):
    print("PY callback redefinition", a)
    return 0


def main(argv):
    dll_name = argv[0] if argv else DLL_NAME
    dll = ctypes.CDLL(dll_name)
    multiply = dll.multiply
    multiply.argtypes = [ctypes.c_int, ctypes.c_int]
    multiply.restype = ctypes.c_int
    #dll.custom_callback(3)
    #dll.custom_callback = CALLBACKFUNCTYPE(py_callback_func)
    #dll.custom_callback(3)
    res = multiply(5, 10)


if __name__ == "__main__":
    print("Python {:s} on {:s}\n".format(sys.version, sys.platform))
    main(sys.argv[1:])
    print("Done.")

输出

[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q055357490]> ~/sopr.sh
*** Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ***

[prompt]> ls
callback.c  code.py  libtest.c
[prompt]> gcc -o libtest0.so -fPIC -shared libtest.c
[prompt]> gcc -o libtest1.so -fPIC -shared libtest.c callback.c
[prompt]> ls
callback.c  code.py  libtest0.so  libtest1.so  libtest.c
[prompt]>
[prompt]> python3 code.py ./libtest0.so
Python 3.5.2 (default, Nov 12 2018, 13:43:14)
[GCC 5.4.0 20160609] on linux

Callback not redefined, a = 5
[prompt]>
[prompt]> python3 code.py ./libtest1.so
Python 3.5.2 (default, Nov 12 2018, 13:43:14)
[GCC 5.4.0 20160609] on linux

C callback redefinition, a = 5
Done.

可见,__attribute__((weak))效果 可见。另外,关于注释的代码(尝试设置 .dll 函数):它仅在 ctypes 代理级别下运行,而不会更改 .dll 代码(这很荒谬)。

关于您的问题,有两种解决方法。
这里是使用带有此类回调函数的指针的。指针可以通过设置函数从外部设置(也可以导出)。

libtest.c

#include <stdio.h>

#define EXEC_CALLBACK(X) \
    if (pCallback) { \
        pCallback(X); \
    } else { \
        fallbackCallback(X); \
    }


typedef int (*CustomCallbackPtr)(int a);

static CustomCallbackPtr pCallback = NULL;


static int fallbackCallback(int a) {
    printf("Callback not redefined, a = %d\n", a);
    return 0;
}


int multiply(int a, int b) {
    EXEC_CALLBACK(a);
    return a * b;
}


void setCallback(CustomCallbackPtr ptr) {
    pCallback = ptr;
}

code.py

#!/usr/bin/env python3

import sys
import ctypes


DLL_NAME = "./libtest.so"
CALLBACKFUNCTYPE = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_int)


def py_callback_func(a):
    print("PY callback redefinition", a)
    return 0


def main():
    dll = ctypes.CDLL(DLL_NAME)
    multiply = dll.multiply
    multiply.argtypes = [ctypes.c_int, ctypes.c_int]
    multiply.restype = ctypes.c_int
    set_callback = dll.setCallback
    set_callback.argtypes = [CALLBACKFUNCTYPE]
    multiply(5, 10)
    set_callback(CALLBACKFUNCTYPE(py_callback_func))
    multiply(5, 10)
    set_callback(ctypes.cast(ctypes.c_void_p(), CALLBACKFUNCTYPE))
    multiply(5, 10)


if __name__ == "__main__":
    print("Python {:s} on {:s}\n".format(sys.version, sys.platform))
    main()
    print("Done.")

输出

[cfati@cfati-ubtu16x64-0:~/Work/Dev/StackOverflow/q055357490]> ~/sopr.sh
*** Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ***

[prompt]> ls
code.py  libtest.c
[prompt]> gcc -o libtest.so -fPIC -shared libtest.c
[prompt]> python3 code.py
Python 3.5.2 (default, Nov 12 2018, 13:43:14)
[GCC 5.4.0 20160609] on linux

Callback not redefined, a = 5
PY callback redefinition 5
Callback not redefined, a = 5
Done.