用 ctypes 替换共享库中的函数指针 [英] Replace a function pointer in a shared library with ctypes
问题描述
我正在尝试通过 ctypes 用 Python 中定义的回调替换共享库中现有的函数指针.
I am trying to replace an existing function pointer in a shared library with a callback defined in Python, through ctypes.
C 中共享库的来源:
#include <assert.h>
#include <stdio.h>
void (*plot)();
int c_main(int argc, void** argv) {
printf("plot is %p
", (void*)plot);
assert(plot != NULL);
plot();
return 0;
}
Python 脚本的来源:
The source of the Python script:
from sys import platform
from pathlib import Path
import ctypes
import _ctypes
FUNCTYPE = ctypes.WINFUNCTYPE if platform == 'win32' else ctypes.CFUNCTYPE
def dlclose(obj):
if platform == "win32":
_ctypes.FreeLibrary(obj._handle)
else:
_ctypes.dlclose(obj._handle)
def enc_args(args):
C_ARGS = (ctypes.POINTER(ctypes.c_char) * len(args))()
for idx, arg in enumerate(args):
C_ARGS[idx] = ctypes.create_string_buffer(arg.encode("utf-8"))
return C_ARGS
@FUNCTYPE(None)
def plotxy():
print("plotxy")
C_ARGS = enc_args([str(Path(__file__))])
CAUXDLL = ctypes.CDLL("./test.so")
print(plotxy)
print(CAUXDLL.plot)
CAUXDLL.plot = plotxy
print(CAUXDLL.plot)
print(CAUXDLL.c_main(len(C_ARGS), C_ARGS))
测试脚本:
gcc -fPIC -shared -o test.so test.c
python3 test.py
我得到的输出:
# ./test.sh
<CFunctionType object at 0x7fb1f0abb1c0>
<_FuncPtr object at 0x7fb1f0abb640>
<CFunctionType object at 0x7fb1f0abb1c0>
plot is (nil)
python3: test.c:8: c_main: Assertion `plot != NULL' failed.
./test.sh: line 3: 21171 Aborted python3 test.py
所以,看起来Python中定义的函数(plotxy)是CFunctionType
类型,而C中定义的函数指针是_FuncPtr
类型.虽然应用了CAUXDLL
中的替换,但是在调用main函数的时候好像没有效果.
So, it seems that the function defined in Python (plotxy) is of type CFunctionType
, while the function pointer defined in C is of type _FuncPtr
. Although the replacement in CAUXDLL
is applied, it seems to have no effect when the main function is called.
除了阅读https://docs.python.org/3/library/ctypes.html#module-ctypes,我发现了其他问题(例如 如何在 ctypes 中使用 typedef 或 pythoncytpes 创建回调函数 - 分段错误(核心转储)),但我找不到如何将 CFunctionType
(plotxy)转换为 _FuncPtr
.
Apart from reading https://docs.python.org/3/library/ctypes.html#module-ctypes, I found other questions (e.g. How to use typedef in ctypes or python cytpes creating callback function - Segmentation fault (core dumped)), but I cannot find how to convert CFunctionType
(plotxy) to _FuncPtr
.
编辑
我相信这可能不是经常使用 ctypes 的问题.这是我成功实现的目标,并且在文档中得到了充分的解释.这个问题超越了.我不想执行 C 函数.我希望 Python 用 Python 编写的回调替换现有的函数指针.请注意,可以使用辅助 C 函数来完成此操作(请参阅 https://github.com/ghdl/ghdl-cosim/blob/master/vhpidirect/shared/pycb/caux.c#L32-L40).因此,这个问题是关于如何在没有辅助函数的情况下实现它(如果可能).
I believe this might not be an issue with the regular usage of ctypes. That's something I have successfully achieved and which is sufficiently explained in the docs. This question goes beyond. I don't want to execute the C function. I want Python to replace an existing function pointer with a callback written in Python. Note that it is possible to do it by using a helper C function (see https://github.com/ghdl/ghdl-cosim/blob/master/vhpidirect/shared/pycb/caux.c#L32-L40). Hence this question is about how to achieve it without that helper function (if possible).
推荐答案
从ctypes
访问全局变量的方式是使用in_dll
,但是好像没有成为更改函数指针的公开方式.我只能读取和调用它,所以我认为没有辅助函数是不可能的.
The way to access global variables from ctypes
is to use in_dll
, but there doesn't seem to be an exposed way to change a function pointer. I was only able to read it and call it, so I don't think it is possible without a helper function.
下面的例子改变了一个 int
全局变量,但是 CFUNCTYPE
实例没有一个 value
成员来改变它.我添加了一个 C 帮助程序来设置全局以解决该问题,并添加了回调的默认值以在更改之前验证它是否被正确访问.
The example below alters an int
global variable, but CFUNCTYPE
instances don't have a value
member to alter it. I added a C helper to set the global to work around the issue and a default value of the callback to verify it was accessed correctly before changing it.
test.c:
#include <stdio.h>
#define API __declspec(dllexport)
typedef void (*CB)();
void dllplot() {
printf("default
");
}
API CB plot = dllplot;
API int x = 5;
API int c_main() {
printf("x=%d (from C)
",x);
plot();
return 0;
}
API void set_cb(CB cb) {
plot = cb;
}
test.py:
from ctypes import *
PLOT = CFUNCTYPE(None)
dll = CDLL('./test')
dll.c_main.argtypes = ()
dll.c_main.restype = c_int
dll.set_cb.argtypes = PLOT,
dll.set_cb.restype = None
@PLOT
def plotxy():
print("plotxy")
x = c_int.in_dll(dll,'x')
plot = PLOT.in_dll(dll,'plot')
print(f'x={x.value} (from Python)')
x.value = 7
print('calling plot from Python directly:')
plot()
print('calling c_main():')
dll.c_main()
dll.set_cb(plotxy)
print('calling plot from Python after setting callback:')
plot()
print('calling plot from C after setting callback:')
dll.c_main()
输出:
x=5 (from Python)
calling plot from Python directly:
default
calling c_main():
x=7 (from C)
default
calling plot from Python after setting callback:
plotxy
calling plot from C after setting callback:
x=7 (from C)
plotxy
注意全局指针使用 .contents
来访问它们的值,所以我尝试使用 POINTER(CFUNCTYPE(None))
和 plot.contents= plotxy
但这并没有正确分配全局变量并且 C 崩溃了.
Note that global pointers use .contents
to access their value, so I experimented with using a POINTER(CFUNCTYPE(None))
and using plot.contents = plotxy
but that doesn't assign the global variable correctly and C crashed.
我什至尝试向函数指针添加一个实际的全局指针:
I even tried adding an actual global pointer to function pointer:
API CB plot = dllplot;
API CB* pplot = &plot;
然后使用:
PLOT = CFUNCTYPE(None)
PPLOT = POINTER(PLOT)
plot = PPLOT.in_dll(dll,'pplot')
plot.contents = plotxy
那让我通过 .contents
分配函数,但 c_main
仍然调用默认绘图值.因此,将 CFUNCTYPE
用作函数参数以外的任何功能的功能似乎并未实现.
That let me assign the function via .contents
, but c_main
still called the default plot value. So the functionality of using CFUNCTYPE
as anything but a function parameter doesn't appear to be implemented.
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