Python ctypes，将c_void_p作为out参数传递给c函数 [英] Python ctypes, pass c_void_p as an out parameter to c function

问题描述

我正在编写一个包装一些C ++代码的Python模块。我曾经使用此函数将指针从C ++传递到Python（该指针将由其他函数释放

I'm writing a Python module that wraps some C++ code. I used to use this function to pass a pointer from C++ to Python (the pointer would be freed by a different function

DLLEXPORT void* run(double input1, double input2)

我添加了错误返回代码，所以我的新函数看起来像

I've added error return codes so my new function looks like

DLLEXPORT int run(double input1, double input2, void* output)

但是现在我似乎无法获得指针的值，在Python中按如下所示使用ctypes设置函数

But now I can't seem to get the value of the pointer, in Python I set up the function with ctypes as follows

from ctypes import *
mydll = cdll.mydll

mydll.run.argtypes = [c_double, # input 1
                      c_double, # input 2
                      c_void_p] # output pointer
mydll.run.restype  = c_int   

然后我通过在Python和passin中创建一个新的void指针来使用该函数将其添加到dll函数

Then I use the function by creating a new void pointer in Python and passing it to the dll function

p = c_void_p()

retval = mydll.run(1.2, 3.4, p)

print p

运行这段代码后，剩下的 p 等于 c_void_p（None）。

After running this code I am left with p equal to c_void_p(None).

将此指针传递给其他函数会导致地址0x0处的非法访问异常，因此我认为它没有被更新。

Passing this pointer to other functions causes illegal access exceptions at address 0x0, so I don't think it is being updated.

我是预计执行后会在 p 中填充一些地址。我缺少ctypes吗？我可以通过分配（c_double * 10）（）来创建一个双精度数组，并将其传递给要写入的c函数，为什么不能为

I am expected some address to populate p after execution. I am missing something with ctypes? I can create a double array by allocating (c_double * 10)() and pass that to c function to be written to, why can't I pass a void pointer for the same purpose?

推荐答案

正如eryksun的评论所指出的， void * 要作为输出参数，应为 void ** ：

As eryksun's comment points out, for void* to be an output parameter, it should be void**:

# DLLEXPORT int run(double input1, double input2, void** output)

from ctypes import *
mydll = cdll.mydll

mydll.run.argtypes = [c_double, # input 1
                      c_double, # input 2
                      POINTER(c_void_p)] # output pointer
mydll.run.restype  = c_int

p = c_void_p()
retval = mydll.run(1.2,3.4,byref(p))

print p.contents

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