Python对象作为ctypes回调函数中的userdata [英] Python objects as userdata in ctypes callback functions

问题描述

C函数 myfunc 对较大的数据块进行操作。结果以块的形式返回到回调函数：

The C function myfunc operates on a larger chunk of data. The results are returned in chunks to a callback function:

int myfunc(const char *data, int (*callback)(char *result, void *userdata), void *userdata);

使用 ctypes ，调用 myfunc 从Python代码，并将结果返回到一个Python回调函数。此回调工作正常。

Using ctypes, it's no big deal to call myfunc from Python code, and to have the results being returned to a Python callback function. This callback work fine.

myfunc = mylib.myfunc
myfunc.restype = c_int
myfuncFUNCTYPE = CFUNCTYPE(STRING, c_void_p)
myfunc.argtypes = [POINTER(c_char), callbackFUNCTYPE, c_void_p]

def mycb(result, userdata):
    print result
    return True

input="A large chunk of data."
myfunc(input, myfuncFUNCTYPE(mycb), 0)

给一个Python对象（说一个列表）作为userdata到回调函数？为了存储结果块，我想做例如：

But, is there any way to give a Python object (say a list) as userdata to the callback function? In order to store away the result chunks, I'd like to do e.g.:

def mycb(result, userdata):
    userdata.append(result)

userdata=[]

但我不知道如何将Python列表转换为c_void_p，以便它可以用于调用myfunc。

But I have no idea how to cast the Python list to a c_void_p, so that it can be used in the call to myfunc.

我目前的解决方法是实现一个链接列表作为ctypes结构，这是相当麻烦。

My current workaround is to implement a linked list as a ctypes structure, which is quite cumbersome.

推荐答案

我想你可以使用 Python C API 来做这个...也许你可以使用一个PyObject指针。

I guess you could use the Python C API to do that... maybe you could use a PyObject pointer.

编辑：正如在注释中指出的，已经有一个 py_object 类型可以在ctypes中使用，所以解决方案从python列表中创建一个 ctypes.py_object 对象，然后将其转换为 c_void_p 以将其作为参数传递到C函数（我认为这个步骤可能是不必要的，因为键入 void * 应该接受任何指针，并且它会更快地传递一个 byref ）。在回调中，完成相反的步骤（从void指针转换为 py_object 的指针，然后获取内容的值）。

edit: As the op pointed out in the comments, there's already a py_object type readily available in ctypes, so the solution is to create first a ctypes.py_object object from the python list and then casting it to c_void_p to pass it as an argument to the C function (I think this step might be unnecessary as a parameter typed as void* should accept any pointer, and it would be faster to pass just a byref). In the callback, the reverse steps are done (casting from the void pointer to a pointer to py_object and then getting the value of the contents).

一个解决方法可能是为你的回调函数使用一个闭包，因此它已经知道在哪个列表中它必须附加项...

A workaround could be to use a closure for your callback function so it already knows in which list it has to append the items...

myfunc = mylib.myfunc
myfunc.restype = c_int
myfuncFUNCTYPE = CFUNCTYPE(STRING)
myfunc.argtypes = [POINTER(c_char), callbackFUNCTYPE]


def mycb(result, userdata):
    userdata.append(result)

input="A large chunk of data."
userdata = []
myfunc(input, myfuncFUNCTYPE(lambda x: mycb(x, userdata)))

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