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

问题描述

C 函数 myfunc 对更大的数据块进行操作.结果以块的形式返回给回调函数:

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，从Python 代码调用myfunc 并将结果返回给Python 回调函数没什么大不了的.这个回调工作正常.

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)

但是，有没有办法将 Python 对象(比如列表)作为用户数据提供给回调函数?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，以便它可以在对 myfunc 的调用中使用.

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将链表实现为ctypes结构，比较麻烦.

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 来做到这一点……也许你可以使用 PyObject 指针.

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

edit:正如 op 在评论中指出的那样，ctypes 中已经有一个 py_object 类型，因此解决方案是先创建一个 ctypes.py_object 对象，然后将其转换为 c_void_p 以将其作为参数传递给 C 函数(我认为这一步可能是不必要的，因为作为参数键入为 void* 应该接受任何指针，并且只传递一个 byref 会更快).在回调中，完成了相反的步骤(从 void 指针转换为指向 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).

一种解决方法可能是为您的回调函数使用闭包，以便它已经知道它必须将项目附加到哪个列表中...

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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