在回调中回播ctypes.py_object

Question

我正在尝试使用ctypes包装一个C库。该库的一个功能是ondestroy回调，当该库返回的句柄即将被销毁时会调用该回调。

回调具有签名：

void cb(f *beingdestroyed);

void *当库返回API时，该API允许将用户指定的内容与f 关联。因此，我可以关联用于将其包装为用户数据的py_object。我的计划是要有一个is_valid字段，当触发回调以提取user_data并将此字段设置为false时。

我的问题是如何着手提取我的高级知识py_object；我可以将用户数据转换为a ctypes.void_p并转换为a，ctypes.py_object但是随后我只有Python C API可以使用。可以通过写入将内容转发到我可以使用的高级对象user_object.is_valid = 0。

Answer 1

详细阐述托马斯·海勒的答案：

• 回调原型应c_void_p为context参数指定
• 库函数的argtypes应该py_object为context参数指定
• 该函数应使用 py_object(my_python_context_object)
• 您的回调函数的Python实现应将上下文转换为py_object，并提取其值： cast(context, py_object).value

这是一个工作示例。从一个简单的DLL的C源代码开始：

// FluffyBunny.c
// Compile on windows with command line
//      cl /Gd /LD FluffyBunny.c
// Result is FluffyBunny.DLL, which exports one function:
//      FluffyBunny() uses __cdecl calling convention.

#include <windows.h>

BOOL APIENTRY DllMain(HMODULE, DWORD, LPVOID) {
  return TRUE;
}

typedef int (*FLUFFYBUNNY_CALLBACK)(void *context);

__declspec(dllexport) int FluffyBunny(FLUFFYBUNNY_CALLBACK cb, void *context) {
  int result = 0;
  int count = 0;
  if (cb) {
    while (result == 0) {
      result = (*cb)(context);
      ++count;
    }
  }
  return count;
}

这是一个调用DLL的Python程序：

# FluffyBunny.py
from ctypes import *

# Declare a class that will be used for context info in calls to FluffyBunny()
class Rabbit:
    def __init__(self):
        self.count = 0

# FluffyBunny() wants a callback function with the following C prototype:
#     typedef int (*FLUFFYBUNNY_CALLBACK)(void *context);
FLUFFYBUNNY_CALLBACK = CFUNCTYPE(c_int, c_void_p)

# This DLL has been compiled with __cdecl calling convention.
FluffyBunny_dll = CDLL('FluffyBunny.dll')

# Get the function from the library. Its C prototype is:
#     int FluffyBunny(FLUFFYBUNNY_CALLBACK cb, void *context);
# Note that I use "py_object" instead of "c_void_p" for the context argument.
FluffyBunny          = FluffyBunny_dll.FluffyBunny
FluffyBunny.restype  = c_int
FluffyBunny.argtypes = [FLUFFYBUNNY_CALLBACK, py_object]

# Create Python version of the callback function.
def _private_enumerateBunnies(context):
    # Convert the context argument to a py_object, and extract its value.
    # This gives us the original Rabbit object that was passed in to 
    # FluffyBunny().
    furball = cast(context, py_object).value
    # Do something with the context object.
    if furball:
        furball.count += 1
        print 'furball.count =', furball.count
        # Return non-zero as signal that FluffyBunny() should terminate
        return 0 if (furball.count < 10) else -1
    else:
        return -1

# Convert it to a C-callable function.
enumerateBunnies = FLUFFYBUNNY_CALLBACK(_private_enumerateBunnies)

# Try with no context info.
print 'no context info...'
result = FluffyBunny(enumerateBunnies, None)
print 'result=', result

# Give it a Python object as context info.
print 'instance of Rabbit as context info...'
furball = Rabbit()
result = FluffyBunny(enumerateBunnies, py_object(furball))
print 'result=', result