python ctypes callback c++

Question

C ++ interface： unsigned long _stdcall NAPI_JsonCommand(char* szCommand, unsigned long* pulRet, char* pBuf, int nLen, unsigned long* pulDevCode);

Callback function definition： typedef long (_stdcall fMsgCallback)(int nMsg, void pUserData, char* pBuf, int nLen, int nParam);

python(3.5.2) code

#coding=utf-8
__author__ = 'wjyang'
import ctypes
from ctypes import *
import json
dll = ctypes.WinDLL("napi.dll")
CMPFUNC=WINFUNCTYPE(c_long,c_int,c_void_p,c_char_p,c_int,c_int)
def py_cmp_func(nMsg,pUserData,pBuf,nLen,nParam):
    print("this is callback")
    return 1

addr=hex(id(CMPFUNC(py_cmp_func)))

dict={"KEY":"CONNECTREGISTER","PARAM":{"CALLBACK":addr,"AUTOPIC":0,"IP":"192.168.150.30","PORT":5556}}

dicts=json.dumps(dict)

commd=c_char_p(dicts.encode())

print(dll.NAPI_JsonCommand(commd,c_int(0),None,c_int(0),None))

、

The result is that the callback function py_cmp_func is not executed

if used

 dict={"KEY":"CONNECTREGISTER","PARAM":{"CALLBACK":CMPFUNC(py_cmp_func),"AUTOPIC":0,"IP":"192.168.150.30","PORT":5556}}
 dicts=json.dumps(dict)

will be wrong! TypeError: WinFunctionType object at 0x021BB8A0 is not JSON serializable

Callback function in the json string, how the callback function memory address passed to c ++?

Answer 1

Not sure this will work, but I see two problems:

addr=hex(id(CMPFUNC(py_cmp_func)))

In the above line, CMPFUNC(py_cmp_func) is destroyed after executing the line because there will be no more references. Also, id() gives the address of the CMPFUNC object, not the internal function pointer. Try either:

callback = CMPFUNC(py_cmp_func) # keep a reference
addr = addressof(callback)      # Not sure you need hex...

or use a decorator which will keep the reference to the function as well:

@WINFUNCTYPE(c_long,c_int,c_void_p,c_char_p,c_int,c_int)
def py_cmp_func(nMsg,pUserData,pBuf,nLen,nParam):
    print("this is callback")
    return 1

addr = addressof(py_cmp_func) # Again, not sure about hex.