ctypes.ArgumentError when calling DLL functions with Python

Question

I'm using ctypes to call functions in a DLL file according to a description file that describe the DLL functions' parameters and returns. Here is one function in this DLL called InitNetwork. Below is its description:

Function：BOOL InitNetwork(char  LocalIP[],char  ServerIP[],int  LocalDeviceID);
Arguments：LocalIP
           ServerIP
           LocalDeviceID
Return：Success：TRUE;
        Faile：FALSE;

What I'm doing in Python is like this:

from ctypes import *
import os

#Load Dll:
cppdll = CDLL("C:\\VS_projects\\MusicServer_Flask\\NetServerInterface.dll")

#Get the function：
initnetwork = getattr(cppdll, "?InitNetwork@@YAHQAD0H@Z")  # The function can be successfully accessed.

#specify arguments' types:
initnetwork.argtypes = [c_char_p,c_char_p,c_int]

After this, I try:

initnetwork("192.168.1.103", "192.168.1.103", 1111)

Get an Error:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ctypes.ArgumentError: argument 1: <class 'TypeError'>: wrong type

If I try this:

initnetwork(LocalIP = "192.168.1.103", ServerIP = "192.168.1.103", LocalDeviceID = 1111)

I will get:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: this function takes at least 3 arguments (0 given)

Why these errors comes out? How can I call this function successfully? Any suggestion is appreciated. Thank you for your attention!

Answer 1

ctypes official doc: [Python 3.5]: ctypes - A foreign function library for Python.

Created a dummy .dll to mimic your behavior.

dll.c:

#include <stdio.h>
#include <Windows.h>


__declspec(dllexport) BOOL InitNetwork(char LocalIP[], char ServerIP[], int LocalDeviceID) {
    printf("From C:\n\tLocalIP: [%s]\n\tServerIP: [%s]\n\tLocalDeviceID: %d\n", LocalIP, ServerIP, LocalDeviceID);
    return TRUE;
}

Check [SO]: How to compile a 64-bit dll written in C? (@CristiFati's answer) for details how to build it (took the exact same steps, command in this case was: cl /nologo /LD /DWIN64 /DWIN32 /Tp dll.c /link /OUT:NetServerInterface.dll).

The (main) problem is that in Python3, char* no longer maps to a string, but to a bytes object.

code.py:

import sys
import ctypes
from ctypes import wintypes


FILE_NAME = "NetServerInterface.dll"
FUNC_NAME = "?InitNetwork@@YAHQEAD0H@Z"  # Name different than yours: if func was declared as `extern "C"`, the "normal" name would be required


def main():
    net_server_interface_dll = ctypes.CDLL(FILE_NAME)
    init_network = getattr(net_server_interface_dll, FUNC_NAME)

    init_network.argtypes = [ctypes.c_char_p, ctypes.c_char_p, ctypes.c_int]
    init_network.restype = wintypes.BOOL

    init_network(b"192.168.1.103", b"192.168.1.103", 1111)


if __name__ == "__main__":
    print("Python {:s} on {:s}\n".format(sys.version, sys.platform))
    main()

Output:

(py35x64_test) e:\Work\Dev\StackOverflow\q050325050>"e:\Work\Dev\VEnvs\py35x64_test\Scripts\python.exe" code.py
Python 3.5.4 (v3.5.4:3f56838, Aug  8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)] on win32

From C:
        LocalIP: [192.168.1.103]
        ServerIP: [192.168.1.103]
        LocalDeviceID: 1111

Answer 2

I think the problem is in your C++ interface. I think if you change it to Function：BOOL InitNetwork(char * LocalIP,char * ServerIP,int LocalDeviceID);

your first call (without the keywords) should work but I think you might need to remove the quotes around the last argument (it is an int after all).