Question

I am trying to use a dll in Python source using the ctypes. I began reading it from Python documentation as I am new to this. After successfully loading the dll in Python, when I try to pass a string to a function which expects a char *, it gives me

“ValueError: Procedure probably called with too many arguments (4
bytes in excess)”.

I also tried looking at other posts but couldn’t resolve the problem.

I tried different approaches to pas this string such as using byref() and pointer() but it didn’t change the outcome. I also tried with WINFUNCTYPE but failed. The dll that I’m using is windll.

Here’s a test program I wrote in python:

from ctypes import *

lpDLL=WinDLL("C:\some_path\myDll.dll")
print lpDLL

IP_ADDR = create_string_buffer('192.168.100.228')
#IP_ADDR = "192.168.100.228"
#IP_ADDR = c_char_p("192.168.100.228")

print IP_ADDR, IP_ADDR.value

D_Init=lpDLL.D_Init
D_InitTester=lpDLL.D_InitTester

#D_InitTesterPrototype = WINFUNCTYPE(c_int, c_char_p)
#D_InitTesterParamFlags = ((1, "ipAddress", None),)
#D_InitTester = d_InitTesterPrototype(("D_InitTester", lpDLL), D_InitTesterParamFlags)

try:
    D_Init()
    D_InitTester("192.168.100.254")
except ValueError, msg:
    print "Init_Tester Failed"
    print msg

Here’s how the D_InitTester is implemented in a cpp file which is available in dll exports,

D_API int D_InitTester(char *ipAddress)
{
    int err = ERR_OK;

    if (LibsInitialized)
    {
        ...
        some code;
        ...

        else
        {
            err = hndl->ConInit(ipAddress);
        }

        if ( 0 < err )
        {
            err = ERR_NO_CONNECTION;
        }
        else
        {
        nTesters = 1;
            InstantiateAnalysisClasses();
            InitializeTesterSettings();
            if(NULL != hndl->hndlFm)
            {
                FmInitialized = true;
            }
        }
    }
    else
    {
        err = ERR_NOT_INITIALIZED;
    }
    return err;
}

Your help is greatly appreciated.

Answer 1

Most likely the cause of the error is a mismatch of calling conventions. I’m guessing your C++ DLL exports functions with cdecl convention but your use of WinDLL implies stdcall.

Create your library like this to use cdecl:

lpDLL=CDLL("C:\some_path\myDll.dll")