ctypes, python3.8 : OSError: exception: access violation writing 0x00000000

i want to run a dll via python 3.8 i have a function in my dll part which is :

void DLLAPI __stdcall GenerateReport (
    int             SeqContext,
    char*           szHost,
    char*           szDBName,
    char*           szUser,
    char*           szPasswd,
    const char*     szUut_Result_Id,
    char            szStylesheet[MAX_PATHNAME_LEN],
    char            szDestinationDir[MAX_DIRNAME_LEN],
    char            szPrefix[MAX_FILENAME_LEN],
    char            szFileName[MAX_PATHNAME_LEN],
    char            szPathName[MAX_PATHNAME_LEN],
    short*          psErrorOccurred,
    long*           plErrorCode,
    char            szErrorMessage[1024]
    );

For a python part i did like this :

def PrintReport():
    szNom_PV_Court = ''
    szErrormessage = ''
    sErrorok = 0
    lErrorcode = 0

    worker_ = cdll.LoadLibrary("D:\\users\\Worker.dll")

    if (worker_ == 0):
        print( " Could not open file DLL")

    worker_.GenerateReport( 0, "localhost" ,  "backend" , "user" , "test12", str( Uut_Result_Id ) , "D:\\users\\py\\ResourceFiles\\Gen.xsl" , "D:\\users\\py","" ,szNom_PV_Court , "D:\\users\\py" ,sErrorok ,lErrorcode ,szErrormessage )

when i execute my code i get error : OSError: exception: access violation writing 0x0000000

I'dont understand this error please help

Strings like "localhost" in Python 3 are Unicode strings and translated to wchar_t* by ctypes. Use b'localhost' for byte strings that are translated to char*.

Also, define .argtypes and .restype for your function to for ctypes to marshal your parameters properly:

worker_.argtypes = c_int,c_char_p,c_char_p,c_char_p,c_char_p,c_char_p,c_char_p,c_char_p,c_char_p,c_char_p,c_char_p,POINTER(c_short),POINTER(c_long),c_char_p
worker_.restype = None

You also only have 13 parameters in your call but 14 are defined for the function.

Use the following for the short* and long* to create instances that can be passed to the call. Right now you are passing a zero as the default c_int which is probably the source of the null pointer exception.

sErrorok = c_short()
lErrorcode = c_long()

Call using byref(sErrorok) and byref(lErrorcode) to pass to the call as pointers.

Here's a complete example. I skipped the many input strings and just show one for reference:

test.c

#include <stdio.h>
#include <string.h>

#define DLLAPI __declspec(dllexport)

void DLLAPI __stdcall GenerateReport (
    int             SeqContext,
    char*           szHost,
    short*          psErrorOccurred,
    long*           plErrorCode,
    char            szErrorMessage[1024]
    )
{
    printf("SeqContext = %d szHost = %s\n",SeqContext,szHost);
    *psErrorOccurred = 5;
    *plErrorCode = 123;
    strcpy_s(szErrorMessage,1024,"error message");
}

test.py

from ctypes import *

szNom_PV_Court = ''
szErrormessage = ''
sErrorok = 0
lErrorcode = 0

dll = WinDLL('./test.dll')
dll.GenerateReport.argtypes = c_int,c_char_p,POINTER(c_short),POINTER(c_long),c_char_p
dll.GenerateReport.restype = None

err_occurred = c_short()
errcode = c_long()
errmsg = create_string_buffer(1024)

dll.GenerateReport(7,b'localhost',byref(err_occurred),byref(errcode),errmsg)
print(err_occurred.value,errcode.value,errmsg.value.decode())

Output:

SeqContext = 7 szHost = localhost
5 123 error message