Resultpointer in function call

Question

I want to use functions in dll's via ctype. I can call the function without errors and even the error code of the function is 0 meanig function successfuly finished. But when I try to acces the result variable ist is empty.

I have been implemented the lookup in free pascal severeal years ago and would transfer it to python right now. The interface allow to access via cdel convention and I tied to reimplement in python 3.7.4 with ctypes now

The last working Pascal Prototype have been:

PROCEDURE pGetCallInfo(DriveInfo: pointer; ACall: pointer; AInfo: pointer;
    var AErrorCode: SmallInt); pascal; external 'raccd32a.dll';

My best version in python have been the following:

from ctypes import *
callBookDLL = CDLL('raccd32a')
AInfo = create_string_buffer(400)
err = callBookDLL.cGetCallInfo("self.txt_CallBookPath.text()","DG1ATN",AInfo)

The result ist:

err 0 AInfo.value b''

AInfo should contain a max. 400 char long stringbuffer with an result containing Name, Adress and so on.

As I have a second library I have to acces same way I search for my fault but I was not able to find it. I think my problem is the work with pointer and the type conversion.

I checked teh ctypes howto allready but I can noht solve this trouble.

Thanks a lot so far ...

Answer 1

Check [Python 3.Docs]: ctypes - A foreign function library for Python. It contains (almost) every piece of info that you need.

There are a number of problems:

1. ctypes doesn't support pascal calling convention, only cdecl and stdcall (applies to 32bit only). That means (after reading the manual) that you shouldn't use the p* functions, but the c* (or s*)

2. You didn't specify argtypes (and restype) for your function. This results in UB. Some effects of this:

   • [SO]: Python ctypes cdll.LoadLibrary, instantiate an object, execute its method, private variable address truncated (@CristiFati's answer)
   • [SO]: python ctypes issue on different OSes (@CristiFati's answer)

3. It is a procedure (a function that returns void). Anyway this is a minor one

Here's some sample code (of course it's blind, as I didn't test it):

#!/usr/bin/env python3

import sys
import ctypes


dll = ctypes.CDLL("raccd32a.dll")

cGetCallInfo = dll.cGetCallInfo
cGetCallInfo.argtypes = [ctypes.c_char_p, ctypes.c_char_p, ctypes.c_char_p, ctypes.POINTER(ctypes.c_short)]
cGetCallInfo.restype = None

ADriveInfo = self.txt_CallBookPath.text().encode()
#ADriveInfo = b"C:\\callbook2019\\"  # Notice the double bkslashes
ACall = b"DG1ATN"
AInfo = ctypes.create_string_buffer(400)
result = ctypes.c_short(0)
cGetCallInfo(ADriveInfo, ACall, AInfo, ctypes.byref(result))

@EDIT0:

From the beginning, I wanted yo say that the 1^st argument passed to the function doesn't make much sense. Then, there are problems regarding the 2^nd one as well. According to the manual ([AMT-I]: TECHNICAL INFORMATION about RACCD32a.DLL (emphasis is mine)):

ADriveInfo, ACall and AInfo are pointers to zero-terminated strings. These strings has to exist at the moment of calling xGetCallInfo. The calling program is responsible for creating them. AInfo must be long enough to comfort xGetCallInfo (at least 400 characters). Note: "Length of AInfo" refers to the length of the string AInfo points at. ADriveInfo and ACall are treated in the same manner for short.

In ADriveInfo the procedure expects the path to the CD ROM drive. Use "G:\" if "G:" designates the CD ROM drive with the callbook CD ROM. Keep in mind that this information is a *must* and the calling program has to know it. Note: If the active directory on drive G: is not the root, ADriveInfo = "G:" will lead to an error 3. So always use "G:\". The calling program has to ensure that the length of ADriveInfo does not exceed 80 characters.

ACall contains the call you are looking for, all letters in lower case, no additional spaces etc. The calling program has to ensure that ACall is not longer than 15 characters. However, there is no call longer than 6 characters in the database.