Python ctypes, char** and DLL

I have a dll function in c++:

void get_DLLVersion(CAxClass* obj, char ** pVal);

In pVal get_DLLVersion write c string like "1.0.0.1"

In c++ its like:

char *strdll = (char*)malloc(50);
	
	get_DLLVersion(tst, &strdll);

	cout << "strdll = "<<strdll<<endl;

I need to use this function in python.

The main problem is how to create char** and put as 2nd argument of dll function.

I use next code:

import ctypes

libc = ctypes.CDLL("AxECR.so")
ecr = libc.create_object() #return CAxClass* obj
print (libc.get_DLLVersion) 
libc.get_DLLVersion.argtypes = [c_void_p, ctypes.POINTER(ctypes.POINTER(c_char))]
dll = ctypes.POINTER(ctypes.POINTER(c_char))
libc.get_DLLVersion(ecr,dll) #don`t work Segmentation fault (core dumped)

Listing [Python.Docs]: ctypes - A foreign function library for Python.

Notes:

• To fix this, a buffer (array) can be created via create_string_buffer, then its address passed (via byref) to the function

  • An explicit cast (from char array to char pointer) is required

• For the 1^st argument, I create singleton CAxClass object that is returned by every createObject call. I could also have the function creating the new instance, but another one would be then required to destroy it, in order to prevent memory leaks ⁽¹⁾

• Looking at the way the function is called from C++, it just populates the memory at the address given as an argument (if not NULL, hopefully).<br>
  In this case, using a double pointer doesn't make much sense, as the same goal could be achieved using a simple one (I added another function in the example below to prove this)

Example:

• dll00.cpp:

  #include <cstring>
  #include <iostream>
  
  #if defined(_WIN32)
  #  define DLL00_EXPORT_API __declspec(dllexport)
  #else
  #  define DLL00_EXPORT_API
  #endif
  
  #define BUF_LEN 50
  
  
  class CAxClass {};
  
  static CAxClass *gObj = new CAxClass();  // nullptr;
  
  
  #if defined(__cplusplus)
  extern "C" {
  #endif
  
  DLL00_EXPORT_API void* createObject();
  DLL00_EXPORT_API void dllVersion(CAxClass *pObj, char **ppVer);
  DLL00_EXPORT_API void dllVersionSinglePtr(CAxClass *pObj, char *pVer);
  
  #if defined(__cplusplus)
  }
  #endif
  
  
  void* createObject() {
      return gObj;
  }
  
  
  void dllVersion(CAxClass *pObj, char **ppVer)
  {
      if ((ppVer) && (*ppVer)) {
          strncpy(*ppVer, "1.22.333.4444", BUF_LEN);
      } else {
          std::cout << "C - NULL pointer\n";
      }
  }
  
  
  void dllVersionSinglePtr(CAxClass *pObj, char *pVer)
  {
      if (pVer) {
          strncpy(pVer, "55555.666666.7777777.88888888", BUF_LEN);
      } else {
          std::cout << "C - NULL pointer\n";
      }
  }
  

• code00.py:

  #!/usr/bin/env python
  
  import ctypes as cts
  import sys
  
  
  CharPtr = cts.c_char_p  # More generic: cts.POINTER(cts.c_char) ?
  CharPtrPtr = cts.POINTER(CharPtr)
  
  BUF_LEN = 50
  
  DLL_NAME = "./dll00.{:s}".format("dll" if sys.platform[:3].lower() == "win" else "so")
  
  
  def main(*argv):
      dll = cts.CDLL(DLL_NAME)
  
      createObject = dll.createObject
      createObject.argtypes = ()
      createObject.restype = cts.c_void_p
  
      dllVersion = dll.dllVersion
      dllVersion.argtypes = (cts.c_void_p, CharPtrPtr)
      dllVersion.restype = None
  
      # @TODO - cfati: Testing purposes
      dllVersionSinglePtr = dll.dllVersionSinglePtr
      dllVersionSinglePtr.argtypes = (cts.c_void_p, CharPtr)
      dllVersionSinglePtr.restype = None
  
      obj = createObject()
      print("Object: {:}".format(obj))
  
      buf = cts.create_string_buffer(BUF_LEN)
      dllVersion(obj, cts.byref(cts.cast(buf, CharPtr)))
      print("Version: {:}".format(buf.value))
  
      dllVersionSinglePtr(obj, cts.cast(buf, CharPtr))
      print("Version: {:}".format(buf.value))
  
  
  if __name__ == "__main__":
      print("Python {:s} {:03d}bit on {:s}\n".format(" ".join(elem.strip() for elem in sys.version.split("\n")),
                                                     64 if sys.maxsize > 0x100000000 else 32, sys.platform))
      rc = main(*sys.argv[1:])
      print("\nDone.\n")
      sys.exit(rc)
  

output:

>
> (qaic-env) [cfati@cfati-5510-0:/mnt/e/Work/Dev/StackOverflow/q075446745]> ~/sopr.sh
> ### Set shorter prompt to better fit when pasted in StackOverflow (or other) pages ###
>
> [064bit prompt]> ls
> code00.py dll00.cpp
> [064bit prompt]>
> [064bit prompt]> g++ -fPIC -shared -o dll00.so dll00.cpp
> [064bit prompt]>
> [064bit prompt]> ls
> code00.py dll00.cpp dll00.so
> [064bit prompt]>
> [064bit prompt]> python ./code00.py
> Python 3.8.10 (default, Nov 14 2022, 12:59:47) [GCC 9.4.0] 064bit on linux
>
> Object: 34716928
> Version: b'1.22.333.4444'
> Version: b'55555.666666.7777777.88888888'
>
> Done.
>

Might also check:

• [SO]: C++ & Python: Pass and return a 2D double pointer array from python to c++ (@CristiFati's answer) for the use of a double pointer when passing a 2D array

• [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer) for a common pitfall when working with CTypes (calling functions)

• [SO]: Python ctypes cdll.LoadLibrary, instantiate an object, execute its method, private variable address truncated (@CristiFati's answer) for an example of footnote #1