Search code examples
pythonc++dllctypescpython

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)
Solution

  • 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 1st 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).
      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: