Python ctypes to return an array of function pointers

Question

I am working with a .dll that contains a single call which returns an array of function pointers. GetMyApi() returns a pointer to a struct, which is an array of function pointers. The functions themselves have different individual inputs and outputs. What I have tried so far:

C code that I can't easily alter:

• Code in C :

  typedef struct My_Api_V2
  {
      int                 (__cdecl *IsValidInt)(int i);
      int                 (__cdecl *InvalidInt)();
      int                 (__cdecl *IsValidSize)(size_t i);
  } my_Api_V2;
  
  const my_Api_V2* GetMyApi(int version);   // This function is accessed from DLL
  

• Python effort:

  from ctypes import *
  
  my_dll = cdll.LoadLibrary(path_to_my_dll)
  my_api = my_dll.GetMyApi
  my_api.argtypes[c_int]  #version number
  my_api.restypes = c_void_p
  
  firstfuncptr = my_api(2)
  firstfunc = prototype(firstfuncptr)
  firstfunc.argtypes[c_int]
  firstfunc.restypes = c_int
  
  test = firstfunc(23)
  

At this point, I am just trying to get the first function of the function list returned to work. Any help pointing me in better direction is appreciated.

Answer 1

Things are not as easy as one might think at 1^st glance. I'm going to post a dummy example that happens to contain the 2 ways of working with functions from .dlls (.sos) (as explained in [Python 3.Docs]: ctypes - A foreign function library for Python).

dll00.c:

#include <stdio.h>

#if defined(_WIN32)
#  define DLL00_EXPORT __declspec(dllexport)
#  pragma warning(disable: 4477)  // !!! Just to avoid having additional code (macro for size_t), do NOT do this !!!
#else
#  define DLL00_EXPORT
#endif

#define PRINT_MSG_0() printf("        [%s] (%d) - [%s]\n", __FILE__, __LINE__, __FUNCTION__)
#define PRINT_MSG_1I(ARG0) printf("        [%s] (%d) - [%s]:  ARG0: %d\n", __FILE__, __LINE__, __FUNCTION__, ARG0)


static int IsValidInt(int i) {
    PRINT_MSG_1I(i);
    return -i;
}

static int InvalidInt() {
    PRINT_MSG_0();
    return 0;
}

static int IsValidSize (size_t i) {
    PRINT_MSG_1I(i);
    return -i;
}


typedef struct DllInterfaceV2Struct {
    int (__cdecl *IsValidIntFuncPtr)(int i);
    int (__cdecl *InvalidIntFuncPtr)();
    int (__cdecl *IsValidSizeFuncPtr)(size_t i);
} DllInterfaceV2;


static DllInterfaceV2 intfV2 = { IsValidInt, InvalidInt, IsValidSize };


#if defined(__cplusplus)
extern "C" {
#endif

DLL00_EXPORT const DllInterfaceV2 *GetInterfaceV2(int version);

#if defined(__cplusplus)
}
#endif


DLL_EXPORT const DllInterfaceV2 *GetInterfaceV2(int version) {
    if (version == 2) {
        return &intfV2;
    } else {
        return NULL;
    }
}

code00.py:

#!/usr/bin/env python3

import sys
import ctypes


DLL_NAME = "test00.dll"
DLL_FUNC_NAME = "GetInterfaceV2"

# "Define" the Python counterparts for C stuff in order to be able to use it

IsValidIntFuncPtr = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_int)
InvalidIntFuncPtr = ctypes.CFUNCTYPE(ctypes.c_int)
IsValidSizeFuncPtr = ctypes.CFUNCTYPE(ctypes.c_int, ctypes.c_size_t)

class DllInterfaceV2(ctypes.Structure):
    _fields_ = [
        ("is_valid_int", IsValidIntFuncPtr),
        ("invalid_int", InvalidIntFuncPtr),
        ("is_valid_size", IsValidSizeFuncPtr)
    ]


# Now, play with C stuff

def test_interface_ptr(intf_ptr):
    print("Testing returned interface: {:}\n".format(intf_ptr))
    if not intf_ptr:
        print("    NULL pointer returned from C\n")
        return
    intf = intf_ptr.contents  # Dereference the pointer
    res = intf.is_valid_int(-2718281)
    print("    `is_valid_int` member returned: {:d}\n".format(res))
    res = intf.invalid_int()
    print("    `invalid_int` member returned: {:d}\n".format(res))
    res = intf.is_valid_size(3141592)
    print("    `is_valid_size` member returned: {:d}\n\n".format(res))


def main():
    test_dll = ctypes.CDLL(DLL_NAME)
    get_interface_v2_func = getattr(test_dll, DLL_FUNC_NAME)  # Or simpler: test_dll.GetInterfaceV2
    get_interface_v2_func.argtypes = [ctypes.c_int]
    get_interface_v2_func.restype = ctypes.POINTER(DllInterfaceV2)

    pintf0 = get_interface_v2_func(0)
    test_interface_ptr(pintf0)
    pintf2 = get_interface_v2_func(2)
    test_interface_ptr(pintf2)


if __name__ == "__main__":
    print("Python {:s} on {:s}\n".format(sys.version, sys.platform))
    main()

Notes:

• C part:

  • I had to add some dummy code in order to test and illustrate the behavior
  • Although you mentioned that it's not modifiable, I changed stuff (mostly naming / coding style, ...):
    • Both letter cases + underscores don't look nice (at least to me)
    • "my" (or any of its variants) in (functions, classes, or any other) names simply scratches my brain

• Python part:

  • As I stated in my comment, the C stuff would have to be "duplicated" in Python

• Although I consider this a major design flaw, in order to keep things as close as possible to the question, I simply followed it (GetInterfaceV2 (the V2 part) doesn't make any sense considering its arg (version))

  • My personal opinion (without having all the context, though) is that (in order to ensure scalability) the function should return a generic structure, with an additional field (e.g. version) that could be checked by client apps.

Output:

(py35x64_test) e:\Work\Dev\StackOverflow\q051507196>"c:\Install\x86\Microsoft\Visual Studio Community\2015\vc\vcvarsall.bat" x64

(py35x64_test) e:\Work\Dev\StackOverflow\q051507196>dir /b
code00.py
dll00.c

(py35x64_test) e:\Work\Dev\StackOverflow\q051507196>cl /nologo dll00.c  /link /DLL /OUT:test00.dll
dll00.c
   Creating library test00.lib and object test00.exp

(py35x64_test) e:\Work\Dev\StackOverflow\q051507196>dir /b
code00.py
dll00.c
dll00.obj
test00.dll
test00.exp
test00.lib

(py35x64_test) e:\Work\Dev\StackOverflow\q051507196>"e:\Work\Dev\VEnvs\py35x64_test\Scripts\python.exe" code00.py
Python 3.5.4 (v3.5.4:3f56838, Aug  8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)] on win32

Testing returned interface: <__main__.LP_DllInterfaceV2 object at 0x00000219984EBAC8>

   NULL pointer returned from C

Testing returned interface: <__main__.LP_DllInterfaceV2 object at 0x00000219984EBB48>

        [dll00.c] (16) - [IsValidInt]:  ARG0: -2718281
    `is_valid_int` member returned: 2718281

        [dll00.c] (22) - [InvalidInt]
    `invalid_int` member returned: 0

        [dll00.c] (28) - [IsValidSize]:  ARG0: 3141592
    `is_valid_size` member returned: -3141592