Reading double c structures of Dll with Ctypes in Python

Question

I'v done researched a lot about this problem.. But there's no where and I couldn't find it. I'm trying to call double c structure by calling c dll.

My question is, did i right way to declare "Class Structure" in python? I couldn't think that i'm right on my way. Because even though the Functions that I want to call from dll, It didn't come output anything.

[Visual C++/C]

I did try to C Syntax code,

typedef sturct {
    int nBoardNum;
    struct{
        char  pBoardName[16];
        int   nBoardID;
    }BOARDINDEX[8];
}AAPBOARDINFO, *PAAPBOARDINFO;

HANDLE AcapOpen(char* cpBoardName, int nBoardNo, int nCh)

[Python]

I changed Python Syntax like this.

import ctypes as c

class BOARDINDEX(c.Structure):
    _field_ = [("nBoardName", c.c_char_p * 16),("nBoardID", c.c_int)]

class AAPBOARDINFO(c.Structure):
    _field_ = [("nBoardNum", c.c_int), ("BOARDINDEX", BOARDINDEX * 8)]

AapLib2 = c.WinDLL("AapLib2.dll")

BoardName = ["ABC","FWD","HGW"]
BoardNo = 0
ch = 1

output = Open(BoardName, BoardNo, ch)

def Open(BoardName, BoardNo, ch)

    func = AapLib2.AcapOpen
    func.argtypes = [c.POINTER(BOARDINDEX),c.c_int, c.c_int]
    func.restype = c.c_int

    ref = BOARDINDEX()

    res = func(c.byref(ref.nBoardName),BoardNo, ch)
    return res

Nothing outcomes when call Open() function...

please consider my request and any answer would be great...

Answer 1

Everything you need to know, can be found in the [Python 3.Docs]: ctypes - A foreign function library for Python .

There are a couple of problems with the code:

• Structure members are specified in the _fields_ ( not _field_ ) attribute
• char pBoardName[16] maps to ctypes.c_char * 16 ( not c_char_p )
• HANDLE should be mapped to wintypes.HANDLE . This kind of mismatch can (and most likely will) generate:
  • [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)
• Function prototype differs between C and Python
• Using globals like AapLib2 is best to be be avoided, but I left them unchanged as they are outside the question scope

Here's a modified version of your code. Needless to say that I didn't actually test it, as I don't have the .dll :

#!/usr/bin/env python3

import sys
import ctypes
from ctypes import wintypes


class BOARDINDEX(ctypes.Structure):
    _fields_ = [
        ("nBoardName", ctypes.c_char * 16),
        ("nBoardID", ctypes.c_int),
    ]


class AAPBOARDINFO(ctypes.Structure):
    _fields_ = [
        ("nBoardNum", ctypes.c_int),
        ("BOARDINDEX", BOARDINDEX * 8),
    ]


def open_board(board_name, board_no, ch):
    AcapOpen = aaplib2.AcapOpen
    AcapOpen.argtypes = [ctypes.c_char_p, ctypes.c_int, ctypes.c_int]
    AcapOpen.restype = wintypes.HANDLE
    ref = BOARDINDEX(board_name, board_no)  # Probably this line should be replaced by the 3 (commented) ones below (AcapGetBoardInfo prototype would have to be specified as well)
    #abi = AAPBOARDINFO()
    #AcapGetBoardInfo(ctypes.byref(abi))
    #ref = abi.BOARDINDEX[0]
    res = AcapOpen(ref.nBoardName, ref.nBoardID, ch)
    return res


def main():
    board_names = ["ABC", "FWD", "HGW"]
    board_no = 0
    ch = 1
    aaplib2 = ctypes.WinDLL("AapLib2.dll")
    output = open_board(board_names[0], board_no, ch)
    print(output)


if __name__ == "__main__":
    print("Python {0:s} {1:d}bit on {2:s}\n".format(" ".join(item.strip() for item in sys.version.split("\n")), 64 if sys.maxsize > 0x100000000 else 32, sys.platform))
    main()
    print("\nDone.")

Let me know how this works out for you.