How to pass the StringBuilder parameter in C# to ctypes in python, which is not the normal ctypes supported datatype

Question

I am working on the project, which there is c# project use the dll like that:

    public string GetMachineKey()
    {
        StringBuilder buff = new StringBuilder();
        buff.Length = 128;
        ZwCommDll.GetCPUMachineKey(buff, 128);
        string mk = buff.ToString();
        return mk;
    }

and I want do it similily in python use the ctypes.

But i am so comfused by the StringBuilder DataType.

Thanks very much your help.

You need to add a bit more information to your question. I think you mean to port the C# code to python. I also think you are importing code from a dll, but you didn't specify what dll that may be. You are also referencing ZwCommDll but it is not explained where this comes from. — Hintham
– Hintham, Commented Oct 29, 2018 at 8:35
The Dll is from anthoer company, I can not tell more info about it. — wanze
– wanze, Commented Oct 29, 2018 at 8:39
The basic ctypes datatype how to pass it I already know it . My problem is focused on the StringBuilder Datatype. — wanze
– wanze, Commented Oct 29, 2018 at 8:42
@wanze Even if the DLL is from another company you should have a header file describing the parameter and return types. — Mark Tolonen
– Mark Tolonen, Commented Oct 29, 2018 at 15:49

Mark Tolonen · Accepted Answer · 2018-10-29 15:54:28Z

1

Use ctypes.create_unicode_buffer() to generate a writable text string buffer (wchar_t*) for an API. Use ctypes.create_string_buffer() for a writable byte string buffer (char*). Something like the following should work if the function takes a char*:

>>> import ctypes
>>> buff = ctypes.create_string_buffer(128)
>>> ZwCommDll.GetCPUMachineKey(buff,128)
>>> buff.value
b'<returned string>'

edited Oct 29, 2018 at 15:54

answered Oct 29, 2018 at 15:48

Mark Tolonen

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2 Comments

wanze Over a year ago

Thanks that's should be the answer, if the dll function doesnot have some problem. but here I still got some error. Did you know what's the situation going on? Traceback (most recent call last): File "tests\test_dll2.py", line 21, in <module> lib_tool.GetCPUMachineKey(buff,128) ValueError: Procedure probably called with not enough arguments (84 bytes missing)

Mark Tolonen Over a year ago

@wanze Use WinDLL instead of CDLL. If you still have issues update your question with your Python code. You should do that anyway otherwise we’re just guessing. See the minimal reproducible example guidelines. It would help to see the function prototype as well.

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