Import C dll function in python

Question

Which data type should be used in this stringUpcase function in my DLL file

void __cdecl stringUpcase(char IN_str[], char OUT_str[], int32_t len);

I am trying ctype.c_char_p in Python 3.6 and This function should return the uppercase string "HELO" in OUT_str variable.

dl = ctypes.cdll.LoadLibrary("path/of/dll/file.dll")

IN_str = 'helo'
OUT_str = ''
IN_len = len(IN_str)

dl.stringUpcase.restype = ctypes.c_void_p
dl.stringUpcase.argtypes = [ctypes.c_char_p, ctypes.c_char_p, ctypes.c_int32]

dl.stringUpcase(IN_str, OUT_str, IN_len);

Console error output is

 line 21, in <module>
    dl.stringUpcase(IN_str, OUT_str, IN_len);
ctypes.ArgumentError: argument 1: <class 'TypeError'>: wrong type

Thank you for any help you can provide.

strCnt or stringUpcase?

Mathieu
– Mathieu

2017-07-04 13:45:14 +00:00
Commented Jul 4, 2017 at 13:45 — Mathieu
– Mathieu, Commented Jul 4, 2017 at 13:45
sorry my mistake ... It is stringUpcase function

Michal Marcin
– Michal Marcin

2017-07-04 15:27:59 +00:00
Commented Jul 4, 2017 at 15:27 — Michal Marcin
– Michal Marcin, Commented Jul 4, 2017 at 15:27

tgregory · Accepted Answer · 2017-07-04 16:01:08Z

2

You are trying to pass python string as an argument of type c_char_p. Python3 unlike python2 is pretty strict about typing in ctypes.

The working sample would look like this.

import ctypes

ins = ctypes.c_char_p(b'helo') 
lens = len(ins.value)
outs = ctypes.create_string_buffer(lens+1)
lib = ctypes.cdll.LoadLibrary("./upper.so")

lib.stringUpcase.restype = None
lib.stringUpcase.argtypes = [ctypes.c_char_p, ctypes.c_char_p, ctypes.c_int32]

lib.stringUpcase(ins, outs, lens)

print(outs.value)

Pay attention to the fact that c_char_p points to a zero-terminated string, so len argument is redundant here. Moreover to construct c_char_p you need to pass bytes object or and integer address rather than just a string, to be able to use string as an argument you need to use c_wchar_p type and wchar_t* in your library respectively.

One more thing to pay attention to is the fact that your C function does not allocate memory so you need for outs to be large enough to contain the result.

answered Jul 4, 2017 at 16:01

tgregory

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2 Comments

Michal Marcin Over a year ago

Thank you it works really good, but the output is b'HELO' and I would like to transform it back to "python" string. so the result will be only HELO How is it possible?

tgregory Over a year ago

@MichalMarcin decode them in some encoding, like this print(outs.value.decode('ascii'))

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