How to pass a string buffer from python function to C Api via ctype

First, you import the ctypes module. Then you need to load your c dll containing the said function. After that you need to set the argument types and the result type of that function. Finally you create your destination buffer and call your function.

Python:

import ctypes as ct

MAX_BUFSIZE = 100
  
mycdll = ct.CDLL("path_to_your_c_dll")  # for windows you use ct.WinDLL(path)

mycdll.function_name.argtypes = [ct.c_char_p, ct.c_int,
                                 ct.c_char_p, ct.c_char_p]

mycdll.function_name.restype = ct.c_int

mystrbuf = ct.create_string_buffer(MAX_BUFSIZE)
result = mycdll.function_name(mystrbuf, len(mystrbuf), 
                              b"my_input", b"my_second_input")

Working example using strncpy:

import ctypes as ct

MAX_BUFSIZE = 100

mycdll = ct.CDLL("libc.so.6")  # on windows you use cdll.msvcrt, instead

mycdll.strncpy.argtypes = [ct.c_char_p, ct.c_char_p, ct.c_size_t]

mycdll.strncpy.restype = ct.c_char_p

mystrbuf = ct.create_string_buffer(MAX_BUFSIZE)
dest = mycdll.strncpy(mystrbuf, b"my_input", len(mystrbuf))

print(mystrbuf.value)

Python 3 output:

user@Mint20:~/Dokumente/Programmieren/sites/Stackoverflow$ python3 --version
Python 3.8.5

user@Mint20:~/Dokumente/Programmieren/sites/Stackoverflow$ python3 python_ctypes.py 
b'my_input'

Python 2 output:

user@Mint20:~/Dokumente/Programmieren/sites/Stackoverflow$ python2 --version
Python 2.7.18

user@Mint20:~/Dokumente/Programmieren/sites/Stackoverflow$ python2 python_ctypes.py 
my_input