Question

I am trying this simple ctypes example and getting the error mentioned

>>> from ctypes import create_string_buffer
>>> str = create_string_buffer("hello")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "C:\Python32\lib\ctypes\__init__.py", line 59, in create_string_buffer
buf.value = init
TypeError: str/bytes expected instead of str instance

Does anyone know what am I doing wrong?

On the same note, I am trying to pass a pointer to a string to a C function from my python code so that I can perform some string operation there and return another string. Can someone give me some sample code on how to do that?

extern "C" __declspec(dllexport) char * echo(char* c)
{
      // do stuff
    return c;
}

Answer 1

With regards to getting it working, if you pass it a bytes object, it works:

>>> import ctypes
>>> ctypes.create_string_buffer(b'hello')
<ctypes.c_char_Array_6 object at 0x25258c0>

Looking at the code for create_string_buffer:

def create_string_buffer(init, size=None):
    """create_string_buffer(aBytes) -> character array
    create_string_buffer(anInteger) -> character array
    create_string_buffer(aString, anInteger) -> character array
    """
    if isinstance(init, (str, bytes)):
        if size is None:
            size = len(init)+1
        buftype = c_char * size
        buf = buftype()
        buf.value = init
        return buf
    elif isinstance(init, int):
        buftype = c_char * init
        buf = buftype()
        return buf
    raise TypeError(init)

Doing it directly,

>>> (ctypes.c_char * 10)().value = b'123456789'

This works fine.

>>> (ctypes.c_char * 10)().value = '123456789'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: str/bytes expected instead of str instance

This shows that same behaviour. Looks to me as though you’ve found a bug.

Time to visit http://bugs.python.org. There are a few bugs related to c_char and create_string_buffer which are in the same field, but none reporting that giving it a str now fails (but there are definite examples showing it used to work in Py3K).