Different behaviour of ctypes c_char_p?
ctypes, python, python-2.7, python-3.x
Solution
`c_char_p` is a subclass of `_SimpleCData`, with `_type_ == 'z'`. The `__init__` method calls the type's `setfunc`, which for simple type `'z'` is `z_set`.
In Python 2, the `z_set` function (2.7.7) is written to handle both `str` and `unicode` strings. Prior to Python 3, `str` is an 8-bit string. CPython 2.x `str` internally uses a C null-terminated string (i.e. an array of bytes terminated by `\0`), for which `z_set` can call `PyString_AS_STRING` (i.e. get a pointer to the internal buffer of the `str` object). A `unicode` string needs to first be encoded to a byte string. `z_set` handles this encoding automatically and keeps a reference to the encoded string in the `_objects` attribute.
>>> c = u'spam'
>>> a = c_char_p(c)
>>> a._objects
'spam'
>>> type(a._objects)
<type 'str'>
On Windows, the default ctypes string encoding is `'mbcs'`, with error handling set to `'ignore'`. On all other platforms the default encoding is `'ascii'`, with `'strict'` error handling. To modify the default, call `ctypes.set_conversion_mode`. For example, `set_conversion_mode('utf-8', 'strict')`.
In Python 3, the `z_set` function (3.4.1) does not automatically convert `str` (now Unicode) to `bytes`. The paradigm shifted in Python 3 to strictly divide character strings from binary data. The ctypes default conversions were removed, as was the function `set_conversion_mode`. You have to pass `c_char_p` a `bytes` object (e.g. `b'spam'` or `'spam'.encode('utf-8')`). In CPython 3.x, `z_set` calls the C-API function `PyBytes_AsString` to get a pointer to the internal buffer of the `bytes` object.
Note that if the C function modifies the string, then you need to instead use `create_string_buffer` to create a `c_char` array. Look for a parameter to be typed as `const` to know that it's safe to use `c_char_p`.
Problem
I am confused with this behaviour of different versions of python and dont understand why ? ``` Python 2.7.5 (default, Aug 25 2013, 00:04:04) [GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> c="hello" >>> a=ctypes.c_char_p(c) >>> print(a.value) hello Python 3.3.5 (default, Mar 11 2014, 15:08:59) [GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.2.79)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> c="hello" >>> a=ctypes.c_char_p(c) Traceback (most recent call last): File "<stdin>", line 1, in <module> TypeError: bytes or integer address expected instead of str instance ``` One works while the other gives me an error. Which one is correct ? If both of them are correct, how can i achieve the same behaviour as 2.7 in 3.3.5 ? I want to pass the char pointer to C from python.