.. function:: create_string_buffer(init, size=None)
              create_string_buffer(size)

   Create a mutable character buffer. The returned object is a
   ctypes array of :class:`c_char`.

   If *size* is given (and not None), it must be an :cls:`int.
   It specifies the size of the returned array.

   If the *init* argument is given, it must be :cls:`bytes`. It is used
   to initialize the array items. Bytes not initialized this way are
   set to zero (NUL).

   If *size* is not given (or if it is None), the buffer is made one byte
   larger than *init*, effectively adding a NUL termination byte.

   If both arguments are given, *size* must not be less than ``len(init)``.

   .. warning::

      If *size* is equal to ``len(init)``, a NUL terminator is
      not added. Do not treat such a buffer as a C string.

   For example::

      >>> bytes(create_string_buffer(2))
      b'\x00\x00'

      >>> bytes(create_string_buffer(b'ab'))
      b'ab\x00'

      >>> bytes(create_string_buffer(b'ab', 2))
      b'ab'

      >>> bytes(create_string_buffer(b'ab', 4))
      b'ab\x00\x00'

      >>> bytes(create_string_buffer(b'abcdef', 2))
      Traceback (most recent call last):
         ...
      ValueError: byte string too long