w# requires Py_ssize_t, not int.

This documentation bug has cost me several hours of debugging :/

Doc/c-api/arg.rst

@@ -251,7 +251,7 @@ variable(s) whose address should be passed.
    or use ``w#`` instead.  Only single-segment buffer objects are accepted;
    :exc:`TypeError` is raised for all others.
 
-``w#`` (read-write character buffer) [char \*, int]
+``w#`` (read-write character buffer) [char \*, Py_ssize_t]
    Like ``s#``, but accepts any object which implements the read-write buffer
    interface.  The :ctype:`char \*` variable is set to point to the first byte of
    the buffer, and the :ctype:`int` is set to the length of the buffer.  Only