cdef extern from "Python.h": ctypedef struct PyTypeObject: pass ctypedef struct PyObject: Py_ssize_t ob_refcnt PyTypeObject *ob_type cdef extern from "longintrepr.h": cdef struct _longobject: int ob_refcnt PyTypeObject *ob_type # int ob_size # not in Py3k unsigned int *ob_digit def test(temp = long(0)): cdef _longobject *l l = <_longobject *> temp #print sizeof(l.ob_size) # not in Py3k print sizeof(l.ob_digit[0])