On Tue, Jul 26, 2016 at 6:31 PM, sth <ursch...@gmail.com> wrote: > > The restype is a ctypes Structure instance with a single __fields__ entry > (coords), which
Watch the underscores with ctypes attributes. Your code spells it correctly as "_fields_". > is a Structure with two fields (len and data) which are the FFI array's > length and the void > pointer to its memory: > https://github.com/urschrei/pypolyline/blob/master/pypolyline/util.py#L109-L117 _FFIArray.__init__ isn't properly keeping a reference to the wrapped numpy array: def __init__(self, seq, data_type = c_double): ptr = POINTER(data_type) nparr = np.array(seq, dtype=np.float64) arr = nparr.ctypes.data_as(ptr) self.data = cast(arr, c_void_p) self.len = len(seq) arr doesn't have a reference to nparr, so self.data doesn't have a reference to the numpy array when it goes out of scope. For example, we can trigger a segfault here: >>> nparr = np.array(range(2**24), dtype=np.float64) >>> ptr = ctypes.POINTER(ctypes.c_double) >>> arr = nparr.ctypes.data_as(ptr) >>> arr[0], arr[2**24-1] (0.0, 16777215.0) >>> del nparr >>> arr[0], arr[2**24-1] Segmentation fault (core dumped) > I'm only half-following your explanation of how ctypeslib works, but it seems > clear that I'm doing > something wrong. The library requires the data pointers wrapped in a struct with the length, so I think what you're doing to wrap and unwrap arbitrary sequences is generally fine. But you don't need the bytes copy from string_at. You can cast to a double pointer and create a numpy array from that, all without copying any data. The only copy made is for tolist(). For example: def _void_array_to_nested_list(res, func, args): """ Dereference the FFI result to a list of coordinates """ try: shape = res.coords.len, 2 ptr = cast(res.coords.data, POINTER(c_double)) array = np.ctypeslib.as_array(ptr, shape) return array.tolist() finally: drop_array(res.coords) If you're not hard coding the double data type, consider adding a simple C type string to the _FFIArray struct. For example: >>> ctypes.c_double._type_ 'd' You just need a dict to map type strings to simple C types. -- https://mail.python.org/mailman/listinfo/python-list
