Python3 ctypes copy pointer to ctypes.structure

Question

An ioctl the linux kernel puts a struct inside of a struct. I'm trying to figure out how to do the same with ctypes.

I receive ParentRq back from an ioctl. I want a generic way of copying the data out of Message.data into another Structure like EventID for example.

from ctypes import Structure, c_uint8, c_uint16, POINTER

class EventID(Structure):
    _pack_ = 1
    _fields_ = [
        ('recent_addition_timestamp', c_uint8 * 4),
        ('last_recid', c_uint8 * 2),
        ('last_sw_processed_id', c_uint8 * 2),
        ('last_processed_id', c_uint8 * 2),
    ]
    _eventid_sampledata = [0x88, 0x29, 0xfd, 0x60, 0x3c, 0x02, 0xff, 0xff, 0x3c, 0x02]

class Message(Structure):
    _pack_ = 1
    _fields_ = [
        ('data_len', c_uint16),
        ('data', POINTER(c_uint8))
]

class ParentRq(Structure):
    _pack_ = 1
    _fields_ = [
        ('msg', Message)
    ]

p = ParentRq()
p.msg.data_len = len(EventID._eventid_sampledata)
p.msg.data = (c_uint8 * p.msg.data_len)(*EventID._eventid_sampledata)

# Proof that items got copied into the pointer correctly.
items = []
for i in range(p.msg.data_len):
    items.append(hex(p.msg.data[i]))
# items -> ['0x88', '0x29', '0xfd', '0x60', '0x3c', '0x2', '0xff', '0xff', '0x3c', '0x2']

# Copy p.msg.data into EventID
# How do I copy p.msg.data into EventID?
eid = EventID()

Answer 1

Listing [Python.Docs]: ctypes - A foreign function library for Python.

There are a number of ways of achieving that, one is using memmove:

>>> import ctypes as ct
>>>
>>>
>>> class EventID(ct.Structure):
...     _pack_ = 1
...     _fields_ = [
...         ("recent_addition_timestamp", ct.c_uint8 * 4),
...         ("last_recid", ct.c_uint8 * 2),
...         ("last_sw_processed_id", ct.c_uint8 * 2),
...         ("last_processed_id", ct.c_uint8 * 2),
...     ]
...
>>>
>>> l = [0x88, 0x29, 0xfd, 0x60, 0x3c, 0x02, 0xff, 0xff, 0x3c, 0x02]
>>> size = len(l)
>>> arr = (ct.c_ubyte * size)(*l)
>>> ptr = ct.cast(arr, ct.POINTER(ct.c_ubyte))  # The equivalent of Message.data (ParentRq.msg.data) instance
[hex(ptr[i]) for i in range(size)]
>>> ['0x88', '0x29', '0xfd', '0x60', '0x3c', '0x2', '0xff', '0xff', '0x3c', '0x2']
>>>
>>> eid = EventID()  # Initialize members to default values (0)
>>>
>>> for f, _ in EventID._fields_:
...     print(f, [hex(i) for i in getattr(eid, f)[:]])
...
recent_addition_timestamp ['0x0', '0x0', '0x0', '0x0']
last_recid ['0x0', '0x0']
last_sw_processed_id ['0x0', '0x0']
last_processed_id ['0x0', '0x0']
>>>
>>> ct.memmove(ct.addressof(eid), ptr, size)
2361449631368
>>>
>>> for f, _ in EventID._fields_:
...     print(f, [hex(i) for i in getattr(eid, f)[:]])
...
recent_addition_timestamp ['0x88', '0x29', '0xfd', '0x60']
last_recid ['0x3c', '0x2']
last_sw_processed_id ['0xff', '0xff']
last_processed_id ['0x3c', '0x2']