Mr Temp Mr Temp - 4 months ago 19
Python Question

How to pack and unpack using ctypes (Structure <-> str)

This might be a silly question but I couldn't find a good answer in the docs or anywhere.

If I use struct to define a binary structure, the struct has 2 symmetrical methods for serialization and deserialization (pack and unpack) but it seems ctypes doesn't have a straightforward way to do this. Here's my solution, which feels wrong:

from ctypes import *

class Example(Structure):
_fields_ = [
("index", c_int),
("counter", c_int),
]

def Pack(ctype_instance):
buf = string_at(byref(ctype_instance), sizeof(ctype_instance))
return buf

def Unpack(ctype, buf):
cstring = create_string_buffer(buf)
ctype_instance = cast(pointer(cstring), POINTER(ctype)).contents
return ctype_instance

if __name__ == "__main__":
e = Example(12, 13)
buf = Pack(e)
e2 = Unpack(Example, buf)
assert(e.index == e2.index)
assert(e.counter == e2.counter)
# note: for some reason e == e2 is False...

Answer

The PythonInfo wiki has a solution for this.

FAQ: How do I copy bytes to Python from a ctypes.Structure?

def send(self):
    return buffer(self)[:]

FAQ: How do I copy bytes to a ctypes.Structure from Python?

def receiveSome(self, bytes):
    fit = min(len(bytes), ctypes.sizeof(self))
    ctypes.memmove(ctypes.addressof(self), bytes, fit)

Their send is the (more-or-less) equivalent of pack, and receiveSome is sort of a pack_into. If you have a "safe" situation where you're unpacking into a struct of the same type as the original, you can one-line it like memmove(addressof(y), buffer(x)[:], sizeof(y)) to copy x into y. Of course, you'll probably have a variable as the second argument, rather than a literal packing of x.