Add ability to force alignment of ctypes.Structure · Issue #112433 · python/cpython (original) (raw)
Feature or enhancement
Proposal:
When creating a ctypes.Structure to map data from c/c++ to python, I have been coming up against issues where I have a structure which has an alignment due to a #pragma align.
Currently there is no way to define a ctypes.Structure which can map to an object like this.
I propose we add an _align_ attribute to the ctypes.Structure class which will instruct the code how to align the structure itself in memory.
In the way that the _pack_ attribute indicates the maximum alignment of the fields in the struct, this would indicate the minimum alignment of the struct itself.
An example of such a struct and it's use is as follows:
import ctypes
class IDString(ctypes.Structure): align = 0x10 fields = [ ("string", ctypes.c_char * 0x10), ]
class main(ctypes.Structure): fields = [ ("first", ctypes.c_uint32), ("second", ctypes.c_ubyte), ("string", IDString), ]
data = bytearray( b"\x07\x00\x00\x00\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00" b"\x68\x65\x6c\x6c\x6f\x20\x77\x6f\x72\x6c\x64\x21\x00\x00\x00\x00" ) m = main.from_buffer(data) print(f"first: {m.first}") # first: 7 print(f"second: {m.second}") # second: 1 print(f"string: {m.string.string}") # string: b'hello world!'
Without the _align_ attribute the value of m.string.string would just be an empty bytes object since it would be reading from 8 bytes into the bytearray.
I have already made a (preliminary) implementation of this here.
Because the attribute is optional, I believe there are no potential backward compatibility issues as the default alignment will simply be what it was before (ie. 1).
Has this already been discussed elsewhere?
I have already discussed this feature proposal on Discourse
Links to previous discussion of this feature:
https://discuss.python.org/t/add-ability-to-force-alignment-of-ctypes-structure/39109