Bug report

Using ctypes with variadic functions on ARM64 macOS machines seems to improperly pass the arguments, leading to truncation.

Minimal repro:

>>> import ctypes
>>> from ctypes import util
>>> libc = ctypes.CDLL(util.find_library("c"))
>>> libc.printf(b"hello %d world\n", 128_000_000)
hello 0 world
14

This happens regardless of casting it to a ctypes.c_int explicitly or not.

>>> libc.printf(b"hello %ld world\n", ctypes.c_int(1))
hello 0 world
14

On my regular machine (in this case an x64 Windows machine) it works as expected:

>>> import ctypes
>>> libc = ctypes.cdll.msvcrt
>>> libc.printf(b"hello %d world\n", 128_000_000)
hello 128000000 world
22

Your environment

I do not personally have a macOS machine, but I got a few others who did have a machine test for me. Their versions were as follows:

Machine 1:
Python 3.10.1, macOS 12.3.1 (21E258)

Machine 2:

Python 3.9.13 (v3.9.13:6de2ca5339, May 17 2022, 11:37:23) 
[Clang 13.0.0 (clang-1300.0.29.30)] on darwin
macOS-12.2.1-arm64-arm-64bit

Machine 3:

~ % python3 --version
Python 3.9.10

~ % sw_vers
ProductName:     macOS
ProductVersion:  12.3.1
BuildVersion:    21E258

• CPython versions tested on: 3.9, 3.10
• Operating system and architecture: ARM64 Apple Silicon macOS

Linked PRs

• GH-92892: Add section about variadic functions to ctypes documentation #99529
• [3.10] GH-92892: Add section about variadic functions to ctypes documentation (GH-99529) #99681
• [3.11] GH-92892: Add section about variadic functions to ctypes documentation (GH-99529) #99682