[Python-Dev] ctypes, memory mapped files and context manager

[eryk sun]

On Sun, Jan 8, 2017 at 8:25 AM, Armin Rigo <armin.rigo at gmail.com> wrote:
>
>     c_raw = ctypes.PYFUNCTYPE(ctypes.c_void_p, ctypes.c_void_p)(lambda p: p)

Use ctypes.addressof.

>     addr = c_raw(ctypes.pointer(T.from_buffer(m)))
>     b = ctypes.cast(addr, ctypes.POINTER(T)).contents

ctypes.cast uses an FFI call. In this case you can more simply use from_address:

    b = T.from_address(ctypes.addressof(T.from_buffer(m)))

There's no supporting connection between b and m. If m was allocated
from a heap/pool/freelist, as opposed to a separate mmap
(VirtualAlloc) call, then you won't necessarily get a segfault (access
violation) if b is used after m has been deallocated or internally
realloc'd. It can lead to corrupt data and difficult to diagnose
errors. You're lucky if it segfaults.