Iterating over ctypes pointer causes hang or segmentation fault

[chgnrdv]

Trying to create iterable (list, tuple, set etc.) from PyCPointer instance leads to hang or segmentation fault.
This code causes hang and memory leak which goes until oom-killer kills python process:

import ctypes
x = ctypes.c_int32(12345)
buf4_p_t = ctypes.POINTER(ctypes.ARRAY(ctypes.c_byte, 4))
buf_p = ctypes.cast(ctypes.byref(x), buf4_p_t)
list(buf_p)

Similar code that causes segmentation fault:

import ctypes
x = ctypes.c_int32(54321)
xp = ctypes.pointer(x)
list(xp)

Iterating over pointer using for statement leads to an infinite loop:
for i in buf_p: print(i)
for i in xp: print(i)
list(for i in buf_p)

Tried on 3.10.3 and 3.11.0a6+, fails at both.

The possible reason is that PyCPointer_Type defines sq_item without defining tp_iter.
Despite I can't imagine any reason why anyone might need to use pointer as iterable/sequence, raising TypeError in such situation would be a better option.

Linked PRs

• gh-92347: Throw a warning when converting a pointer to an iterator #126318

[JelleZijlstra]

Similar to #92262. I think we should fix this (by setting tp_iter to None) but only on the main branch in case people rely on calling next() on a pointer object.

[jkloth]

Do note that indexed access to pointer objects is a documented (and useful) behavior. It would be quite surprising to have a sequence-like object raise an exception when used in a for loop, typing constructs notwithstanding.

Perhaps a note in the Pointers section mentioning the unbounded behavior when iterating aka an infinite generator.