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Iterating over ctypes pointer causes hang or segmentation fault #92347

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Iterating over ctypes pointer causes hang or segmentation fault#92347
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topic-ctypestype-crashA hard crash of the interpreter, possibly with a core dump

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@chgnrdv
chgnrdv
opened on May 5, 2022

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

Activity

  1. chgnrdv
    added
    type-bugAn unexpected behavior, bug, or error
    on May 5, 2022
  2. eryksun
    added
    topic-ctypes
    on May 5, 2022

  3. JelleZijlstra commented on May 5, 2022

    @JelleZijlstra
    JelleZijlstra
    on May 5, 2022
    Member

    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.

  4. jkloth commented on May 6, 2022

    @jkloth
    jkloth
    on May 6, 2022
    Contributor

    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.

  5. serhiy-storchaka
    added this to Ctypes issueson Jan 1, 2024
  6. serhiy-storchaka
    added
    type-crashA hard crash of the interpreter, possibly with a core dump
    and removed
    type-bugAn unexpected behavior, bug, or error
    on Jul 3, 2026
  7. ZeroIntensity
    added 3 commits that reference this issue on Sep 21, 2026

    gh-92347: Document pitfalls of `ctypes.pointer` iteration (GH-157801)

    a3e389d

    [3.13] gh-92347: Document pitfalls of `ctypes.pointer` iteration (GH-…

    2ecf528

    [3.14] gh-92347: Document pitfalls of `ctypes.pointer` iteration (GH-…

    6c4cd44
  8. hugovk
    added a commit that references this issue on Sep 28, 2026

    [3.15] gh-92347: Document pitfalls of `ctypes.pointer` iteration (GH-…

    9cf4661
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