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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`_Py_atomic_load_uint16` incorrect in `pyatomic_std.h`","articleBody":"# Bug report\n\nFound by [Mohammed Zuhaib](https://discuss.python.org/u/zuhu2195) in https://discuss.python.org/t/code-explanation-for-absence-of-stdatomic-h-for-python-build/101623/30.\n\nhttps://github.com/python/cpython/blob/6450b1d142b6254d2e3b2eba47d69125ca79b3fe/Include/cpython/pyatomic_std.h#L458-L463\n\nWe cast to a `uint32_t` instead of `uint16_t`. I suspect this probably doesn't have noticeable effects on little-endian systems, but would load the wrong value on big-endian systems. We also mostly use `pyatomic_gcc.h` or `pyatomic_msvc.h`, which is probably why we didn't catch this in big-endian buildbots.\n\nI asked Claude to review these files. In addition to the above bug:\n\n```\n  2. pyatomic_msc.h:974-978 — _Py_atomic_store_uint_release broken on ARM64\n\n  static inline void\n  _Py_atomic_store_uint_release(unsigned int *obj, unsigned int value)\n  {\n      *(volatile unsigned int *)obj = value;\n  }\n\n  This is placed among the relaxed stores and uses only a volatile store. On x86/x64, volatile stores have release semantics, so this happens to work. But on ARM64, volatile accesses are only relaxed\n  (as stated in the file's header comment). Every other _release function in this file (e.g., _Py_atomic_store_int_release at line 1057, _Py_atomic_store_uint32_release at line 1108) properly uses\n  __stlr32 on ARM64. This function is missing the #if/#elif ARM64 branch.\n\n  3. pyatomic_gcc.h — Missing _Py_atomic_store_uint_release\n\n  Both pyatomic_std.h (line 1035) and pyatomic_msc.h (line 975) define _Py_atomic_store_uint_release, but pyatomic_gcc.h does not. If any code calls it when compiled with GCC/Clang, it would fail to\n  link/compile.\n\n  4. pyatomic.h:75-76 — Incorrect pseudo-code comment\n\n  def _Py_atomic_store_ptr_release(obj):\n      return obj  # release\n\n  This is documented as a one-arg function that returns a value, but it's actually a two-arg store (like the real declaration at line 518). Should be:\n  def _Py_atomic_store_ptr_release(obj, value):\n      obj = value  # release\n```\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-146229\n* gh-146232\n* gh-146233\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/colesbury","@type":"Person","name":"colesbury"},"datePublished":"2026-03-20T17:05:15.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":0},"url":"https://github.com/146227/cpython/issues/146227"}