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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"`SystemError` from `termios.tcsetwinsize`passing `NULL` to `PyLong_AsLong`","articleBody":"# Crash report\n\n### What happened?\n\nIt's possible to raise `SystemError` from `termios.tcsetwinsize` by passing a sequence that raises on `__getitem__` to it.\n\n### Automated diagnosis:\n\nBug: `termios.tcsetwinsize` passes `NULL` to `PyLong_AsLong`. `PySequence_GetItem(winsz, 0)` can return `NULL` (e.g., `__getitem__` raises). The result is passed directly to `PyLong_AsLong(NULL)` which calls `PyErr_BadInternalCall()`, masking the real error with `SystemError`.\n\nFile: `Modules/termios.c`, lines 502-510\n\nMRE:\n```python\nimport termios\nimport os\n\nfd = os.open(\"/dev/tty\", os.O_RDWR | os.O_NOCTTY)\n\nclass BadSequence:\n    def __len__(self):\n        return 2\n    def __getitem__(self, idx):\n        raise TypeError\n\ntermios.tcsetwinsize(fd, BadSequence())\n```\n\nBacktrace:\n```python\nTraceback (most recent call last):\n  File \"/home/danzin/crashers/trigger_termios_null_pylong.py\", line 23, in \u003cmodule\u003e\n    termios.tcsetwinsize(fd, BadSequence())\n    ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^\nSystemError: Objects/longobject.c:598: bad argument to internal function\n```\nFound using [cpython-review-toolkit](https://github.com/devdanzin/cpython-review-toolkit) with Claude Opus 4.6, using the `/cpython-review-toolkit:explore Modules/termios.c all deep` command.\n\n\n\n### CPython versions tested on:\n\nCPython main branch\n\n### Operating systems tested on:\n\nLinux\n\n### Output from running 'python -VV' on the command line:\n\nPython 3.15.0a7+ (heads/main:e0f7c1097e1, Mar 17 2026, 18:10:52) [Clang 21.1.2 (2ubuntu6)]\n\n\u003c!-- gh-linked-prs --\u003e\n### Linked PRs\n* gh-146147\n\u003c!-- /gh-linked-prs --\u003e\n","author":{"url":"https://github.com/devdanzin","@type":"Person","name":"devdanzin"},"datePublished":"2026-03-17T22:30:02.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":1},"url":"https://github.com/146091/cpython/issues/146091"}