Hangup when printing empty string to unbuffered stdout

[sideeffect42]

Crash report

What happened?

$ sudo python3 -u -c 'print(""); print("foo")'
Hangup

When printing an empty string to unbuffered stdout, under certain circumstances (e.g. when running on AIX under sudo, screen, tmux), the user session including the Python interpreter will terminate.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Other

Output from running 'python -VV' on the command line:

Python 3.12.13 (main, Mar 6 2026, 03:56:12) [GCC 10.3.0]

Linked PRs

• gh-148773: Fix undefined behaviour in write() call #148774

[cmaloney]

Hi! Thanks for the issue. Reading through the PR a little bit it looks like the issue here is that the first write is of length 0 and returns 0 and the I/O stack continues after that rather than exiting?

Length 0 is a defined behavior in the write manual page, and _Py_write_impl underlies a lot of different I/O cases across many platforms so hesitant to skip the write call. For this particular case it should be going through Buffered I/O and Text I/O which I would have expected to not do zero-length writes....

Really hesitant to change the underlying primitive as things like zero return and 0 length is allowed and can have side effects. I am potentially open to making sure Buffered I/O doesn't call through in this case; its job is to avoid too many writes.