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Date: Thu, 16 Apr 2026 15:37:45 -0700
Message-ID: <CA+1R0Vh-hygR06K4MUbvou8JQwGP4B5YvMCc0rJiQUYzp2kAGg@mail.gmail.com>
Subject: `poll()` returns -1 but errno is 0
To: cygwin AT cygwin DOT com
Content-Type: multipart/mixed; boundary="000000000000d643d6064f9b7b2d"
From: Nahor via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Nahor <nahor.j+cygwin AT gmail DOT com>
Sender: "Cygwin" <cygwin-bounces~archive-cygwin=delorie DOT com AT cygwin DOT com>

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Hi,

In some cases, `poll()` return -1 but errno is still 0.
Attached is a way to reproduce it. On my system, it fails immediately
(from a wall clock point of view, there are quite a few iterations
with the app).

This was first reported against the fish shell and then against MSYS2
(https://github.com/msys2/msys2-runtime/issues/308). The MSYS2 issue
also has a reproducible sample. It's not as quick to fail, but it's
less artificial (more similar to how fish works).

Potentially related:
Sometimes `poll()` fails with a EFAULT errno. I haven't found a way to
reproduce that outside the fish test suite. And there, it's fairly
rare (maybe 5% of the time for the full test suite, which has ~150
tests, some of which spawn lots of processes)

Regards,
Nahor

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