sigfastblock(2) on FreeBSD
The text f4d0b6420170, as FreeBSD 16.0-CURRENT (2026-10-27), FreeBSD 15.1-RELEASE, FreeBSD 15.0-RELEASE, FreeBSD 14.5-RELEASE, FreeBSD 14.4-RELEASE, FreeBSD 14.3-RELEASE shipped it. The page shows this text now.
controls signals blocking with a simple memory write