Function unix_o_nonblock 

pub const fn unix_o_nonblock() -> i32

O_NONBLOCK for OpenOptions::custom_flags. Opening a FIFO read-only WITHOUT this flag blocks until a writer appears — a pre-planted FIFO (swapped in after any pathname precheck; pathname checks cannot close that race) would hang the caller forever before the post-open handle check could refuse it. With the flag, the open returns immediately and the handle check sees a non-regular file. Regular files ignore O_NONBLOCK entirely, so the flag is free on the happy path. Same per-arch caution as unix_o_nofollow above: Linux asm-generic uses 0o4000; the PowerPC layout swaps it; BSD/macOS use 0x4.