PR_SET_IO_FLUSHER(2const)

💖 PR_SET_IO_FLUSHER — change the IO_FLUSHER state 💖

okay bestie, welcome to the PR_SET_IO_FLUSHER(2const) page 🌸 she's from section 2const of the Linux man-pages, and she's very niche but honestly? iconic.

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🎀 LIBRARY

Standard C library (libc, -lc) ✨

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🦋 SYNOPSIS

#include <linux/prctl.h>  /* Definition of PR_* constants */
#include <sys/prctl.h>

int prctl(PR_SET_IO_FLUSHER, long state, 0L, 0L, 0L);

so the vibe here is: you're calling prctl with PR_SET_IO_FLUSHER as the first arg, your state as a long, and then literally three 0Ls just chillin there. no notes 💅

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✨ DESCRIPTION ✨

so here's the tea 🍓 if a user process is involved in the block layer or filesystem I/O path, and it can allocate memory while processing I/O requests, it must set state to 1. that puts the process into the IO_FLUSHER state 👑 which grants it special treatment so it can keep making progress while it's allocating memory. no cap, this is the priority-lane situation.

if state is 0, the process clears the IO_FLUSHER state and the default behavior is used. back to being a regular civilian 🙈

⚠️ heads up bestie: the calling process must have the CAP_SYS_RESOURCE capability. no cap, no entry.

the IO_FLUSHER state is inherited by a child process created via fork(2) 🍼 and it is preserved across execve(2). so it follows you around, lowkey clingy but in a useful way 💜

examples of IO_FLUSHER applications are FUSE daemons 🫶, SCSI device emulation daemons, and daemons that perform error handling — like multipath path recovery applications. yeah, the serious infrastructure girlies 💀🔥

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🔥 RETURN VALUE

• on success, 0 is returned ✨ slay
• on error, -1 is returned, and errno is set to indicate the error 😭

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😭 ERRORS

• EINVAL — state is not a valid value. (it's giving "girl what is that number" 💀)

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🌈 STANDARDS

Linux. 🐧 periodt.

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👑 HISTORY

Linux 5.6. so she's a relative newcomer, born 2020, already carrying the whole I/O path on her back 💅

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🌸 SEE ALSO

• prctl(2)
• PR_GET_IO_FLUSHER(2const)

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okay that's the whole page bestie, you ate 🎀 mwah 💖

sign-off time: this lil rewrite is brought to you by Linux man-pages 6.20, dated 2026-02-08, for PR_SET_IO_FLUSHER(2const) ⭐ go forth and allocate memory while processing I/O requests responsibly, iykyk 🦋