F_GET_RW_HINT(2const)

💖 F_GET_RW_HINT(2const) — the girlies guide to telling your kernel how long your data is gonna live ✨

okay bestie, sit down, hydrate, because we're talking about F_GET_RW_HINT, F_SET_RW_HINT, F_GET_FILE_RW_HINT and F_SET_FILE_RW_HINT — the little fcntl(2) const squad that handles get/set file read/write hints. this is a 2const page from the Linux man-pages, and it's lowkey one of my faves 🫶

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🌸 NAME 🌸

F_GET_RW_HINT, F_SET_RW_HINT, F_GET_FILE_RW_HINT, F_SET_FILE_RW_HINT — get/set file read/write hints. that's it, that's the name, you're welcome 💅

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🎀 LIBRARY 🎀

Standard C library (libc, -lc). the classic. the blueprint. the moment. ✨

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🦋 SYNOPSIS 🦋

#include <fcntl.h>

int fcntl(int fd, F_GET_RW_HINT, uint64_t *arg);
int fcntl(int fd, F_SET_RW_HINT, uint64_t *arg);
int fcntl(int fd, F_GET_FILE_RW_HINT, uint64_t *arg);
int fcntl(int fd, F_GET_FILE_RW_HINT, uint64_t *arg);

yes bestie, fcntl(2) four times, no cap. also notice the last two lines are literally twins — I'm not touching that, I'm just reporting it verbatim like the loyal narrator I am 🫡 (the original says F_GET_FILE_RW_HINT twice, so that's exactly what you're getting. iykyk.)

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💜 DESCRIPTION 💜

okay so. write lifetime hints are the little whispers you send to the kernel about the relative expected lifetime of writes — either on a given inode, or through a particular open file description. (if u don't know what an open file description is, open(2) has the whole lore. go read it, I'll wait. 💖)

and in this context, "write lifetime" = the expected time the data is gonna live on the media, before somebody overwrites it or erases it. it's giving "how long is this gonna sit here" energy. 😌

an application can use the different hint values down below to split writes into different write classes, so that multiple users or apps vibing on a single storage back-end can aggregate their I/O patterns in a consistent way. HOWEVER — and this is important, bestie — there are no functional semantics implied by these flags. different I/O classes can use the write lifetime hints in arbitrary ways, as long as they use them consistently. so it's a social contract, not a law. 🤝

here's what you can actually do to the file descriptor, fd:

• F_GET_RW_HINT — hands you back the read/write hint value associated with the underlying inode that fd points at. 👀
• F_SET_RW_HINT — sets the read/write hint value on the underlying inode that fd points at. this hint persists until either it gets explicitly modified, or the underlying filesystem gets unmounted. so it's a commitment, not a situationship. 💀
• F_GET_FILE_RW_HINT — hands you back the read/write hint value associated with the open file description that fd points at.
• F_SET_FILE_RW_HINT — sets the read/write hint value associated with the open file description that fd points at.

and here's the fallback tea: if an open file description has not been assigned a read/write hint, then it shall use the value assigned to the inode, if any. so it inherits, like a good kid. 🥹

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✨ the read/write hints that are supported ✨

• RWH_WRITE_LIFE_NOT_SET — no specific hint has been set. this is the default value. the "i have no opinion" option. iconic neutrality. 😐
• RWH_WRITE_LIFE_NONE — no specific write lifetime is associated with this file or inode. as in, none. zero. nada.
• RWH_WRITE_LIFE_SHORT — data written to this inode, or via this open file description, is expected to have a short lifetime. blink and it's gone ✨
• RWH_WRITE_LIFE_MEDIUM — data written to this inode, or via this open file description, is expected to have a lifetime longer than data written with RWH_WRITE_LIFE_SHORT. a lil more staying power 💅
• RWH_WRITE_LIFE_LONG — data written to this inode, or via this open file description, is expected to have a lifetime longer than data written with RWH_WRITE_LIFE_MEDIUM. now we're settling in 🛋️
• RWH_WRITE_LIFE_EXTREME — data written to this inode, or via this open file description, is expected to have a lifetime longer than data written with RWH_WRITE_LIFE_LONG. this one's in it for the long haul, main character energy fr 👑

⚠️ okay so this part matters: all the write-specific hints are relative to each other, and no individual absolute meaning should be attributed to them. they're not "X seconds" or "Y bytes" or anything. it's pure vibes-based ranking. do not read more into them bestie, they will not deliver 📢

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🔥 RETURN VALUE 🔥

Zero. that's it. that's the success case. zero. 🫶

on error, -1 is returned, and errno is set to indicate the error. classic. 💀

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🌈 ERRORS 🌈

see fcntl(2). that's the whole section, go look over there, the answers live at their house 🏠

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⭐ STANDARDS ⭐

Linux. periodt. 🐧

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🧚 HISTORY 🧚

Linux 4.13. a vintage, honestly. a classic era. 🎀

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👑 SEE ALSO 👑

fcntl(2) — the parent of all of this. go read the mothership.

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and that's the whole page, bestie!! this was pulled from Linux man-pages 6.20, dated 2025-07-20, and the page is F_GET_RW_HINT(2const) in case u forgot where we started 💖 slay responsibly, set ur hints consistently, and never attribute absolute meaning to a relative hint. mwah 🫶✨🌸