field_prep(9)

📦 version 7.3.1, in nixpkgs unstable

💅 field_prep(9) — Kernel API Manual ✨

okay bestie, welcome to field_prep(9) — it lives in the Kernel API Manual and it's genuinely one of the sweetest little helpers in the whole bitfield family. let me put you on 🫶

🌸 NAME 🌸

field_prep — prepare a bitfield element

lowkey does exactly what it says on the tin: it gets a field ready. no cap.

✨ SYNOPSIS ✨

field_prep (mask , val );

that's it. two things in, one gorgeous masked-and-shifted thing out. iconic.

🎀 ARGUMENTS 🎀

  • mask — the shifted mask that defines the field's length and its position. and bestie, listen: it must be non-zero. fr fr, zero is not the vibe here. 💀
  • val — the value you wanna put in the field. your main character value, if you will. 👑

🦋 RETURN 🦋

the field value, masked and shifted all the way to its final destination. it's giving "packed and ready to ship". ✈️

💖 DESCRIPTION 💖

so here's the tea: field_prep masks the value and shifts it up. that's the whole move. ✨

then — and this is the important part — the result should be combined with the other fields of the bitfield using logical OR. think of it like assembling your fit: each piece gets its own spot, then you OR them all together. 🔥

now, the difference from FIELD_PREP (iykyk): with field_prep, mask is not limited to a compile-time constant. she's flexible. 💅 typical usage patterns are:

  • a value that's stored in a table 📋
  • a value calculated by shifting a constant by a variable number of bits 🌀
but if you do want to make sure mask is a compile-time constant, please use FIELD_PREP directly instead. bestie, don't fight it — just call the right function. 🙈

🌈 SEE ALSO 🌈

Kernel file ./include/linux/bitfield.h, plus the whole squad:

go read them, they all ate. 👑

and that's the whole page, bestie — no crumbs left. this one lives in ./include/linux and was stamped October 2026 ⭐, so go forth and prep those fields. mwah 💋🧚‍♀️

📦 where this page comes from