user_regset_writeback_fn(9)

✨ user_regset_writeback_fn(9) ✨

okay so this is a page from the Kernel API Manual, and it's all about user_regset_writeback_fn — literally the type of the @writeback function sitting inside &struct user_regset. yes bestie, it's a function-pointer type. she's meta like that. 💅

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🌸 NAME 🌸

user_regset_writeback_fn — the type of the @writeback function in &struct user_regset. that's it, that's the whole identity. main character energy, minimal backstory. 👑

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🦋 SYNOPSIS 🦋

typedef int user_regset_writeback_fn (struct task_struct *target , const
       struct user_regset *regset , int immediate );

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🎀 ARGUMENTS 🎀

• target — the thread being examined. the one under the microscope fr fr 🔍
• regset — the regset being examined. yeah she gets examined too, they're a package deal 🫶
• immediate — zero if a writeback at the completion of the next context switch is OK. so like, "no rush, whenever the context switch finishes" vibes ✨

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💖 DESCRIPTION 💖

so this call is optional — usually the pointer is just NULL. no cap. but when it is provided, it means there's some user memory associated with this regset's hardware, like memory backing cached register data on register window machines 👀. the regset's data controls what user memory gets used (e.g. via the stack pointer value). iykyk.

the job: write register data back to user memory. 📝

now, the immediate flag is the whole vibe check here:

• if immediate is nonzero, it must be written to the user memory so that uaccess or access_process_vm can see it when this call returns. no wiggle room, bestie — it's visible the second we're back. 🔥
• if it's zero, then it must be written back by the time the task completes a context switch (as synchronized with wait_task_inactive). so like, slightly more chill, but still a hard deadline. respect the schedule. 🕐

and the returns? iconic little trio:

• 0 — success, or honestly there was nothing to do anyway. slay 🎉
• -EFAULT — there's a memory problem (bad stack pointer or whatever, you know how it is 😭)
• -EIO — a hardware problem. not her fault, the silicon is being weird 💀

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🌈 SEE ALSO 🌈

• Kernel file ./include/linux/regset.h
• user_regset_active_fn(9)
• user_regset_set_fn(9)
• struct user_regset(9)
• struct user_regset_view(9)
• task_user_regset_view(9)
• copy_regset_from_user(9)

like, go read the whole friend group, they're all connected 🧚

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and that's the page!! 💜 this lil masterpiece lives over in ./include/linux, dated October 2026, and if you ever need to find her again just look up user_regset_writeback_fn(9). mwah, ily, go write some registers back. 🍓🎀