UFFDIO_WAKE(2const)

💖 UFFDIO_WAKE 💖

okay so bestie, welcome to UFFDIO_WAKE(2const) — yes that's a real one, it lives in section 2const of the Linux manual, and today we're gonna make friends with it. grab a matcha, we're doing kernel-flavored lore 🍓✨

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✨ NAME ✨

UFFDIO_WAKE — wake up a thread waiting for page-fault resolution 😴➡️👑

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📚 LIBRARY 📚

Standard C library — that's libc, -lc. she's a classic, she's been here forever 💅

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🛠️ SYNOPSIS 🛠️

#include <linux/userfaultfd.h>  /* Definition of UFFD* constants */
#include <sys/ioctl.h>

int ioctl(int fd, UFFDIO_WAKE, const struct uffdio_range *argp);

like, you call ioctl(2) with UFFDIO_WAKE, hand her the fd, and point argp at a const struct uffdio_range. that's the whole vibe 🦋

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🌸 DESCRIPTION 🌸

So. Wake up the thread waiting for page-fault resolution on a specified memory address range. that's the elevator pitch, no cap.

Here's when you actually reach for this: UFFDIO_WAKE is used in conjunction with the UFFDIO_COPY and UFFDIO_ZEROPAGE operations — specifically the ones that have UFFDIO_COPY_MODE_DONTWAKE or UFFDIO_ZEROPAGE_MODE_DONTWAKE set in the mode field. yes, the DONTWAKE bits. those ones. 🙈

Why would anyone do that? because the userfault monitor can batch a bunch of UFFDIO_COPY and UFFDIO_ZEROPAGE operations together, and then explicitly wake up the faulting thread with UFFDIO_WAKE. it's giving efficiency. it's giving "I'll handle all of you at once, besties." 🔥👑

so like, DONTWAKE = "hold that thought", and UFFDIO_WAKE = "okay go, we're done here". iconic teamwork fr fr.

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🔁 RETURN VALUE 🔁

• 0 — success, we ate, we devoured 💖
• -1 — error, and errno gets set to tell you what went wrong. always check your errno, bestie, she's the group chat receipts 😭

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💀 ERRORS 💀

• EINVAL — the start or the len field of the ufdio_range structure was not a multiple of the system page size; or len was zero; or the specified range was otherwise invalid.

okay so real talk: page-size multiples only, len cannot be 0, and if the range is just weird/invalid in any other way, you're getting EINVAL. she is not subtle about it. respect the page size or perish 🧚✨

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📜 STANDARDS 📜

Linux. that's the standard. that's the whole standard. periodt 🎀

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🕰️ HISTORY 🕰️

Linux 4.3 — she's been around since 2015 and still serving looks 👑

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🧪 EXAMPLES 🧪

See userfaultfd(2) — yeah bestie, the whole example situation lives over there, go peek 🫶

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👀 SEE ALSO 👀

• ioctl(2)
• ioctl_userfaultfd(2)
• UFFDIO_REGISTER(2const)
• userfaultfd(2)
• linux.git/Documentation/admin-guide/mm/userfaultfd.rst

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💜 cute sign-off: this lil page came straight from Linux man-pages 6.20, dated 2026-02-08. go forth and wake your threads, angel 🌈⭐💅