io_uring_register_region(3)

✨ io_uring_register_region — bestie edition ✨

okay so this is a page from the liburing Manual (yes, the liburing girlies stay winning 💅) and it's all about io_uring_register_region, a little function that registers a memory region with io_uring. she's giving setup-era main character energy, not runtime chaos. let's get into it 🫶

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🌸 NAME 🌸

io_uring_register_region — register a memory region. that's it, that's the whole vibe. short, sweet, iconic. ✨

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🎀 SYNOPSIS 🎀

#include <liburing.h>

int io_uring_register_region(struct io_uring *ring,
                             struct io_uring_mem_region_reg *reg);

like yes she takes two args and she's gonna return an int, no surprises, no drama (well, -errno drama, but we'll get there 😭).

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💖 DESCRIPTION 💖

so the way that io_uring_register_region(3) works is: you hand io_uring a memory region and you register it. once that goes through successfully, that region is now usable — for example, to pass waiting parameters to the io_uring_enter(2) system call in a more efficient manner, because it avoids copying wait-related data for every single wait event. no cap, that's the whole point: less copying, more slay. 🔥

the ring argument should point to the ring in question (the ring you're working with, duh 💅), and the reg argument should be a pointer to a struct io_uring_mem_region_reg.

the reg argument must be filled in with the appropriate information. it looks as follows:

struct io_uring_mem_region_reg {
    __u64 region_uptr;
    __u64 flags;
    __u64 __resv[2];
};

so let's break her down, bestie:

• the region_uptr field must contain a pointer to an appropriately filled struct io_uring_region_desc. (yes, struct in struct, it's giving nesting doll realness 🪆)
• the flags field must contain a bitmask of the following values:
  • IORING_MEM_REGION_REG_WAIT_ARG — allows use of the region to pass waiting parameters to the io_uring_enter(2) system call. if set, the registration is only allowed while the ring is in a disabled mode. see IORING_SETUP_R_DISABLED. ⚠️ like, that's a hard requirement, not a suggestion, so don't come crying to me when it fails.
• the __resv fields must be filled with zeroes. all of them. every last one. zeroes only, no freelancing. 🙈

now, struct io_uring_region_desc is defined as following:

struct io_uring_region_desc {
    __u64 user_addr;
    __u64 size;
    __u32 flags;
    __u32 id;
    __u64 mmap_offset;
    __u64 __resv[4];
};

field-by-field, here's the tea ☕:

• the user_addr field must contain a pointer to the memory the user wants to register. it's valid only if IORING_MEM_REGION_TYPE_USER is set, and should be zero otherwise. so if you're not using that flag, put a zero there, periodt.
• the size field should contain the size of the region. (it's giving "obviously" but we're being thorough, okay 😌)
• the flags field must contain a bitmask of the following values:
  • IORING_MEM_REGION_TYPE_USER — tells the kernel to use memory specified by the user_addr field. if not set, the kernel will allocate memory for the region, which can then be mapped into the user space. so it's basically your memory vs kernel's memory, pick your fighter. 👑
• on successful registration of a region with kernel provided memory, the mmap_offset field will contain an offset that can be passed to the mmap(2) system call to map the region into the user space. that's your ticket to actually getting at that memory, so don't sleep on it. 🎟️
• the id field is reserved and must be set to zero. don't touch. don't be cute. zero. 💀
• the __resv fields must be filled with zeroes. again. same rules as before. zeroes or nothing. 🧚

✨ available since kernel 6.13 ✨ — so if your kernel is older, this is not the moment, bestie, it literally won't be there.

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🌈 RETURN VALUE 🌈

on success, io_uring_register_region(3) returns 0. 🎉 iconic. flawless. no notes.

on failure it returns -errno. 😭 so yeah, check that return value, it's not decorative.

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that's the whole page, you're basically a liburing girlie now 🦋 stay registered, stay zeroed out, and always fill your __resv fields properly 💜

mwah 💋 this was brought to you by liburing-2.14, dated Jan 13, 2026 — that's the source, version, and date, all wrapped up in a little bow 🎀