io_setup(2)

💖 io_setup(2) — bestie edition 💖

okay so this is a page from the System Calls Manual (section 2, the syscall girlies) and we're talking about io_setup, the little syscall that spins up an async I/O context. no cap, it's giving "let me get you set up before you go do eighteen things at once" ✨

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🌸 NAME 🌸

io_setup — create an asynchronous I/O context

🎀 LIBRARY 🎀

Standard C library (libc, -lc)

✨ SYNOPSIS ✨

#include <linux/aio_abi.h>
#include <sys/syscall.h>    /* Definition of SYS_* constants */
#include <unistd.h>

long syscall(SYS_io_setup, unsigned int n, aio_context_t *ctx_idp);

🦋 DESCRIPTION 🦋

note note note bestie: you probably want to use the io_setup(3) wrapper function that libaio hands you instead of rawdogging the syscall. go peek at VERSIONS below, i'm serious 💅

so the io_setup() system call creates an asynchronous I/O context — and this context is suitable for concurrently processing n operations. like, n of them. all at once. the multitasking queen behavior of it all 👑

now, the ctx_idp argument:

• it must not point to an AIO context that already exists, and
• it must be initialized to 0 prior to the call.

then, once the AIO context is created successfully, *ctx_idp gets filled in with the resulting handle. that's your lil ticket, your receipt, your golden ticket to the async I/O factory 🎟️

🔥 RETURN VALUE 🔥

• on success, io_setup() returns 0. iconic.
• on error, -1 is returned, and errno is set to indicate the error. sad but informative 😭

💀 ERRORS 💀

• EAGAIN — the specified n exceeds the limit of available events, as defined in /proc/sys/fs/aio-max-nr (see proc_sys_fs(5)).
• EFAULT — an invalid pointer is passed for ctx_idp.
• EINVAL — *ctx_idp is not initialized to 0.
• EINVAL — the specified n exceeds internal limits. n should be greater than 0.
• ENOMEM — insufficient kernel resources are available.
• ENOSYS — io_setup() is not implemented on this architecture.

for real, two whole different EINVAL moments up there. read them both, don't get caught slipping 🙈

🧚 VERSIONS 🧚

libaio provides a wrapper function with the same name, but a different prototype and a different return value. you probably want to use that wrapper. fr fr.

👑 STANDARDS 👑

Linux. that's it, that's the standard 💅

🌈 HISTORY 🌈

Linux 2.5.

🫶 SEE ALSO 🫶

io_cancel(2), io_destroy(2), io_getevents(2), io_submit(2), io_setup(3), aio(7)

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and that's the whole page bestie, go forth and set up your contexts 💖 this lil cutie comes to you courtesy of Linux man-pages 6.20, dated 2026-04-11. mwah 🍓✨