setup(2)

✨ setup(2) — the system call manual 💖

okay bestie, so this is setup(2) from the System Calls Manual and I need you to know upfront: you cannot use this. Like, at all. It's the syscall equivalent of a VIP-only backstage pass and you are not on the list. But the lore is so good, so let's get into it 🦋

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🌸 NAME 🌸

setup — set up devices and filesystems, mount the root filesystem

like, that's the whole job description. girlboss behaviour fr fr.

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📚 LIBRARY 📚

Standard C library (libc, -lc)

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✨ SYNOPSIS ✨

#include <unistd.h>

[[deprecated]] int setup(void);

see that [[deprecated]]? yeah. the compiler is literally side-eyeing it. 💀

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💅 DESCRIPTION 💅

okay so here's the tea: setup() gets called exactly once, from inside linux/init/main.c. one time. that's it. it's the opening number of the whole boot musical 🎭

what it does while it's up there:

• calls the initialization functions for the devices and filesystems that are configured into the kernel
• then mounts the root filesystem

and then it retires, because its moment has passed. periodt.

now the part you actually need to hear: no user process may call setup(). not you, not me, not your friend who thinks they know everything. any user process — and I mean any, even one with superuser permission — will receive EPERM. so don't even try it, the kernel said no and the kernel meant it 🙈

warning bestie: this is not a "well maybe if I'm root" situation. root gets EPERM too. superuser privilege does NOT unlock this door. it's not a permission issue, it's a you are a user process issue. 💀

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🫶 RETURN VALUE 🫶

for a user process, setup() always returns -1. always. no exceptions, no lucky days, no "but I asked nicely." 😭

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⭐ ERRORS ⭐

• EPERM — Always, for a user process.

(note the word always. not "sometimes." not "in certain conditions." ALWAYS. iconic honestly.)

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👑 STANDARDS 👑

Linux.

just Linux. that's the whole vibe. 🐧

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🧚 VERSIONS 🧚

• Removed in Linux 2.1.121. she's gone, she's retired, we remember her fondly 🎀
• and the calling sequence was messy the whole time ngl — at some times setup() had a single argument void *BIOS, and at other times it had a single argument int magic. different eras, different signatures, no consistency, we love a chaotic queen 💜

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🌈 and that's the whole page 🌈

lowkey the shortest man page you'll ever read but it ate. remember: you can't call it, you can't have it, it left in 2.1.121. iykyk 💅

this was brought to you by Linux man-pages 6.20, dated 2026-02-08 — mwah, stay hydrated, read your man pages, bye bestie 🦋💖✨