ENOSYS(1)

✨ ENOSYS(1) ✨

okay bestie so you know how this whole thing lives in the User Commands section of the manual? yeah. and it's like, the enosys page. let's get into it 💅

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🌸 NAME 🌸

enosys — a lil utility that makes syscalls fail with ENOSYS. literally its whole job. iconic.

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🎀 SYNOPSIS 🎀

enosys [--syscall|-s syscall] command

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🦋 DESCRIPTION 🦋

so enosys is a simple command that runs a child process where certain syscalls just... fail. with errno set to ENOSYS. not me making your syscalls go poof 💀

it's giving vibe check for old kernels — you use it to test how an app behaves when syscalls are missing, like what would happen if you were running on an ancient kernel. fr fr.

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✨ OPTIONS ✨

• -s, --syscall — the syscall you wanna block with ENOSYS. you can specify this multiple times, no cap. and if you want a different error number or name, just tack it on with a colon. 🫶
• -i, --ioctl — the ioctl you wanna block with ENOTTY. also stackable, specify it as many times as you want. alternative error number or name? colon. same deal. 💖
• -l, --list — list out the syscalls that enosys knows about. a little roster, if you will. 📋
• -m, --list-ioctl — list out the ioctls that enosys knows about. the ioctl roster. ✨
• -d, --dump[=file] — dump the seccomp bytecode filter to standard output.

the dump can, for example, be used by setpriv --seccomp-filter. so like, you can hand it off to your other fave tool. 🎁

• -h, --help — show the help text and dip. 👋
• -V, --version — show the version and dip. 👑

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🌈 EXAMPLES 🌈

# fail syscall "fallocate" with ENOSYS
enosys -s fallocate ...

# fail syscall "fallocate" with ENOMEM
enosys -s fallocate:ENOMEM ...

# fail syscall "fallocate" with value 12/ENOMEM
enosys -s fallocate:12 ...

# fail ioctl FIOCLEX with ENOTTY
enosys -i FIOCLEX ...

# fail ioctl FIOCLEX with ENOMEM
enosys -i FIOCLEX:ENOMEM ...

# fail ioctl FIOCLEX with value 12/ENOMEM
enosys -i FIOCLEX:12 ...

see the pattern? colon = "actually, fail it with this instead". the way that's so clean 😭

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💜 EXIT STATUS 💜

enosys exits with the status code of the process it executed. periodt. but these two values have special meanings, so pay attention bestie:

1. 1 — internal error. something inside went wrong. 🙈
2. 2 — the system does not provide the necessary functionality. like, the machine literally can't do the thing. 💀

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👑 AUTHORS 👑

Thomas Weißschuh <thomas@t-8ch.de> — shoutout to the main character who wrote this 🫶

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🌟 SEE ALSO 🌟

syscall(2)

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🍓 REPORTING BUGS 🍓

found a bug? for bug reports, use the issue tracker: <https://github.com/util-linux/util-linux/issues>. go tell them, they wanna know 💌

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🧚 AVAILABILITY 🧚

the enosys command is part of the util-linux package, which can be downloaded from the Linux Kernel Archive: <https://www.kernel.org/pub/linux/utils/util-linux/>. go grab it bestie 🛍️

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and that's the whole page, no notes 💖 this was util-linux 2.42.4, dated 2026-09-21 — mwah, slay, bye 👑✨