SD_PIDFD_GET_INODE_ID(3)

💖 sd_pidfd_get_inode_id(3) — the manual page, bestie edition 💖

okay so this is the sd_pidfd_get_inode_id(3) page from the systemd manual (section 3, the library-functions girlies). it's short, it's sweet, it's giving "one function, big opinions." let's get into it 🎀

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✨ NAME ✨

sd_pidfd_get_inode_id — grab the 64-bit inode ID of a PID file descriptor (aka a PIDFD), no cap 🦋

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✨ SYNOPSIS ✨

#include <systemd/sd-daemon.h>

int sd_pidfd_get_inode_id(int pidfd, uint64_t *ret);

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✨ DESCRIPTION ✨

so you can call sd_pidfd_get_inode_id() when you wanna acquire the 64-bit inode ID of a PID file descriptor (a PIDFD, iykyk). and bestie, the whole point of this number is that you can use it to reliably identify a process for the current boot. that's the vibe. ✨

classic example incoming: the service manager sets the $MAINPIDFDID and $MANAGERPIDFDID environment variables — those are the inode IDs of the service main process and the service manager itself, respectively — but only if the kernel supports that functionality. if the kernel isn't serving, you're not getting the env vars, periodt. 💅

now here's the tea on 64-bit vs 32-bit architectures, because this is literally why the function exists:

• on 64-bit architectures you can just call fstat(2) on a given pidfd and the inode ID falls right out. easy. iconic. 👑
• on 32-bit architectures though? struct stat's .st_ino field is also 32-bit, which — much like PIDs — is subject to reuse. 🫠 not the reliable-boot-identity vibe we're going for.
• so a second mechanism leveraging name_to_handle_at(2) was added to the kernel in v6.14. and this helper? it's here to simplify downstream handling of pidfd/pidfs internals. slay. 🧚

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✨ RETURN VALUE ✨

on success, the function returns 0 or a positive integer. 🎉

on failure, you get a negative errno-style error code back. 😭 (that's the "not me getting an error" moment.)

Errors 🌸

the returned errors may mean any of the following situations, so read carefully bestie:

• -EOPNOTSUPP — the stable PIDFD inode ID is not supported by the running kernel, or the system is 32-bit and name_to_handle_at() is unavailable. 💀
• -EBADF — the file descriptor you handed over is invalid, or it's just... not a PIDFD. wrong vibe entirely. 🙈

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✨ NOTES ✨

the functions described here are available as a shared library, and you can compile against it and link to it with the libsystemd pkg-config(1) file. 🫶 that's the whole note. short and sweet.

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✨ HISTORY ✨

sd_pidfd_get_inode_id() was added in version 258. new-ish girl on the block, but she's doing great. ⭐

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✨ SEE ALSO ✨

• systemd(1)
• sd-daemon(3)
• sd_notify(3)
• systemd.exec(5)

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🌈 friendly reminder from your local unix bestie: every option, error, env var and version number above is exactly what the page says — the personality is mine, the facts are theirs. don't @ me. 💜

and that's the page!! this manual came to you courtesy of systemd 262.1 (the SD_PIDFD_GET_INODE_ID(3) page itself, section 3). go forth and identify your processes reliably for the current boot, queen. 💅✨🎀