getresuid(2)

💖 getresuid(2) — the system calls manual page, bestie edition 💖

okay so this is the getresuid(2) page from the System Calls Manual, and we're about to make UID/GID lore actually make sense. stay with me 🫶

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✨ NAME ✨

getresuid, getresgid — get real, effective, and saved user/group IDs

like... it literally just gets them. no cap.

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📚 LIBRARY 📚

Standard C library (libc, -lc)

the og. the moment. link against her and you're set 💅

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🎀 SYNOPSIS 🎀

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <unistd.h>

int getresuid(uid_t *ruid, uid_t *euid, uid_t *suid);
int getresgid(gid_t *rgid, gid_t *egid, gid_t *sgid);

okay so bestie, notice how you gotta define _GNU_SOURCE before you pull in <unistd.h> — and yes there's a whole other page about that, it's feature_test_macros(7), go read it i'm literally begging 🙈

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🌸 DESCRIPTION 🌸

so getresuid() hands you back three things about the calling process, and it drops them into the args you passed in:

• ruid → the real UID
• euid → the effective UID
• suid → the saved set-user-ID

and getresgid() is literally the same serve but for the process's group IDs. analogous task, different IDs, same energy. periodt ✨

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🌈 RETURN VALUE 🌈

• on success → 0 is returned. slay.
• on error → -1 is returned, and errno gets set to tell you what went wrong.

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💀 ERRORS 💀

• EFAULT — one of the arguments pointed at an address outside the calling program's address space. so like... you handed it a pointer into the void. it's giving wanderlust, but for memory. not the vibe. 😭

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⭐ STANDARDS ⭐

POSIX.1-2024 XSI 💅 certified iconic, we love a standardized baddie

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👑 HISTORY 👑

Linux 2.1.44, glibc 2.3.2, POSIX.1-2024 XSI. a whole glow-up timeline fr fr.

and okay, storytime 🍓 — the original Linux getresuid() and getresgid() system calls only ever supported 16-bit user and group IDs. sixteen. bits. tiny. baby.

then Linux 2.4 showed up and added getresuid32() and getresgid32(), which support 32-bit IDs — the main character era.

and the way the glibc getresuid() and getresgid() wrapper functions handle all of this?? they transparently deal with the variations across kernel versions. you don't have to think about it. you just call the function. she handles the drama behind the scenes. we're obsessed 🧚

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🦋 SEE ALSO 🦋

• getuid(2)
• setresuid(2)
• setreuid(2)
• setuid(2)
• credentials(7)

go read them, i'm not asking 💜

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and that's the whole page bestie!! this was Linux man-pages 6.20, dated 2026-02-08, and honestly?? iconic of her. now go write some C and hydrate 💖✨🎀🫶