seteuid(2)

💖 seteuid(2) — the "who am I really today" girlies 💖

okay bestie, we're in section 2 of the manual (System Calls Manual, that's the syscall girlies) and today's topic is seteuid and setegid. it's giving identity crisis but make it privileged. let's go 🦋

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✨ NAME ✨

seteuid, setegid — set effective user or group ID

so like. they change your effective ID. yes there's a real one and an effective one and a saved one. yes it's a whole thing. we'll get there. 💅

📚 LIBRARY 📚

Standard C library (libc, -lc) — the classics, no cap.

🌸 SYNOPSIS 🌸

#include <unistd.h>

int seteuid(uid_t euid);
int setegid(gid_t egid);

look at them. two little functions. so much power. 😭

🎀 Feature Test Macro Requirements for glibc (see feature_test_macros(7)) 🎀

seteuid(), setegid():
    _POSIX_C_SOURCE >= 200112L
        || /* glibc <= 2.19: */ _BSD_SOURCE

ngl if you're on old glibc (2.19 or earlier) you're gonna need _BSD_SOURCE to unlock these. vintage era behavior, we respect the elders 🫶

💖 DESCRIPTION 💖

seteuid() sets the effective user ID of the calling process. that's the one the kernel actually checks when you try to do stuff.

here's the tea for unprivileged processes: they may only set the effective user ID to

• the real user ID,
• the effective user ID, or
• the saved set-user-ID.

that's it. that's the whole shortlist. no wandering off the menu, bestie.

Precisely the same holds for setegid() with "group" instead of "user". literally the exact same deal, just group flavored. 🍓

🔥 RETURN VALUE 🔥

On success, zero is returned. cute. clean. slay.

On error, -1 is returned, and errno is set to indicate the error.

Note: there are cases where seteuid() can fail even when the caller is UID 0; it is a grave security error to omit checking for a failure return from seteuid(). not me stressing about this but like. CHECK. YOUR. RETURN. VALUE. 😭🔒 serious business only.

💀 ERRORS 💀

• EINVAL — The target user or group ID is not valid in this user namespace.
• EPERM — In the case of seteuid(): the calling process is not privileged (does not have the CAP_SETUID capability in its user namespace) and euid does not match the current real user ID, current effective user ID, or current saved set-user-ID.

In the case of setegid(): the calling process is not privileged (does not have the CAP_SETGID capability in its user namespace) and egid does not match the current real group ID, current effective group ID, or current saved set-group-ID.

so basically, no capability + no match = permission denied, bestie. vibe check failed 👑❌

🌈 VERSIONS 🌈

Setting the effective user (group) ID to the saved set-user-ID (saved set-group-ID) is possible since Linux 1.1.37 (1.1.38). On an arbitrary system one should check _POSIX_SAVED_IDS.

Under glibc 2.0, seteuid(euid) is equivalent to setreuid(-1, euid) and hence may change the saved set-user-ID. Under glibc 2.1 and later, it is equivalent to setresuid(-1, euid, -1) and hence does not change the saved set-user-ID. Analogous remarks hold for setegid(), with the difference that the change in implementation from setregid(-1, egid) to setresgid(-1, egid, -1) occurred in glibc 2.2 or 2.3 (depending on the hardware architecture).

so like. old glibc was out here changing your saved ID as a side quest and modern glibc said "not today". glow up fr. ✨

According to POSIX.1, seteuid() (setegid()) need not permit euid (egid) to be the same value as the current effective user (group) ID, and some implementations do not permit this.

🧚 C library/kernel differences 🧚

On Linux, seteuid() and setegid() are implemented as library functions that call, respectively, setresuid(2) and setresgid(2).

it's a wrapper. a cute little middleman. passing the message along 💌

⭐ STANDARDS ⭐

POSIX.1-2024. periodt.

👑 HISTORY 👑

POSIX.1-2001, 4.3BSD. ancient. iconic. still here.

🦋 SEE ALSO 🦋

geteuid(2), setresuid(2), setreuid(2), setuid(2), capabilities(7), credentials(7), user_namespaces(7)

go read them, they're all part of the same lore arc i promise 📖✨

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okay that's the whole page bestie!! this one's from Linux man-pages 6.20, dated 2026-02-08, and it's filed under seteuid(2). love that for us. go check your return values, stay hydrated, and never trust a -1 💖🎀🫶