SYNOPSIS

#include <sys/types.h>
#include <unistd.h>

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

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

seteuid(), setegid():

_POSIX_C_SOURCE >= 200112L
    || /* Glibc versions <= 2.19: */ _BSD_SOURCE

DESCRIPTION

seteuid() sets the effective user ID of the calling process. Unprivileged user processes may only set the effective user ID to the real user ID, the effective user ID or the saved set-user-ID.

Precisely the same holds for setegid() with "group" instead of "user".

RETURN VALUE

On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

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().

CONFORMING TO

POSIX.1-2001, POSIX.1-2008, 4.3BSD.

NOTES

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).

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, setreuid(2) and setregid(2).

COLOPHON

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