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SETEUID(2) Linux Programmer's Manual SETEUID(2)
NAME
seteuid, setegid - set effective user or group ID
SYNOPSIS
#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 <= 2.19: */ _BSD_SOURCE
DESCRIPTION
seteuid() sets the effective user ID of the calling process. Unprivileged 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 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().
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.
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 implemen‐
tation 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).
SEE ALSO
geteuid(2), setresuid(2), setreuid(2), setuid(2), capabilities(7), credentials(7), user_namespaces(7)
Linux 2021-03-22 SETEUID(2)