seteuid(2)		       System Calls Manual		      seteuid(2)

NAME
     seteuid, setegid - set effective user or group ID

LIBRARY
     Standard C library (libc, -lc)

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  ef-
     fective 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.

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 architec-
     ture).

     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 imple-
     mentations 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).

STANDARDS
     POSIX.1-2024.

HISTORY
     POSIX.1-2001, 4.3BSD.

SEE ALSO
     geteuid(2),  setresuid(2), setreuid(2), setuid(2), capabilities(7), creden-
     tials(7), user_namespaces(7)

Linux man-pages 6.18		   2026-02-08			      seteuid(2)