SETEUID(2) Linux Programmer's Manual SETEUID(2)
seteuid, setegid - set effective user or group ID
int seteuid(uid_t euid);
int setegid(gid_t egid);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
seteuid(), setegid(): _BSD_SOURCE || _POSIX_C_SOURCE >= 200112L ||
_XOPEN_SOURCE >= 600
seteuid() sets the effective user ID of the calling process. Unprivi-
leged 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".
On success, zero is returned. On error, -1 is returned, and errno is
EPERM The calling process is not privileged (Linux: does not have the
CAP_SETUID capability in the case of seteuid(), or the CAP_SET-
GID capability in the case of setegid()) and euid (respectively,
egid) is not the real user (group) ID, the effective user
(group) ID, or the saved set-user-ID (saved set-group-ID).
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 libc4, libc5 and 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. Similar remarks hold for
geteuid(2), setresuid(2), setreuid(2), setuid(2), capabilities(7), cre-
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be found at http://www.kernel.org/doc/man-pages/.
Linux 2007-07-26 SETEUID(2)