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seteuid() - Unix, Linux System Call
seteuid, setegid - set effective user or group ID
int seteuid(uid_t euid);
int setegid(gid_t egid);
seteuid() sets the effective user ID of the current 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".
On success, zero is returned. On error, -1 is returned, and
errno is set appropriately.
The current process is not privileged (Linux: does not have the
CAP_SETUID capability in the case of
seteuid(), or the
CAP_SETGID capability in the case of
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 glibc2.0
seteuid(euid) is equivalent to
setreuid(-1, euid) and hence may change the saved set-user-ID.
Under glibc2.1 it is equivalent to
setresuid(-1, euid,-1) and hence does not change the saved set-user-ID.
Similar remarks hold for