seteuid(2) -- Linux man page
NAMEseteuid, setegid - set effective user or group ID
DESCRIPTIONseteuid 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 user ID.
Precisely the same holds for setegid with "group" instead of "user".
RETURN VALUEOn success, zero is returned. On error, -1 is returned, and errno is set appropriately.
- The current process is not the super-user and euid (resp. egid) is not the real user (group) ID, the effective user (group) ID or the saved user (group) ID.
NOTESSetting the effective user (group) ID to the saved user (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 user ID. Under glibc2.1 it is equivalent to setresuid(-1, euid,-1) and hence does not change the saved user ID. Similar remarks hold for setegid.
CONFORMING TOBSD 4.3
SEE ALSOgeteuid(2), setuid(2), setreuid(2), setresuid(2)