SETREUID(2) BSD System Calls Manual SETREUID(2)
setreuid -- set real and effective user IDs
setreuid(uid_t ruid, uid_t euid);
The real and effective user IDs of the current process are set according
to the arguments. The saved user ID will be set to the new value of the
real user ID if a real user ID is specified and either the new real user
ID value is different from the current value or the new value of the
effective user ID differs from the current saved user ID.
Unprivileged users may change either user ID to the current value of the
real, effective, or saved user ID. Only the superuser may make other
Supplying a value of -1 for either the real or effective user ID forces
the system to substitute the current ID in place of the -1 parameter.
The setreuid() function was intended to allow swapping the real and
effective user IDs in set-user-ID programs to temporarily relinquish the
set-user-ID value. This purpose is now better served by the use of the
When setting the real and effective user IDs to the same value, the
setuid(2) function is preferred.
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
[EPERM] The current process is not the superuser and a change
other than changing the effective user ID to the real
user ID was specified.
getuid(2), seteuid(2), setregid(2), setresuid(2), setuid(2)
The setreuid() function conforms to the IEEE Std 1003.1-2008
(``POSIX.1'') specification, except that the conditions for changing the
saved user ID differ and that, if it is changed, the saved user ID is set
to the real user ID instead of the effective user ID.
The setreuid() system call first appeared in 4.1cBSD, predating POSIX. A
semantically different version appeared in 4.4BSD. The current version,
with the original semantics restored, appeared in OpenBSD 3.3.
BSD April 29, 2017 BSD