SETREUID(2) Linux Programmer's Manual SETREUID(2)
setreuid, setregid - set real and/or effective user or group ID
int setreuid(uid_t ruid, uid_t euid);
int setregid(gid_t rgid, gid_t egid);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
setreuid() sets real and effective user IDs of the calling process.
Supplying a value of -1 for either the real or effective user ID forces the system to
leave that ID unchanged.
Unprivileged processes may only set the effective user ID to the real user ID, the effec-
tive user ID, or the saved set-user-ID.
Unprivileged users may only set the real user ID to the real user ID or the effective user
If the real user ID is set (i.e., ruid is not -1) or the effective user ID is set to a
value not equal to the previous real user ID, the saved set-user-ID will be set to the new
effective user ID.
Completely analogously, setregid() sets real and effective group ID's of the calling
process, and all of the above holds with "group" instead of "user".
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
EPERM The calling process is not privileged (Linux: does not have the CAP_SETUID capabil-
ity in the case of setreuid(), or the CAP_SETGID capability in the case of
setregid()) and a change other than (i) swapping the effective user (group) ID with
the real user (group) ID, or (ii) setting one to the value of the other or (iii)
setting the effective user (group) ID to the value of the saved set-user-ID (saved
set-group-ID) was specified.
POSIX.1-2001, 4.3BSD (the setreuid() and setregid() function calls first appeared in
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).
POSIX.1 does not specify all of possible ID changes that are permitted on Linux for an
unprivileged process. For setreuid(), the effective user ID can be made the same as the
real user ID or the save set-user-ID, and it is unspecified whether unprivileged processes
may set the real user ID to the real user ID, the effective user ID, or the saved set-
user-ID. For setregid(), the real group ID can be changed to the value of the saved set-
group-ID, and the effective group ID can be changed to the value of the real group ID or
the saved set-group-ID. The precise details of what ID changes are permitted vary across
POSIX.1 makes no specification about the effect of these calls on the saved set-user-ID
and saved set-group-ID.
The original Linux setreuid() and setregid() system calls supported only 16-bit user and
group IDs. Subsequently, Linux 2.4 added setreuid32() and setregid32(), supporting 32-bit
IDs. The glibc setreuid() and setregid() wrapper functions transparently deal with the
variations across kernel versions.
getgid(2), getuid(2), seteuid(2), setgid(2), setresuid(2), setuid(2), capabilities(7)
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Linux 2013-12-12 SETREUID(2)