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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 effective 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 ID.
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
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 setreuid(), or the CAP_SET-
GID 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 4.2BSD).
Setting the effective user (group) ID to the saved set-user-ID (saved
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),
Linux 2013-12-12 SETREUID(2)