SETREUID(2)                 Linux Programmer's Manual                SETREUID(2)

       setreuid, setregid - set real and/or effective user or group ID

       #include <unistd.h>

       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)):

       setreuid(), setregid():
           _XOPEN_SOURCE >= 500
               || /* Since glibc 2.19: */ _DEFAULT_SOURCE
               || /* Glibc <= 2.19: */ _BSD_SOURCE

       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 of

       On success, zero is returned.  On error, -1 is returned, and errno is set
       to indicate the error.

       Note: there are cases where setreuid() can fail even when the caller is
       UID 0; it is a grave security error to omit checking for a failure return
       from setreuid().

       EAGAIN The call would change the caller's real UID (i.e., ruid does not
              match the caller's real UID), but there was a temporary failure
              allocating the necessary kernel data structures.

       EAGAIN ruid does not match the caller's real UID and this call would
              bring the number of processes belonging to the real user ID ruid
              over the caller's RLIMIT_NPROC resource limit.  Since Linux 3.1,
              this error case no longer occurs (but robust applications should
              check for this error); see the description of EAGAIN in execve(2).

       EINVAL One or more of the target user or group IDs is not valid in this
              user namespace.

       EPERM  The calling process is not privileged (on Linux, does not have the
              necessary capability in its user namespace: CAP_SETUID in the case
              of setreuid(), or CAP_SETGID 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, POSIX.1-2008, 4.3BSD (setreuid() and setregid() first
       appeared in 4.2BSD).

       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 the UID changes that Linux permits for an
       unprivileged process.  For setreuid(), the effective user ID can be made
       the same as the real user ID or the saved 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 implementations.

       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.

   C library/kernel differences
       At the kernel level, user IDs and group IDs are a per-thread attribute.
       However, POSIX requires that all threads in a process share the same
       credentials.  The NPTL threading implementation handles the POSIX
       requirements by providing wrapper functions for the various system calls
       that change process UIDs and GIDs.  These wrapper functions (including
       those for setreuid() and setregid()) employ a signal-based technique to
       ensure that when one thread changes credentials, all of the other threads
       in the process also change their credentials.  For details, see nptl(7).

       getgid(2), getuid(2), seteuid(2), setgid(2), setresuid(2), setuid(2),
       capabilities(7), credentials(7), user_namespaces(7)

       This page is part of release 5.13 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2021-03-22                        SETREUID(2)