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

       setresuid, setresgid - set real, effective, and saved user or group ID

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <unistd.h>

       int setresuid(uid_t ruid, uid_t euid, uid_t suid);
       int setresgid(gid_t rgid, gid_t egid, gid_t sgid);

       setresuid() sets the real user ID, the effective user ID, and the saved
       set-user-ID of the calling process.

       An unprivileged process may change its real UID, effective UID, and saved
       set-user-ID, each to one of: the current real UID, the current effective
       UID, or the current saved set-user-ID.

       A privileged process (on Linux, one having the CAP_SETUID capability) may
       set its real UID, effective UID, and saved set-user-ID to arbitrary

       If one of the arguments equals -1, the corresponding value is not

       Regardless of what changes are made to the real UID, effective UID, and
       saved set-user-ID, the filesystem UID is always set to the same value as
       the (possibly new) effective UID.

       Completely analogously, setresgid() sets the real GID, effective GID, and
       saved set-group-ID of the calling process (and always modifies the
       filesystem GID to be the same as the effective GID), with the same
       restrictions for unprivileged processes.

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

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

       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 (did not have the necessary
              capability in its user namespace) and tried to change the IDs to
              values that are not permitted.  For setresuid(), the necessary
              capability is CAP_SETUID; for setresgid(), it is CAP_SETGID.

       These calls are available under Linux since Linux 2.1.44.

       These calls are nonstandard; they also appear on HP-UX and some of the

       Under HP-UX and FreeBSD, the prototype is found in <unistd.h>.  Under
       Linux, the prototype is provided by glibc since version 2.3.2.

       The original Linux setresuid() and setresgid() system calls supported
       only 16-bit user and group IDs.  Subsequently, Linux 2.4 added
       setresuid32() and setresgid32(), supporting 32-bit IDs.  The glibc
       setresuid() and setresgid() 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 setresuid() and setresgid()) 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).

       getresuid(2), getuid(2), setfsgid(2), setfsuid(2), setreuid(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                       SETRESUID(2)