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SETREUID(2)                Linux Programmer's ManualSETREUID(2)NAME       setreuid, setregid - set real and/or effective user or group IDSYNOPSIS       #include <sys/types.h>       #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 (seefeature_test_macros(7)):       setreuid(), setregid():           _XOPEN_SOURCE >= 500               || /* Since glibc 2.19: */ _DEFAULT_SOURCE               || /* Glibc versions <= 2.19: */ _BSD_SOURCEDESCRIPTION       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 "user".RETURN VALUE       On success, zero is returned.  On error, -1 is returned, and  errno  is       set appropriately.       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  re-       turn from setreuid().ERRORS       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 ex-ecve(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.CONFORMING TO       POSIX.1-2001, POSIX.1-2008, 4.3BSD (setreuid() and setregid() first ap-       peared in 4.2BSD).NOTES       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  se-       treuid32()  and  setregid32(),  supporting  32-bit  IDs.  The glibc se-       treuid() 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 re-       quirements 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, seenptl(7).SEE ALSOgetgid(2),getuid(2),seteuid(2),setgid(2),setresuid(2),setuid(2),capabilities(7),credentials(7),user_namespaces(7)COLOPHON       This page is part of release 5.05 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       https://www.kernel.org/doc/man-pages/.Linux                             2017-09-15SETREUID(2)