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

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

       #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 (see feature_test_macros(7)):

       setreuid(), setregid():
	   _XOPEN_SOURCE >= 500
	       || /* Since glibc 2.19: */ _DEFAULT_SOURCE
	       || /* Glibc versions <= 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 "user".

       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
       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

       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)

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Linux				  2016-10-08			   SETREUID(2)