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       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-

       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 values.

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

       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 appropriately.

       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

       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 (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


       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 BSDs.


       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)


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