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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.

       Unprivileged user processes may change the 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.

       Privileged processes (on Linux, those having the CAP_SETUID capability)
       may set the 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 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 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  (did  not  have  the
              CAP_SETUID capability) and tried to change  the  IDs  to  values
              that are not permitted.


       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)


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