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       setuid - set user identity


       #include <sys/types.h>
       #include <unistd.h>

       int setuid(uid_t uid);


       setuid()  sets  the  effective  user ID of the calling process.  If the
       effective UID of the caller is root (more precisely: if the caller  has
       the CAP_SETUID capability), the real UID and saved set-user-ID are also

       Under Linux, setuid() is implemented like the POSIX  version  with  the
       _POSIX_SAVED_IDS  feature.  This allows a set-user-ID (other than root)
       program to drop all of its user privileges, do some un-privileged work,
       and then reengage the original effective user ID in a secure manner.

       If  the  user  is root or the program is set-user-ID-root, special care
       must be taken.  The setuid() function checks the effective user  ID  of
       the  caller  and  if it is the superuser, all process-related user ID's
       are set to uid.  After this has occurred,  it  is  impossible  for  the
       program to regain root privileges.

       Thus,  a  set-user-ID-root  program  wishing  to  temporarily drop root
       privileges, assume the identity  of  an  unprivileged  user,  and  then
       regain   root  privileges  afterward  cannot  use  setuid().   You  can
       accomplish this with seteuid(2).


       On success, zero is returned.  On error, -1 is returned, and  errno  is
       set appropriately.

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


       EAGAIN The  call would change the caller's real UID (i.e., uid does not
              match the caller's real UID), but there was a temporary  failure
              allocating the necessary kernel data structures.

       EAGAIN uid  does not match the real user ID of the caller and this call
              would bring the number of processes belonging to the  real  user
              ID  uid  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 The user  ID  specified  in  uid  is  not  valid  in  this  user

       EPERM  The  user is not privileged (Linux: does not have the CAP_SETUID
              capability) and uid does not match the real UID  or  saved  set-
              user-ID of the calling process.


       POSIX.1-2001, POSIX.1-2008, SVr4.  Not quite compatible with the 4.4BSD
       call, which sets all of the real, saved, and effective user IDs.


       Linux has the concept of the filesystem user ID, normally equal to  the
       effective  user ID.  The setuid() call also sets the filesystem user ID
       of the calling process.  See setfsuid(2).

       If uid is different from the old effective UID,  the  process  will  be
       forbidden from leaving core dumps.

       The original Linux setuid() system call supported only 16-bit user IDs.
       Subsequently, Linux 2.4 added setuid32() supporting  32-bit  IDs.   The
       glibc  setuid() wrapper function transparently deals with the variation
       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  the  one  for  setuid()) 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


       getuid(2),  seteuid(2),  setfsuid(2),   setreuid(2),   capabilities(7),
       credentials(7), user_namespaces(7)


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