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

       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
       calling process is privileged (more precisely: if the process  has  the
       CAP_SETUID  capability  in  its user namespace), the real UID and saved
       set-user-ID are also set.

       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 pro-
       gram to regain root privileges.

       Thus, a set-user-ID-root program wishing to temporarily drop root priv-
       ileges, 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 applica-
	      tions  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	names-

       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),  cre-
       dentials(7), user_namespaces(7)

       This  page  is  part of release 4.10 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

Linux				  2016-10-08			     SETUID(2)