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



NAME
       setuid - set user identity

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

       int setuid(uid_t uid);

DESCRIPTION
       setuid() sets the effective user ID of the calling process.  If the effective UID of the caller is root, 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 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 afterwards cannot use setuid().  You can accomplish this with seteuid(2).

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

ERRORS
       EAGAIN The uid does not match the current uid and uid brings process over its RLIMIT_NPROC resource limit.

       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.

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

NOTES
   Linux Notes
       Linux  has  the concept of file system user ID, normally equal to the effective user ID.  The setuid() call also sets the
       file system 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.

SEE ALSO
       getuid(2), seteuid(2), setfsuid(2), setreuid(2), capabilities(7), credentials(7)

COLOPHON
       This page is part of release 3.25 of the Linux man-pages project.  A description of the project,  and  information  about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                      2010-02-21                                                  SETUID(2)