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

NAME
       setuid - set user identity

SYNOPSIS
       #include <unistd.h>

       int setuid(uid_t uid)

DESCRIPTION
       setuid  sets the effective user ID of the current process.
       If the effective userid of the caller is  root,  the  real
       and saved user ID's are also set.

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

       If the user is root or the program is setuid root, special
       care must be taken. The setuid function checks the  effec-
       tive  uid  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 setuid-root program wishing  to  temporarily  drop
       root  privileges,  assume the identity of a non-root user,
       and then regain  root  privileges  afterwards  cannot  use
       setuid.  You can accomplish this with the (non-POSIX, BSD)
       call seteuid.

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

ERRORS
       EPERM  The  user  is  not the super-user, and uid does not
              match the effective or saved user ID of the calling
              process.

CONFORMING TO
       SVr4, SVID, POSIX.1.  Not quite compatible with the 4.4BSD
       call, which sets all of the  real,  saved,  and  effective
       user  IDs.  SVr4 documents an additional EINVAL error con-
       dition.

LINUX-SPECIFIC REMARKS
       Linux has the concept  of  filesystem  user  ID,  normally
       equal to the effective user ID.  The setuid call also sets
       the filesystem user ID of the current process.  See  setf-
       suid(2).

       If  uid  is different from the old effective uid, the pro-
       cess will be forbidden from leaving core dumps.

SEE ALSO
       getuid(2), setreuid(2), seteuid(2), setfsuid(2)

Linux 1.1.36               29 July 1994                         1