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 afterward 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 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.

SEE ALSO

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

COLOPHON

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