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 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: setuid() 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.

       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().

ERRORS

       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 namespace.

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

CONFORMING TO

       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.

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.

   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 nptl(7).

SEE ALSO

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

COLOPHON

       This page is part of release 5.10 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
       https://www.kernel.org/doc/man-pages/.