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
       (more precisely: if the caller has the CAP_SETUID capability), 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.

       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) 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

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