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

       setuid - set user identity

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

       int setuid(uid_t uid);

       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
       re-engage 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 a non-root
       user,  and  then regain root privileges afterwards cannot use setuid().  You can accomplish this with the (non-
       POSIX, BSD) call seteuid(2).

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

       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.

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

   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.

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

       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at

Linux                             2004-05-27                         SETUID(2)