setuid - set user identity


     #include <unistd.h>

     int setuid(uid_t uid))


     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
     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 setuid  root,  special
     care must be taken. The setuid function checks the effective
     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.


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


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


     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 condition.


     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 setfsuid(2).

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


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