With POSIX IDs disabled, implemented for program portability and
    serves no function. It returns 0 (to indicate success).

    With POSIX IDs enabled, sets the user IDs.

    Format

      #include  <types.h>

      #include  <unistd.h>

      int setuid  (__uid_t uid); (_DECC_V4_SOURCE)

      uid_t setuid  (uid_t uid); (not _DECC_V4_SOURCE)