S_ISUID  (04000)  set-user-ID (set process effective user ID on execve(2))
S_ISGID  (02000)  set-group-ID  (set  process effective group ID on execve(2); mandatory locking, as described in fcntl(2);

　　　　　　　　take a new file‘s group from parent directory, as described in chown(2) and mkdir(2))
S_ISVTX  (01000)  sticky bit (restricted deletion flag, as described in unlink(2))

文件模式设置用户ID/设置组ID/sticky bit