using CSCOPE

1. build database: cscope -b -q -k -R
2. invoke: cscope -d ... does not create the database again

3: invoke command line:
      find all functions that calls bad_page()
          cscope -d -3 bad_page -L
      find all functions that bad_page() calls
          cscope -d -2 bad_page -L

exit: ctrl_d