Why ? When segmentation fault,there has no "core" file

["chao yeaj" <yeajchao gmail com>]

Hello everyone
   I compiled my application with the     ``-g ''       options
   and when debug with   gdb,  i can list the source file

   The problem is :  when   segmentation fault ,there has no    core file
   if the has a core file ,i can debug it, but there  has no    core file

  why?

  how can i let the  OS   create core file when  segmentation fault

Any comments would be much appreciated , thanks in   advance