S60: filename access on either phone mem or mem card

How do i declare filenames so that my Series60 application is runs well if it is installed on either phone memory or the memory card..

i have hardcoded a filename in my code like this:

\\system\\apps\\myapp\\filename.dat

Now when i installed the application in the phone memory it works fine.. but when i installed the application in the memory card.. the application panics...