How to write a file inside Isolated Storage in C?

How to write a file inside Isolated Storage in C?

Let's say I open a file for writing in C.
fopen_s(&myFilePointer, "filename.bin", "wb");
If I debug my Windows Phone application from the Windows Phone Runtime
Component side, I see that there's a file somewhere that is written. At
least, there is no crash when writing.
But strangely, if I open a tool like IsoStoreSpy, I don't see any new file
in the application root folder. Does that means my created file doesn't
exist?
Should I specify a special directory when I want to write to the Isolated
Storage?
[Edit]
I'm also opening a file for reading the same way (no directory, no
isolated storage specific operation), and it works, no matter if I add it
as a resource or as a content. Then when the file is converted with a
codec to a destination, I can't see that file when exploring the phone.