using find-grep in emacs

[Rami A]

Hi,
I am trying to setup some default for grep-find in emacs.
I do the following to search in the source code:
M-x find-grep
which give:
find . -type f -exec grep -n  {} /dev/null \;

Now I have to change /dev/null to be the directory which I want to search in.
How is it possible to not do that every time I use find-grep and that it could 
remember the directory I am specifying.

Also, How to make it default to search all three file types .h .s .c and 
nothing else?