I suggest "man mmap" as a start. Notice the reference at the bottom of the page to getpagesize(), and consider the modulus operator (%). That should help you out... :-)

You'll also find sample code that uses mmap() at http://www.redhat.com/~johnsonm/lad/src/map-cat.c.html Other sample source you might find useful is at http://www.redhat.com/~johnsonm/lad/src/