creating/modifying sparse files on linux

[draghuram at gmail.com]

My goal is very simple. Have a mechanism to create sparse files and
modify them by writing arbitratry ranges of bytes at arbitrary offsets.
I did get the information I want (xrange instead of range, and a simple
way to generate 1Mb string in memory). Thanks for pointing out about
using "len" as variable. It is indeed silly.

My only assumption from underlying OS/file system is that if I seek
past end of file and write some data, it doesn't generate blocks for
data in between. This is indeed true on Linux (I tested on ext3).

Thanks,
Raghu.