Using this file http://code.google.com/p/jquery-speedtest/downloads/detail?name=100MB.txt
This is one line full of 0s, so there are 104857600 0s.
wget http://jquery-speedtest.googlecode.com/files/100MB.txt
main.m reads the whole file at once. Doing so it occupies more than 200MB of memory (100MB for the NSString, another 100MB for the char array), as you can see here http://d.pr/i/jopC
main_memory_mapped.m maps the file in-memory, incrementally reading it; therefore its far less memory aggressive, as you can see here http://d.pr/i/YYzi