how to calculate the amount of data had been download
 

hi,
i'm trying to write a program with c socket programming,
what i am trying to reach is a program which will calculate a computer's downloaded data from the internet,just to know how much he/she download?
would anyone please just give the idea of how to do this!
thanks in advance

libpcap is probably what you're after - it'll allow you (so long as you have root privileges) to see all the traffic that goes over the network on any given interface (or all interfaces - though that means you'll also get the loopback interface, which I'm not sure you'll want) and so see how much there is.