how can I get maximum bandwidth of an interface in linux C?

Let's say I have a machine with 3 NICs; wifi, eth, and 3G. I'm trying to get the ESTIMATED speed (or bandwidth) at which a link CAN transmit at a given time for each of these interfaces and determine which one is the fastest. Again at a given time. I'm writing a linux kernel module. I'm using kernel version 2.6.35. Is there anyway to measure the available bandwidth? Even if it meant sending a testing ping to measure something like we do in measuring latency ...

What is interface bandwidth?

Introduction. The Interface Bandwidth report displays maximum and average values for interface inbound and outbound throughputs. This information is presented in a tabular data format and can additionally include graphs per interface.

iperf can help you measure actual bandwidth http://sourceforge.net/projects/iperf/