If you want to see a more complete example of how to use INET_DIAG, check out tcp_closer
For a project I was working on lately, I needed to passively monitor a large number of TCP sockets. One common way to do is to parse the output of /proc/tcp, but my goal was to create a "clean" solution and avoid for example text-parsing.
After doing some digging, I discovered the convenient ss-utility (of the iproute-suite), which does something similar to what I want. ss makes use of NETLINK and the INET_DIAG-sockets for a nice way of extracting connection information. ss has support for all protocols (sockets) supporting exporting information, so the code is very generic and quite large.
Combined with a lack of INET_DIAG-documentation (the kernel source is your friend), I decided to create a small, easy to follow example of how INET_DIAG can be used to passively monitor sockets. A detailed description of the example is available here.