How to monitor Linux UDP buffer available space?

You are trying to solve the wrong problem. UDP is unreliable communication, period. If packet loss is a problem for you, you should either implement your own retransmission/error control algorithm or not using UDP in the first place. Drop it entirely in favor of TCP, or perhaps something more advanced like SCTP or even DCCP.

UDP is datagram-based, socket buffers should be bigger than the maximum length of an UDP datagram that your application may receive, up to 64kiB. If your application may transmit datagrams larger than this, then it is another reason you should not be using UDP. And it really doesn't matter how big is the buffer, you still may lose packets if your application can't read the socket faster than packets arrive.

You say that you want to know how full the UDP buffer is. This is the sort of thing that doesn't really matter. Just read everything that is waiting on the buffer and you can be sure that when it blocks the buffer will be empty.

If you still want to know how full the buffer is, read the file /proc/net/udp, 

             1. column rx_queue. But if you see any value different than zero in that column, it just means that your application is not reading the socket fast enough.

             2. column tx_queue is the same to rx_queue

   /proc/net/tcp  is  tcp info