Socket buffer usage

[Ivan Voras]

Hi,

I'm tracking down an obscure bug in my userland program and it might
have something to do with the way I write&read data through a (Unix
domain) socket. I'm setting SO_SNDBUF and SO_RCVBUF, and what I'm
looking for is some way to query the amount of TX & RX buffered / free
data on a socket. Is there something I can use? I'll even accept
inspecting kernel structures if explained in detail and can be done on
a running system.

Alternatively, is there anything else which could cause poll(2) with
POLLOUT on a socket to return no events ready on such a socket? (my
expectation being that a socket is always ready to be written to if
there is buffer space free...).