>SUMMARY:
>I have two data streams being written to a single socket connection:
>"Important" data (from a data generator), and "Fast" data (read from a
>file). I wanted a way to prevent Fast data from filling up my TCP Send
>Buffer and preventing Important data from getting sent. My thought was to
>discern the amount of free space in the TCP Send Buffer before writing
>each Fast data record. If there weren't at least X bytes free, I would
>delay the write until later. ("Important" data would always get written.)
>But I didn't know how to find the TCP Send Buffer free space. [Note, I'm
>speaking from an application level here.]

	did you consider TCP urgent data?  see sendmsg(2) SEND_OOB case,
	recvmsg(2) MSG_OOB case, and probably SO_OOBINLINE socket option.

itojun