Sun ws/IP socket/router question

[Brian Jay Gould]

How many bytes can I write to a fd wihout blocking after having just been
told by select that the fd can be written to...

The fd in question is to a tcp socket on SUN OS 4.0.3 ( a BSD 4.2
derivative... ) and the tcp socket will in many cases lead to a system
local 19.2Kb ptp InterNet Router connection(Xerox HDLC link) in others its
a local ethernet to another system that has a 19.2Kb InterNet Router
connection...

i'm looking for some general guidance on this issue... ie discussions of
system buffer availability is not the issue...  a 90% of the time correct
answer is acceptable...

The context of the answer is that a large(>23KB) message needs to broken
up to go over these slow links from a process that is multitasked.
Unfortunately use of a third party library package precludes the use of
nonblocking writes/sends since it is doing the io and it has some nasty
side effects when being used with the sun nbio library.

Thanks in advance.