exceptfds in select(2)

[Michael Garzione]

The form of select(2) is
	
     nfound = select(nfds, readfds, writefds, exceptfds, timeout)
     int nfound, nfds;
     fd_set *readfds, *writefds, *exceptfds;
     struct timeval *timeout;

The only references to "exceptfds" I can find say that it "indicates
if an exceptional condition is pending" (man page); "indicates when exceptional
conditions exist for particular file descriptors, useful in networking"
(Bach); and misc. discussions about reading and writing (Leffler et. al.)

My guess is that the appropriate bit in exceptfds is set when the i/o 
channel associated with a descriptor is lost, or in the case of some 
sockets, out of band data is present (or maybe that would be readfds?).

Does anyone know for sure what a set bit in exceptfds indicates (what an
"exceptional condition" is)?

Mike Garzione
Convex Computer Corporation                            
{uunet,sun}!convex!garzione
garzione at convex.com