fd_set limitations

[Greg Munt <greg#uni-corn,demon.co.uk>]

On Mon, 28 Apr 1997 clawrenc#cup,hp.com wrote:

> >Also, an fd_set holds FD_SETSIZE file descriptors. Is it true that a 
> >theroretically infinite number of FDs can be opened by the server,
> >and  that I can handle > FD_SETSIZE connections by having some kind
> >of linked  list of fd_sets?
> 
> Almost always: No.  The exceptions are so rare and so limited as to be
> ignorable (certain classes of list based OS'es etc).

Every connected socket uses up a socket descriptor. I think it is common 
for FD_SETSIZE to be 256 (it is under GNU, anyway). So how would you be 
able to maintain, say, 300 simultaneous connections? 

Is this anything to do with port concentrators? I've heard of them, but have 
no idea what they are, or what they are used for..

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