>> 	- we can have file descriptor larger than FD_SETSIZE.
>> 	- we can pass fd_set bitmap larger than FD_SETSIZE okay.
>> 	  (see usr.sbin/traceroute6/traceroute6.c for instance)
>> 	so i don't see the need for returning error when nfds > FD_SETSIZE.

> No - that code is broken, or at least not fully portable.

> Not least because is assumes the format of fd_set - which is not
> public.

Well, FD_SETSIZE is not a constant system-wide.  As the manpage for
select(2) notes, if you define FD_SETSIZE as a larger-than-default
value before your includes, you can raise the effective limit and still
use the FD_* macros without the code having to know anything about the
format of an fd_set.

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