here is the ideA: we want to map fd to pointers to an object.
the search has to be *FAST*. the fastest search method is the binary search
on an array.

add user event only happens every so often, but lookups happen more often.

so we do this:
typedef struct map {
int fd;
Obj *o;
} map;

then create an array (dynamic, or whatever):

map users[500];

then we keep this array sorted according to the 'fd' variable.
when we need to do a lookup we do a binary search on the array for 
'fd' then we have our match.