io_setcookie associates a cookie (pointer to some anonymous data
structure) with this descriptor. Only one cookie can be associated with
Use io_getcookie(3) to retrieve the cookie for a descriptor (usually
after io_canread or io_canwrite brought it to your attention).
Please note that io_close does not deallocate your cookie. You need to
do that yourself.
The idea is that you put the state associated with a TCP connection you
serve in a common struct. Then you do not need to have your own data
structure to retrieve the state for a connection, you can just use the
data structure io_wait already maintains. The lookup works in constant
time and should not cause any cache misses.