io_setcookie man page
io_setcookie — associate cookie with descriptor
void io_setcookie(int64 fd,void* cookie);
io_setcookie associates a cookie (pointer to some anonymous data structure) with this descriptor. Only one cookie can be associated with a descriptor.
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.