How to match the async result? (dns_submit_a4 returns 0 instead of a pointer)

[Iñaki Baz Castillo]

Hi, using async mode how is supposed the callback to match its query?
In some other libraries the async call returns an identifier (an
integer) and when it's replied, its callback function contains, in one
of its arguments, such identifier (so I can know which query the
result belongs to).