Thanks, module Glib.Io is what I needed.

Now I have another question. I've started writing some code,

module Channel =
  struct
    type event =
      | Connected of Unix.sockaddr
      | Dropped of Unix.sockaddr
      | Received of Unix.sockaddr * x
      | Sent of Unix.sockaddr * x
    type action =
      | Associate of Unix.sockaddr
      | Dissociate of Unix.sockaddr
      | Send of Unix.sockaddr * x

where x will be something returned by Marshal.from_string.

How do I properly specify its type?