Hi all,
experimenting with kioslave I found that the kdemain receives always four parameters (when called via kioclient):
0 => the slave executable name (kio_<protocol>)
1 => the protocol
2 => a local socket connected to klauncher
3 => a local socket connected to kioclient

Can anyone explain me what are the purposes of the last two sockets? If I get it right klauncher is waiting for incoming connections to serve via such socket, but how am I supposed to use them?

In general you do not need to concern yourself with how the KIO infrastructure communicates commands and data.

You should follow the general guides on how to write a KIO slave, which are available on KDE Techbase.

My guess would be that these arguments are given to a generic launcher - which then loads the appropriate slave plugin and renames itself as kio_<protocol>

The two sockets are for communication purposes. The KLauncher socket is used to manage requests as slaves can be reused by more than one application. The second socket is for the transfer of the retrieved data (which only the application itself is interested in - sending it via KLauncher would be overkill)