Thanks for your response. I quote Solaris Internals by Jim Mauro and Richard McDougall:

A new, fast, lightweight mechanism for calling procedures between processes is available in Solaris: doors. Dorrs are a low-latency method of invoking a procedure in local process. A door server contains a thread that sleeps, waiting for an invocation for the door client. A client makes a call to the server through the door, along with a small (16 Kbyte) payload. When the call is made from a door client to a door server, scheduling control is passed directly to the thread in the door server. Once a door server is finished handling the request, it passes control and response back to the calling thread. The scheduling control allows ultra-low-latency turnaround because the client does not need to waint for the server thread to be scheduled to complete the request.

Doors were introduced in Solaris 2.5.1, but API was not stabilized until 2.6. (Or so I hear. I have only seen the API on Solaris 8 and 9 so far.)