Burroughs MCP - Port Files

Port Files

Another technique for inter-process communication (IPC) is port files. They are like Unix pipes, except that they are generalized to be multiway and bidirectional. Since these are an order of magnitude slower than other IPC techniques such as libraries, it is better to use other techniques where the IPC is between different processes on the same machine.

The most advantageous use of port files is therefore for distributed IPC. Port files were introduced with BNA (Burroughs Network Architecture), but with the advent of standard networking technologies such as OSI and TCP/IP, port files can be used with these networks as well.

A server listening for incoming connections declares a port file (a file with the KIND attribute equal to PORT). Each connection that is made from a client creates a subfile with an index, so each port file represents multiple connections to different clients around the network.

A server process receives client requests from anywhere on the network by issuing a read on the port file (subfile = 0 to read from any subfile). It issues a response to the client that issued the request by writing to the particular subfile from which the request was read.