We recommend that if you are just starting out, do not use this package. The TCP package is much faster and more powerful. IPC/RPC is not available under Windows.

In releases prior to Quintus Prolog Release 2.5, this package was simply known as IPC. It is now called IPC/RPC to distinguish it from another interprocess communication package, which is called IPC/TCP. That package is more general than this one since its facilities can be used to implement the functionality of this package.

This package has some interesting facilities for calling a Prolog servant from C. Before Release 3.0, this was the only way to call Prolog from C. Now Prolog can be fully embedded in a C application.

This Interprocess Communication (IPC) package provides tools for allowing programs written in Prolog or C to remotely call predicates in a Prolog program that is running as a separate process, possibly on a different machine. The communication between the processes is implemented using sockets or pipes to send goals to the remote process and to retrieve answers back.

We refer to the Prolog process that is being invoked by some other program as a servant, because it provides a goal evaluation service at the request of another program: it is given a goal to invoke, invokes it, and then returns the answers to the caller. It is called a servant, as opposed to a server, because it serves a single master. We refer to the program that is calling the servant as the master. The interface described here permits a program (the master) to call one servant and use it to evaluate many subgoals. The characteristics of the interface vary somewhat with the programming language of the master. If the master is itself a Prolog program, then the interface can be much more flexible than when the calling program is written in a procedural language, such as C. We divide the description of the interface into two parts: (1) when the master program is written in Prolog, and (2) when the master program is written in C. Although only C calling Prolog is documented, other languages can also call Prolog if they adhere to the specified protocol.

Please note: On System V versions of UNIX or VMS, the master and server processes are currently required to run on the same machine, and the communication is via pipes rather than sockets. On UNIX systems based on BSD, such as SunOS 4.x, the user can choose to use either pipes or sockets, provided that the two processes are on the same machine. Sockets must be used when the processes are on separate machines. When sockets are used, there needs to be an entry in the /etc/hosts file(s) for each machine that is used.