6 Mixing C/C++ and Prolog

SICStus Prolog provides several ways of interfacing with code written in other languages:

• A uni-directional, loosely coupled interface from client components written in any language to a Prolog server. This is library('jsonrpc/jsonrpc_server') (see lib-jsonrpc). It uses JSON in its process communication. This is probably the easiest way to expose a Prolog program to a client written in some other programming language.
• A uni-directional, loosely coupled interface from client components written in Java or .NET to a Prolog server. This is library(prologbeans) (see lib-prologbeans). It uses a custom protocol in its process communication.
• A bi-directional, tightly coupled interface between program components written in Java and Prolog. This is library(jasper) (see lib-jasper).
• A bi-directional, tightly coupled interface for program components written in C/C++ and Prolog. This is the topic of the current chapter.

The C side of the interface defines a number of functions and macros for various operations. On the Prolog side, you have to supply declarations specifying the names and argument/value types of C functions being called as predicates. These declarations are used by the predicate load_foreign_resource/1, which performs the actual binding of functions to predicates. They are also needed when the functions are unloaded, for example when SICStus is halted.

In most cases, the argument/value type declarations suffice for making the necessary conversions of data automatically as they are passed between C and Prolog. However, it is possible to declare the type of an argument to be a Prolog term, in which case the receiving function will see it as a “handle” object, called an SP_term_ref, for which access functions are provided.

The C support routines are available in a development system as well as in runtime systems. The support routines include:

• Static and dynamic linking of C code into the Prolog environment.
• Automatic conversion between Prolog terms and C data with foreign/[2,3] declarations.
• Functions for accessing and creating Prolog terms, and for creating and manipulating SP_term_refs.
• The Prolog system may call C predicates, which may call Prolog back without limits on recursion. Predicates that call C may be defined dynamically from C.
• Support for creating stand-alone executables.
• Support for creating user defined Prolog streams.
• Functions to read and write on Prolog streams from C.
• Functions to install interrupt handlers that can safely call Prolog.
• Functions for manipulating mutual exclusion locks.
• User hooks that can be used to perform user defined actions e.g. for customizing the memory management bottom layer.

In addition to the interface described in this chapter, library(structs) and library(objects) (see lib-structs and lib-objects) allow Prolog to hold pointers to C data structures and arrays and access and store into fields in those data structures in a very efficient way, allowing the programmer to stay completely inside Prolog.