Design of the Amzi! Prolog C/C++ API

Technical Note: CPDAPI June 1995 (updated with name change) This article first appeared in the PCAI Vendor's Forum. Please do not redistribute without acknowleding PCAI. The article used the old name, Cogent Prolog.


Today's software developer is faced with a wide array of computing environments, and, in each of these, numerous tool kits for application development. The environments range from real-mode DOS on the PC with its 16-bit addresses to the 64-bit address space of DEC's Alpha AXP workstations, and include GUI tools such as Windows and Motif, as well as interfaces to database, multimedia, and a host of other specialized services.

The 3.0 release of Amzi! Prolog is designed to allow C/C++ developers to take advantage of the expressiveness of Prolog for dealing with the complexities of these environments, and, at the same time, allow the Prolog programmer to take advantage of the rich array of tools available in C/C++. It does this through a full application program interface (API) that allows for both call-in and call-out access between C/C++ and Prolog, and which runs in a variety of environments.

The three main design criteria for Amzi! 3.0 and its API are

For portability, the core Prolog implementation sticks closely to "standard" Prolog and does not include environment specific built-in predicates (these are included as "add-on" libraries for the different environments). In this way the Prolog portions of an application will run in multiple environments without change. In addition to source code portability, Amzi! Prolog provides compiled Prolog object code portability as well. This means Prolog code can be developed and compiled in one environment for deployment in another.

Embedding Prolog in C/C++ is not quite like interfacing C/C++ with another language, such as FORTRAN or BASIC. A Prolog program is actually closer to a database than it is to a program. As such, the API reflects this fact and presents a "logic server" interface to C/C++. The C/C++ program interacts with the logic server in much the same way a Prolog programmer interacts with Prolog at the standard '?-' prompt of a Prolog listener.

Extending Prolog to access C/C++ functions has the opposite problem. The query-like nature of Prolog predicates is mapped to procedural function calls in C. The API provides the tools to make this mapping as natural as possible.

Amzi! Prolog Architecture

From its inception, Amzi! Prolog was designed to be an efficient, portable Prolog. It is, like most compiled Prologs, based on the Warren Abstract Machine (WAM). A WAM is a software simulation of a computer specifically designed for running Prolog. Like any hardware computer, the WAM is programmed with primitive op-codes and arguments. (For an excellent description of a WAM see 'Warren's Abstract Machine' by Ait-Kaci, published by MIT Press.)

How the WAM interacts with the compiler and Prolog source code varies from vendor to vendor. In the case of Amzi! Prolog, the compiler generates a file containing a stream of WAM op codes. A Prolog linker is used to combine the compiled files of multiple Prolog modules into a single file. That file can then be loaded and run by the Amzi! engine (a WAM).

The Amzi! engine is implemented in ANSI C, and is the only code that needs to be ported to move Amzi! Prolog to a new environment. The Amzi! compiler and interpreter are both implemented in Amzi! Prolog, and, because Amzi! is object code compatible across environment, both run immediately in a new environment as soon the engine is up and running.

Prolog as a Logic Server for C/C++

The simplest form of embedding Prolog in C/C++ is to load and run a compiled Prolog module from C/C++. The following minimal C program does just that, in this case running the Prolog program FOO.

This code initializes the Prolog engine, loads the compiled file, calls the main/0 predicate of the Prolog program, and then frees the resources used by the Prolog engine. Obviously, this same code could be part of a larger application, called, for example from a user's menu choice.

Once a Prolog program is loaded you can also query it just as you would a 'consult'ed program in a Prolog listener. To understand what is involved, you need to understand what a Prolog term is. It is, simply, anything you express in Prolog. For example all of the following are terms.

As you can see, there are simple terms and complex terms, which are made up of simple terms. Internally, Prolog stores terms on the heap, so in C/C++ a term is no more or less than a pointer to a heap location where a term begins

The main task in going from C to Prolog and back is the mapping of C variables to and from Prolog terms. For the Amzi! API we wanted this to be simple, and yet powerful enough to map any complex Prolog term. Because this type of dual capability is often impossible, we provide two ways to construct and deconstruct Prolog terms from C.

The first is the more intuitive, and uses a technique similar to that used in the standard C 'printf' and 'scanf' functions. A format specification describes the term, and C variables are mapped to the format specifications. The following example illustrates this mechanism.

Running this program produces

There are four main points illustrated by the example. First, the C program can easily assert facts to the dynamic Prolog database using the API call, lsAssertaStr(), which builds a Prolog term based on a format specification and a list of C variables. In the example, there are no variables, but the next function described illustrates how variables could be used here as well.

Second, the query term, t, is built using the function call

It maps the C string, sPerson, into the query pattern, creating the query term 'ancestor(X, dennis)'. The term is returned in 't', which is the pointer to the heap where the term is stored. The lsCallStr() function has the exact same effect as entering

in the Prolog listener. That is, the term is unified with the Prolog database. As a result the variable, X, is unified with some value in the database. To extract the value of X into a C string, a function that works similar to C's scanf is used.

Between these two functions, the C programmer can easily build a Prolog query from C variables, and map the result of the query back into C variables.

The next thing the C programmer needs to do is iterate through all the results from the query, initiating backtracking. That is done with the call

which causes Prolog to redo the last query (still using the same term t). Notice that both lsRedo() and lsCallStr() return a value of either TRUE or FALSE. This is used to drive the C loop that keeps on issuing lsRedo's until there are no more answers.

While the formatted string approach to building terms is easy and intuitive, a more formal mechanism is included in the API for constructing and deconstructing arbitrarily complex Prolog terms. There are functions that create a new term with a functor and arity, and other functions that allow you to specify the arguments for that functor. You can also directly access any of the arguments of a complex term, as well as determine what type of term you are dealing with.

For example, the query term, 'sibling(mary, Y)', can be created with the calls

The created term can be called with

and the now unified second argument retrieved with

Lists are created head first with lsPushList(), and the heads of lists retrieved with lsPopList(). The following C code pops elements from a Prolog list until the list is empty.

Note the use of a C 'while' loop to retrieve all the elements of the list. The term that points to the list in lsPopList is automatically updated to point to the tail of the list after the head is popped off. This makes it easy to loop through Prolog lists from C.

Extending Prolog

The above examples are presented in the context of C calling Prolog. The API also lets you write your own built-in predicates. The following excerpt is from a sample program that extends Prolog with simple array handling. The function listed here is mapped to the Prolog predicate array_elem/3, whose arguments are

  1. the array identifier, a C pointer in this case,
  2. the index, and
  3. either a variable which is unified with the value of the array element, or an integer used to set the value of the array element.

It is used in conjunction with make_array/2 which creates an array of a given size by allocating storage for it. So the following Prolog code might create an array, store the third element, and later retrieve the value of the third element.

Here is the C code that implements the extended predicate array_elem/3. It uses API function calls to map the first two arguments of the predicate to C variables. It uses another API function to determine whether the third argument is an integer or not, and takes the correct action based on that information.

Extended predicates such as these can be used for just a specific application, or included with the Prolog environment so they are accessible from Amzi! listener as well.

While the API is designed as a C library interface to the Amzi! engine, we have also implemented a C++ wrapper around the API so that an instance of a Prolog engine becomes an object. In this way, the Prolog modules of a C++ application become objects, receiving messages like any other C++ object.

We've also implemented a C++ wrapper around the API, so the Prolog engine becomes an object, initialized with the name of the compiled program to run. In this way, Prolog modules in a C++ application becomes objects, receiving messages like any other C++ object. The first code example, which loaded and ran the Prolog program FOO, can be rewritten using the C++ wrapper as follows.

Notice that C++ constructors and destructors in the CLSEngine class take care of the initialization and shut down work. API messages are sent to particular instances of the Amzi! engine.


We have already made use of the 3.0 architecture in implementing custom extensions to Prolog and ports to other environments. In addition to the DOS 16- and 32- bit versions supported in the 2.0 Amzi! release, we've ported the engine to both Windows and OSF/1 on the 64- bit DEC Alpha AXP workstation. Other ports are in progress as well, based on customer demand.

Of course we're providing a certain level of extended- predicate support for Windows, but the same GUI services can be provided using other tool kits as well, as Pacific AI did by extending Prolog to include support for a DOS GUI library, PCX files, and graphics printing. This allows them to use Prolog for the industrial education and expert systems they develop, and still provide state-of-the-art GUI interfaces in a DOS environment.

We also see tremendous opportunities for using this technology for dealing with the complexity of today's computing environments. Embedded expert advisors are a natural next step to the embedded help systems currently offered with today's application.

If you've ever called for technical support on any of the new array of complex Windows products, you've probably noticed the support individual is often reading from a script. That type of first-line expertise could easily be embedded directly in the product through Prolog modules with custom extensions that let them directly probe the application in question.

It is our hope that this new release will make Prolog a practical tool for C/C++ programmers to add the needed intelligence into the increasingly complex applications of today, and, at the same time make it practical for Prolog programmers to access the rich array of tools available to the C/C++ programmer in a variety of environments.