Defining Prolog predicates in C

To define a predicate p(a1,...,an) in C and make it callable from Prolog, follow the following steps.

1. Write a C function, say p(), and store it in a file, say myCPreds.c, in the directory Emulator. As described above, the C function should take no argument. The macro ARG(i,arity) is used to fetch an argument passed from a Prolog call. The C function should return a value to the Prolog call, 1 meaning a success, 0 meaning a failure, and -1 meaning error.

2. Add another function, say myCBoot(), into the file myCPreds.c, and add the following line into the body of the function.

   
          insert_cpred("p",n,p);
   

   The function insert_cpred establishes a relation between the Prolog predicate p and the C function p(). The name p inside the quotations is visible from Prolog, n is the arity, and the last argument p is the name of the C function defining the predicate. The name of the Prolog predicate and that of the C function are not required to be identical.

3. Add myCPreds.o into the variable OBJ in Makefile and remake the B-Prolog emulator.