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8.5 Defining a new C main() function

GNU Prolog allows the user to define his own main() function. This can be useful to perform several tasks before starting the Prolog engine. To do this simply define a classical main(argc, argv) function. The following functions can then be used:

int  Start_Prolog         (int argc, char *argv[])
void Stop_Prolog          (void)
void Reset_Prolog         (void)
Bool Try_Execute_Top_Level(void)
The function Start_Prolog(argc, argv) initializes the Prolog engine (argc and argv are the command-line variables). This function collects all linked objects (issued from the compilation of Prolog files) and initializes them. The initialization of a Prolog object file consists in adding to appropriate tables new atoms, new predicates and executing its system directives. A system directive is generated by the Prolog to WAM compiler to reflect a (user) directive executed at compile-time such as op/3 (section 5.1.10). Indeed, when the compiler encounters such a directive it immediately executes it and also generates a system directive to execute it at the start of the executable. When all system directives have been executed the Prolog engine executes all initialization directives defined with initialization/1 (section 5.1.13). The function returns the number of user directives (i.e. initialization/1) executed. This function mst be called only once.

The function Stop_Prolog() stops the Prolog engine. This function must be called only once after all Prolog treatment have been done.

The function Reset_Prolog() reinitializes the Prolog engine (i.e. reset all Prolog stacks).

The function Try_Execute_Top_Level() executes the top-level if linked (section 2.4.3) and returns TRUE. If the top-level is not present the functions returns FALSE.

Here is the definition of the default GNU Prolog main() function:

int main(int argc, char *argv[])

{
 int  nb_user_directive;
 Bool top_level;

 nb_user_directive=Start_Prolog(argc, argv);

 top_level=Try_Execute_Top_Level();

 Stop_Prolog();

 if (top_level)
     return 0;

 if (nb_user_directive)
     return 0;

 printf("Warning: no initial goal executed\n"
        "   use a directive :- initialization(Goal)\n"
        "   or remove the link option --no-top-level"
        " (or --min-bips or --min-size)\n");

 return 1;
}
8.5.1 Example: asking for ancestors

In this example we use the following Prolog code (in a file called prog.pl):

parent(bob,   mary).
parent(jane,  mary).
parent(mary,  peter).
parent(paul,  peter).
parent(peter, john).

anc(X, Y):-
        parent(X, Y).

anc(X, Z) :-
        parent(X, Y),
        anc(Y, Z).
The following main() function asks for the name of a person and displays all successors of that person (this is equivalent to the Prolog query: anc(Result, Name)).

int main(int argc, char *argv[])

{
 int     func;
 WamWord arg[10];
 char    str[100];
 char   *sol[100];
 int     i,nb_sol=0;
 Bool    res;

 Start_Prolog(argc,argv);

 func=Find_Atom("anc");
 for(;;)
    {
     printf("\nEnter a name (or 'end' to finish): ");
     scanf("%s",str);

     if (strcmp(str, "end")==0)
         break;

     arg[0]=Mk_Variable();
     arg[1]=Mk_String(str);
     nb_sol=0;
     res=Pl_Query_Start(func, 2, arg, TRUE);
     while(res)
        {
         sol[nb_sol++]=Rd_String(arg[0]);
         res=Pl_Query_Next_Solution();
        }
     Pl_Query_End(PL_RECOVER);

     for(i=0;i<nb_sol;i++)
         printf("  solution: %s\n", sol[i]);
     printf("%d solution(s)\n", nb_sol);
    }

 Stop_Prolog();
 return 0;     
}
The compilation produces an executable called prog:

% gplc prog.pl utils.c
Examples of use:

Enter a name (or 'end' to finish): john
  solution: peter
  solution: bob
  solution: jane
  solution: mary
  solution: paul
5 solution(s)

Enter a name (or 'end' to finish): mary
  solution: bob
  solution: jane
2 solution(s)

Enter a name (or 'end' to finish): end
Copyright (C) 1999,2000 Daniel Diaz

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