10.22.10 Example

The following example shows how to use library(structs) in a simple package for handling integer arrays. We define a module minivec with exported predicates for creating and disposing arrays, accessing its elements, and computing their sum. The summing operation is implemented in C and the rest in Prolog. Arrays are created using the array(Type) foreign type.

Note that the type declaration int32 does not have to be given in the C source code, as it appears in the automatically generated header file minivec_glue.h. Note also how the foreign type specification +pointer(int_array) corresponds to the C type declaration int32 *.

                               
% minivec.pl
     
     :- module(minivec, [
             new_array/2,
             get_array/3,
             put_array/3,
             dispose_array/1,
             sum_array/2
             ]).
     
     :- load_files(library(str_decl), [when(compile_time)]).
     :- use_module(library(structs)).
     
     :- foreign_type
             int32           = integer_32,
             int_array       = array(int32).
     
     foreign(c_sum_array, c_sum_array(+integer,
                                      +pointer(int_array),
                                      [-integer])).
     
     foreign_resource(minivec, [c_sum_array]).
     
     :- load_foreign_resource(minivec).
     
     new_array(Size, array(Size,Mem)) :-
             new(int_array, Size, Mem).
     
     get_array(Index, array(_,Mem), Value) :-
             get_contents(Mem, Index, Value).
     
     put_array(Index, array(_,Mem), Value) :-
             put_contents(Mem, Index, Value).
     
     dispose_array(array(_,Mem)) :-
             dispose(Mem).
     
     sum_array(array(Size,Mem), Sum) :-
             c_sum_array(Size, Mem, Sum).

                            
/* minivec.c */
     
     #include "minivec_glue.h"
     
     long c_sum_array(long cnt, int32 *mem)
     {
       int i;
       long sum = 0;
     
       for (i=0; i<cnt; i++)
         sum += mem[i];
       return sum;
     }

                                  
# session
     
     % splfr --struct minivec.pl minivec.c
     % sicstus -l minivec
     % compiling /home/matsc/sicstus4/Suite/minivec.pl...
     % [...]
     % compiled /home/matsc/sicstus4/Suite/minivec.pl in module minivec, 30 msec 68388 bytes
     SICStus 4.0.0 ...
     Licensed to SICS
     | ?- new_array(4, A),
          put_array(0,A,1),
          put_array(1,A,10),
          put_array(2,A,100),
          put_array(3,A,1000),
          sum_array(A,S),
          dispose_array(A).
     A = array(4,int_array(1264224)),
     S = 1111

A fragment from the generated header file:

                       
/* minivec_glue.h */
     
     #include <sicstus/sicstus.h>
     #include <stdlib.h>
     typedef int int32;
     typedef int32 *(int_array)/* really an unknown-size array */;
     extern long c_sum_array( long, int32 *);