Re: access list elements from C code

[Daniel Diaz]

Le 23/06/2012 13:57, dmaugis a écrit :
> Well...I m trying to access list element from C code, and obviously there is
> something I miss-understood, because I crash. any help/explanation would be
> welcome.
>
> On the gprolog side I have:
>
> :-foreign(sdl_init(+term)).
>
> initialise :-
>                 sdl_init([timer,audio,video]).
>
> On the C side:
>
> int             sdl_init(PlTerm list)
> {
>         unsigned long flags=0;
>         if(Pl_Builtin_List(list)) {
>                 PlTerm *terms=NULL;
>                 int len=Pl_List_Length(list);
>                 terms=malloc(sizeof(PlTerm)*len);
>                 if(terms) {
>                         unsigned i=Pl_Rd_Proper_List_Check(list, terms);
>                         for(i=0; i<len; i++) {
>                                 char* s;
>                                 switch(Pl_Type_Of_Term(terms[i])) {
>                                 case PL_INT:
>                                         printf("integer.\n");
>                                         flags|=Pl_Rd_Integer(terms[i]);
>                                         break;
>                                 case PL_ATM:
>                                         s=Pl_Atom_Name(terms[i]);
>                                         printf("atom '%s'\n",Pl_Is_Valid_Atom(terms[i]) ? 
> "valid":"invalid");
>                                         break;
>                                 }
>                         }
>                         
> And I get:
>
> | ?- go.
> atom 'invalid'
> Fatal Error: Segmentation Violation
>
> it seems when doing :
>
> s=Pl_Atom_Name(terms[i]);
Your code is incomplete and cannot be compiled... Next time, please 
provide a usable code...

However there is clearly an error here:

s=Pl_Atom_Name(terms[i]);
printf("atom '%s'\n",Pl_Is_Valid_Atom(terms[i]) ? "valid":"invalid");

Because you don't "obtain" the atom. Use Pl_Rd_Atom() for this. Example:

int atom = Pl_Rd_Atom(terms[i]);
s=Pl_Atom_Name(atom);
printf("atom %s is '%s'\n",s,Pl_Is_Valid_Atom(Pl_Rd_Atom(terms[i])) ? 
"valid":"invalid");

Daniel


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