Re: access list elements from C code

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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