access list elements from C code

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]);



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