Question

i dont know why the list returned is NULL, this is the code:

In my List.h

struct nodo_ {
    char* dato;

    struct nodo_ *next;
};
struct nodo_ *Lista;
/*Def list */
void createList(struct nodo_ **Lista);

in my main.c

struct nodo_ *Lista;

int main(){
    createList(Lista);
    while(Lista != NULL){        
         printf("The date is %s\n  ",Lista->dato); //Error here now
         Lisa = Lista->next;
    }

    return 0 ;
}

in my List.c im create the List :

void createList(struct nodo_ *Lista){
    struct nodo_ *Aux_List = list_D;
    aux_List = malloc(sizeof(struct nodo_));

    char* path_a = "Hello"; 
    char* path_B = "Minasan";

    /* Store */
    aux_List->dato = path_a;
    aux_List = Aux_List->next;    
    aux_List = malloc(sizeof(struct nodo_));
    aux_List->dato = path_b;
    aux_List->next = NULL;

}

Thanks.

Answer 1

That pointer is being passed by value, i.e., a copy is made. If you wish to initialize the pointer to a completely new value then you must use another level of indirection (i.e., a nodo_**).

On a side note, typedefing pointer types is almost always a bad idea unless the type is truly opaque (which yours is not). One reason for this “rule” is evident when you consider another bug in your code:

auxList = (Lista*)malloc(sizeof(Lista));

You’re allocating space for a pointer to noda_, not enough for a noda_ object. Also, don’t cast the return value of malloc in C. It is redundant as a void* is safely and implicitly converted to any other pointer type and, if you forget to include stdlib.h, malloc will be assumed to be a function which returns int, and the cast hides the error. (only applies to compilers which implement C89 or an older version)

EDIT:

To initialize a pointer argument within a function:

void init(struct node **n) {
    if(n)
        *n = malloc(sizeof(struct node));
}

int main() {
    struct node *n;
    init(&n);
}