Question

When I run the program below, it prints “one: 1”, instead of “one : 1, two: 2”, as I had expected. Anyone know what’s going on here? I am trying to create a function that will allow me to create as many linked lists as possible instead of just declaring global heads.

struct Node {
  int value;
  char label[10];
  node *next;
};
typedef struct Node node;

int add(int data, char name[], node *head) {
  node *newNode = (node *)malloc(sizeof(node));
  if (newNode != NULL) {
    newNode->value = data;
    strcpy(newNode->label, name);
    newNode->next = head;
    head = newNode;
  }
}

node* createNewLinkedList(int d, char *name) {
  node *newNode = (node *)malloc(sizeof(node));
  newNode->value = d;
  strcpy(newNode->label, name);
  newNode->next = NULL;
  return newNode;
}

int main() {
  node *head1 = createNewLinkedList(1, "one");
  add(2, "two", head1);
  iterate(head1);
}

Answer 1

You are passing a pointer to the node. Like all parameters in C, it is passed by value, therefore head = newNode; has no effect in the caller.

You need to change the signature to accept node **head, and add a level of indirection in order to make the changes in add be reflected in the main.

int add(int data, char name[], node **head) {
    node *newNode = (node *)malloc(sizeof(node));
    if (newNode != NULL) {
        newNode->value = data;
        strcpy(newNode->label, name);
        newNode->next = *head;
        *head = newNode;
    }
}

Of course you’ll need to pass &head1 to the add method: add(2, "two", &head1);

P.S. Since you are adding {2,”two”} to the front of the list, your output will be “two: 2, one : 1”