C adding node to head of linked list

Question

I have created a linked list struct in c

struct node{
   int value;
   struct node* next;
};

a method to add a node at the start of the list :

Answer 1

In C, if you want a function to be able to change a value it received in its arguments, you need to pass the address of that value. So, to change the value of the list pointer, you need to pass the address of the list pointer. Your addFirst() function should look like this :

void addFirst(struct node **list, int value){
     struct node *new_node = (struct node*) malloc (sizeof (struct node));
     new_node->value = value;
     new_node->next = *list;
     *list = new_node;
}

And when calling that function, you call it like this :

addFirst(&list, value);

Now, if you want to keep the function's signature, a possibility is to change the way you consider the head node. If you state that your head node is purpose is only to hold a pointer to the first value, but does not contain a value by itself, you can do something like this :

struct node *head;

void addFirst(struct node *list, int value){
     struct node *new_node = (struct node*) malloc (sizeof (struct node));
     new_node->value = value;
     new_node->next = list->next;
     list->next = new_node;
}

addFirst(head, 45);

Now, you only have all your functions that work on the list to be change so they work the same, to consider that 'head' is only pointing to the real first node of the list but is not a member of the list itself. The 'real' head is, for all practical purpose, head->next.