这个链表代码是一个好习惯吗？

Question

Hi everyone I am a newbie in C and trying to learn it. I have a simple query regarding this linkedlist implementation which I found at many places:

void addNode(node **listhead, int data, int pos){
        if(pos<=0 || pos > length(*listhead)+1){
                printf("Invalid position provided, there are currently %d nodes in the list \n", length(*listhead));
                return;
        }else{
                node *current = *listhead;
                node *newNode = (node*)malloc(sizeof(node));
                if(newNode == NULL){
                        printf("Memory allocation error\n");
                        return;
                }
                newNode->data = data;
                newNode->next = NULL;
                if (current == NULL){
                        *listhead = newNode;
                        return;
                }else{
                        int i = 0;
                        while(current->next != NULL && i < pos-1){
                                ++i;
                                current = current->next;
                        }
                        if(current->next == NULL){
                                current->next = newNode;
                        }
                        if(i == pos-1){
                                newNode->next = current->next;
                                current->next = newNode;
                        }
                }
        }
}




int main(){
        node *head = NULL;
        node **headref = &head;
        addNode(headref, 1, 1);
        addNode(headref, 2, 2);
        addNode(headref, 3, 3);
        printList(head);
        return 0;
    }

my query is here we are creating a pointer to a pointer which is pointing to NULL. This code works, however I wanted to know if this is a good practice. If it is not, how should I create my head pointer and pass its reference to the addNode function.

Answer 1

Suggested alternative:

int main() {
  node *head = addNode(NULL, 1, 1);
  node *current = head;
  current = addNode(current, 2, 2);
  current = addNode(current, 3, 3);
  printList(head);
  return 0;
}

In other words:

1) addNode() becomes a function that takes the current value as a parameter (so it doesn't have to traverse the entire list just to add a new element)...

2) ... and it returns a pointer to the new node.

3) This means at ANY point in the program you can access ANY of a) the list head, b) the previous pointer (before "add") and/or c) the next pointer (after add).

Answer 2

We pass a double pointer to addNode() for cases, where the headref pointer needs to be updated. For such cases, with "addNode(headref, 1, 1);", the addNode would most likely be storing the address of the malloced element inside addNode() in the headref. If you were to pass headref as a pointer, then after the call headref would continue to point to the address in the main and you would lose the malloced address.

Answer 3

For single linked list, it is actually a good practice, which simplifies the addNode implementation. I guess the call to addNode(node_x, 1, 1) will add a node before node_x . If you pass only the pointer to node_x . Then the function will need to loop over the entire list and find the node before node_x and modify its pointer to the new constructed node. While if you pass a pointer to pointer, let's say node** p then the function only needs to assign the address of the new node to *p .