How to obtain a pointer to pointer in C? - c

Question

I'm supposed to write a function to remove the first node in a linked list.
List is defined like so:
struct ListNode{
int nInfo;
struct ListNode *next;
};
struct ListNode *createNode(int nInfo) {
ListNode *node;
node->nInfo = nInfo;
node->next = NULL;
return node;
}
void insertNode(struct ListNode **list, struct ListNode *node) {
//Sorting list after nInfo
struct ListNode *temp;
struct ListNode *tmpList = *list;
if(*list != NULL) { //List exists
while((tmpList->next != NULL)) {
if((tmpList->nInfo >= node->nInfo) && (tmpList->next->nInfo < node->nInfo)) {
break;
}
tmpList = tmpList->next;
}
//Found where to insert the node
temp = tmpList->next; //Saving old nextnode
tmpList->next = node; //Assigning new nextnode
node->next = temp; //Re-inserting old node
}
else{
*list = node;
}
}
The function for removing the first node looks like this:
void deleteFirst(struct ListNode **list) {
//Delete first node
struct ListNode *temppointer = *list;
if(temppointer == NULL)
return; //List is NULL
*list = temppointer->next;
}
I use the functions like so:
struct ListNode *list = createNode(100);
struct ListNode *node1 = createNode(50);
insertNode(list, node1); //Gives error, cannot convert ListNode* to ListNode**
deleteFirst(list); //Same error
I can't figure out how to obtain a pointer to the list pointer.

Answer 1

As we suspected, you forget to allocate memory for your nodes:
struct ListNode *createNode(int nInfo) {
ListNode *node;
node->nInfo = nInfo;
node->next = NULL;
return node;
}
Your *node is a pointer, but it still points to nothing. You must ask the heap for memory for your node:
ListNode *node = malloc(sizeof(ListNode));
Then in DeleteNode, you must return the memory to the heap, as you no longer need it:
void deleteFirst(struct ListNode **list) {
//Delete first node
struct ListNode *temppointer = *list;
if(temppointer == NULL)
return; //List is NULL
*list = temppointer->next;
free(temppointer); // release the memory.
}

Answer 2

Pay attention! The function you wrote for creating a node cannot work as it is: the node is allocated on the stack, in the context of the function, and it is invalid after the function has exited.
You have to allocate the memory for the node on the heap, using (p.e.) malloc. The function that remove the node from the list is responsable for its deallocation, usually using free.

Answer 3

[posting as an answer to get the formatting right]
Note: your insert() function is overly complex. It can be reduced to
void insertNode(struct ListNode **list, struct ListNode *node) {
for( ; *list ; list = &(*list)->next ) { //List exists
if(*(list)->nInfo >= node->nInfo) break;
}
//Found where to insert the node
node->next = *list;
*list = node;
}

Answer 4

#include <stdlib.h>
struct ListNode{
int nInfo;
struct ListNode *next;
};
struct ListNode *createNode(int nInfo) {
struct ListNode *node=malloc(sizeof(*node));
if(node){
node->nInfo = nInfo;
node->next = NULL;
}
return node;
}
void insertNode(struct ListNode **list, struct ListNode *node) {
// for safety
if(!list) return;
if(!node) return;
//Sorting list after nInfo
struct ListNode *temp;
struct ListNode *tmpList = *list;
if(tmpList!= NULL) { //List exists
while(tmpList->next) {
if(
((tmpList->nInfo)>= (node->nInfo))
&&
((tmpList->next->nInfo) < (node->nInfo))
) {
break;
}
tmpList = tmpList->next;
}
//Found where to insert the node
temp = tmpList->next; //Saving old nextnode
tmpList->next = node; //Assigning new nextnode
node->next = temp; //Re-inserting old node
}
else{
*list = node;
}
}
void deleteFirst(struct ListNode **plist) {
if(!plist) return;
struct ListNode *list=*plist;
if(!list) return;
*plist=list->next;
free(list);
return ;
}
void printNodes(char *title,struct ListNode *list){
printf("\n== %s\n",title);
while(list){
printf("\t%d\n",list->nInfo);
list=list->next;
}
printf("\n");
}
int main(){
struct ListNode *list = createNode(100);
struct ListNode *node1 = createNode(50);
insertNode(&list, node1);
printNodes("on start",list);
insertNode(&list, createNode(70));
printNodes("after add 70",list);
deleteFirst(&list);
printNodes("after del first",list);
}