Double linked list prepend element by ref in C - c

Question

I try to write double linked list in C.
This is my implementation:
typedef struct
{
void* value;
struct Dlist* prev;
struct Dlist* next;
} Dlist;
Dlist* createDlist()
{
Dlist* newList = (Dlist*)malloc (sizeof(Dlist));
newList->value = NULL;
newList->next = NULL;
newList->prev = NULL;
return newList;
}
/*
* Get last element from Dlist
*/
Dlist* getLast(Dlist* list)
{
if (list)
{
while(list->next)
list = (Dlist*)list->next;
}
return list;
}
/*
* add element to list at start
*/
Dlist* addItemAtStart(Dlist* list, Pair* value)
{
Dlist* newList = NULL;
Dlist* last = NULL;
newList = createDlist ();
newList->value = value;
if (list)
{
last = getLast(list);
last->next = newList;
newList->prev = last;
return list;
}
else
return newList;
}
Now, when i try to add element to my list, i need assign a new value every time:
list = addItemAtStart(list, "Hello");
But i want only
addItemAtStart(list, "Hello");
Without list = How can i make so that list will change without assign?
p.s. I get segfaut with Dlist* addItemAtStart(Dlist **list, void* value)
I try to insert so:
Dlist **list = NULL;
addItemAtStart(&list, "Hello");
Thank you.

Answer 1

If you handle the list by pointing to its first element, maybe you should use double indirection:
void addItemAtStart(Dlist** plist, Pair* value)
{
// replace all list with *plist
}
addItemAtStart(&list, "Hello");

Answer 2

You can write your function to accept a pointer to a pointer to a List:
Dlist* addItemAtStart(Dlist** list, Pair* value)
Just make sure you add another level of indirection inside of addItemAtStart when using list.
The function can be called using
addItemAtStart(&list, "Hello");

Answer 3

Give reference of the head node to insert node at start.
Dlist* addItemAtStart(Dlist** list, Pair* value)
{
Dlist* newList = NULL;
Dlist* last = NULL;
newList = createDlist();
newList->value = value;
if (list)
{
last = getLast(*list);
last->next = newList;
newList->prev = last;
}
else
*list = newList

Answer 4

for achieving that, you need to create a static linked list node pointer.
make sure that the atitematstart method updates that node.

Answer 5

Using a global list will solve your problem. In the function, assign the value to the list object. I have tried it.

Answer 6

Your code with some corrections and inclusions:
#include <stdio.h>
#include <stdlib.h>
struct node{
void* value;
struct node *prev, *next;
};
/*This Function is Unnecessary
struct node * createList(){
struct node *newNode=malloc(sizeof(struct node));
if (newNode==NULL) return NULL;
newNode->value = NULL;
newNode->next = NULL;
newNode->prev = NULL;
return newNode;
}
*/
/*
*This Function is also Unnecessary
*Get last element from Dlist
struct node* getLast(struct node* node){
if (node==NULL) return NULL;
while (node->next!=NULL) node=node->next;
return node;
}
*/
/*
*Description: add element to list at start
*
*Return Values:
*0:Failed
*1:Succeeded
*/
int addItemAtStart(struct node **head, void *value){
struct node *newNode=malloc(sizeof(struct node));
/*Sometimes malloc can't allocate memory and returns NULL so you have to check it and if malloc couldn't allocate memory you have to exit the function*/
if (newNode==NULL) return 0;
newNode->value=value;
newNode->prev=NULL;
newNode->next=*head;
if (*head!=NULL) (*head)->prev=newNode;
*head=newNode;
return 1;
}
int main(){
struct node *list=NULL;
addItemAtStart(&list, "apple");
addItemAtStart(&list, "lemon");
addItemAtStart(&list, "orange");
addItemAtStart(&list, "peach");
return 0;
}