在 C 中 Linkedlist 的实现中，head 没有被设置为 NULL

Question

I am trying to implement linkedlist in C, using struct linkedlist and struct node (as implemented below).

While i call new_list() method, i explicitly set list->head as NULL . Now, I try to add the first element using add_elem() method. And later, I print the first element of the list using print_list() function.

In the add_elem() function I check: if the head is NULL and index (where the node is being added) is zero, then create the head and set the value. But, while I execute the code, list->head==NULL is FALSE .

Why head is not NULL , despite even after setting it NULL explicitly?

#include<stdio.h>
#include<stdlib.h>

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

struct linkedlist{
    int size;
    struct node *head;
};

void new_list(struct linkedlist *input);
int get_elem(struct linkedlist *list, int index);
int add_elem(struct linkedlist *list, int index, int value);
void remove_elem(int index);
void print_list(struct linkedlist *list);

int main(){

struct linkedlist *mylist;

printf("starting the program \n");

    // Creating an empty list
    new_list(mylist);

    int index = 0;
    int value = 2;

    // adding an element (currently only supports of adding a head)
    add_elem(mylist, index, value);

    // print the head
    print_list(mylist);
    return 0;
}


void new_list(struct linkedlist* input){
    printf("Creating new list\n");
    input = (struct linkedlist*) malloc(sizeof(struct linkedlist));
    input->size = 0;
    //input->head = (struct node *)malloc(sizeof(struct node));
    input->head = NULL;
}

int add_elem(struct linkedlist  *list, int index, int value){

    // If i remove "list->head==NULL" condition, it works
    //  otherwise it goes into else block, instead of executing if block
    if(list->head==NULL && index==0){
        printf("Adding first elem\n");
        struct node *nodeptr;
        nodeptr =(struct node *) malloc(sizeof(struct node));
        nodeptr->value=value;
        nodeptr->next = NULL;
        list->head = nodeptr;
        list->size=1;
        return 1;
    }
    else {

        // handle later
        return -1;
    }

}


void print_list(struct linkedlist *list){

if(list!=NULL && list->head!=NULL){
    struct node *ptr = list->head;
    //while(ptr!=NULL)
    {
        printf("Head value:%d\n",ptr->value);
        ptr= ptr->next;
    }
}
}

EDIT:

I changed the function new_list( ) as suggested to return the newly allocated address directly as a return value. And now the code works correctly.

struct linkedlist* new_list(void){
    printf("Creating new list\n");
    return (struct linkedlist*) malloc(sizeof(struct linkedlist));
}

Answer 1

new_list allocates a new head node, but assigning it to input does not change the value of mylist . Thus, add_elem isn't looking at the node you allocated and assigned to; its looking at whatever mylist happens to point to, which could be anything, since you never initialized it (expecting new_list to do it for you, I suppose).