#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct node{
    int num;
    int deg;
    struct node* nxt;
    struct node* prv;

};
typedef struct node node_t;


void push(node_t *head, node_t *last, int number, int degree){
    node_t *newNode = (node_t*)malloc(sizeof(node_t));
    newNode->num = number;
    newNode->deg = degree;

    newNode->nxt = NULL;
    newNode->prv = NULL;

    if(head == NULL){
        head = newNode;
        last = newNode;
    }
    else{
       
        last->nxt = newNode;
        newNode->prv = last;
        last = newNode;
    }
}

int main(){
    node_t *pol1 = NULL;
    node_t *pol1F=NULL;
    int dataNum, dataDeg;
    dataNum =1;
    dataDeg =2;
    push(pol1, pol1F, dataNum , dataDeg);
    printf("%d", pol1->num );
    free(pol1);
    free(pol1F);

    return 0;
}

When trying to print a number from node I get status -1073741819. In the function, as long as I know, it should associate head with pol1 and enter the first if, considering head = NULL. In the future I will add other nodes and create another "head" for a second linked list. How can I access data from pol1 ?

CodePudding user response：

Since you want the node pointers in main to be changed by the call to push, you should pass the address of those pointers as arguments. That is how push can know what to change. As you currently have it, it will only modify the value of its local variables.

Secondly, to free up the memory at the end, you should not call free on both pointers, since they may point to the same node. And this is the case when you have only added one node to the list. On the other hand, if your list has more than 2 nodes, you'll need to also free those intermediate nodes. All in all, you should create a loop that visits each node and frees it. Or, why not, create a pop function that removes the tail node from the list. Then you can call that function repeatedly until the list is empty.

Finally, pol1 and pol1F are not very descriptive variable names. Why not just head and tail?

Here is a correction and some related functions you could use:

void push(node_t **head, node_t **last, int number, int degree) {
    node_t *newNode = malloc(sizeof(node_t));
    newNode->num = number;
    newNode->deg = degree;
    newNode->nxt = NULL;
    newNode->prv = NULL;

    if (*head == NULL) {
        *head = newNode;
        *last = newNode;
    } else {
        (*last)->nxt = newNode;
        newNode->prv = *last;
        *last = newNode;
    }
}

void pop(node_t **head, node_t **last) {
    if (*head == NULL) {
        return;
    }
    if (*head == *last ) {
        free(*head);
        *head = NULL;
        *last = NULL;
    } else {
        *last = (*last)->prv;
        free((*last)->nxt);
        (*last)->nxt = NULL;
    }
}

void clearList(node_t **head, node_t **last) {
    while (*head != NULL) {
        pop(head, last);
    }
}

void printList(node_t *head) {
    while (head != NULL) {
        printf("(%d, %d) ", head->num, head->deg);
        head = head->nxt;
    }
    printf("\n");
}

int main() {
    node_t *head = NULL;
    node_t *tail = NULL;
    push(&head, &tail, 1, 20);
    push(&head, &tail, 2, 18);
    push(&head, &tail, 3, 21);
    printList(head);
    clearList(&head, &tail);
    return 0;
}