打印带有节点的链表包含空指针

Question

Im a beginner in c and i need to print a linked list contents but the printing function doesnt work and i dont know why. i checked if the function of creating the linked list works by printf some words after using addFirst function and it seems the addFirst function works and the linked list has been created but the printing function doesnt Here is my code

#include <stdio.h>
#include <stdlib.h>
typedef enum msgtag {
  NoTag = 0, Important, Work, Personal, ToDo, Later
} msgtag;

typedef struct MsgID
{
  char* id ;
} MsgID;
typedef struct MsgDetails
{
  char* id;
  msgtag tag;
  int year;
  int month;
  int day;
} MsgDetails;

typedef struct Node
{
  void *item;
  struct Node* next;
} Node;
Node* addFirst(MsgDetails*ptr,Node* head) {
    Node* newHead = ( Node*)malloc(sizeof(Node));
    newHead->item = malloc(sizeof(MsgDetails));
    ((MsgDetails*)newHead->item)->year = ptr->year;
    ((MsgDetails*)newHead->item)->day = ptr->day;
    ((MsgDetails*)newHead->item)->month = ptr->month;
    ((MsgDetails*)newHead->item)->id = ptr->id;
    ((MsgDetails*)newHead->item)->tag = ptr->tag;
    newHead->next = head;

    return newHead;
}
void printAll(Node* head) {
        Node* current = NULL;
    ((MsgDetails*)current->item)->year = ((MsgDetails*)head->item)->year;
    ((MsgDetails*)current->item)->day = ((MsgDetails*)head->item)->day;
    ((MsgDetails*)current->item)->month = ((MsgDetails*)head->item)->month;
    ((MsgDetails*)current->item)->id = ((MsgDetails*)head->item)->id;
    ((MsgDetails*)current->item)->tag = ((MsgDetails*)head->item)->tag;



    while (current != NULL) {
        printf("%d,%d,%d,%d,%s",((MsgDetails*)current->item)->tag,((MsgDetails*)current->item)->year,((MsgDetails*)current->item)->month,((MsgDetails*)current->item)->day,((MsgDetails*)current->item)->id);
        current = current->next;
    }
    printf("\n");
}


int main()

{
    Node* head = NULL;
    MsgDetails details;
    MsgDetails *ptr =NULL;
    int msg_tag;
    details.id =(char*)malloc(sizeof(char)*40);
    ptr = &details;
    printf("\nenter msg metadata (tag,year,month,day,id):");
    scanf("%d,%d,%d,%d,%s",&msg_tag,&(details.year),&(details.month),&(details.day),details.id);
    
    details.tag = msg_tag;
    head = addFirst(ptr,head);
   
    printAll(head);

}

Answer 1

Your code initialises current to (Node *)NULL, then attempts to dereference the pointer. That's not going to work.

void printAll(Node* head) {
        Node* current = NULL;
    ((MsgDetails*)current->item)->year = ((MsgDetails*)head->item)->year;

The preliminary code that makes assignments to current do not seem necessary. Assign head to current and it will work:

void printAll(Node* head) {
    Node* current = head;

    while (current) {
        printf("%d,%d,%d,%d,%s", ((MsgDetails*)current->item)->tag,((MsgDetails*)current->item)->year,((MsgDetails*)current->item)->month,((MsgDetails*)current->item)->day,((MsgDetails*)current->item)->id);
        current = current->next;
    }
    printf("\n");
}