在C中实现链表时出现分段错误

Question

I'm trying to create a simple linked list and insert a node at the end of the list. I'm getting a segmentation fault.

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

struct node{
    int data;
    struct node *link;
};

void create(struct node *head){
    struct node* second = NULL;
    struct node* last = NULL;

    second = (struct node*)malloc(sizeof(struct node));
    last = (struct node*)malloc(sizeof(struct node));

    second -> data = 2;
    last -> data = 3;

    head -> link = second;
    second -> link = last;
    last -> link = NULL;
}

void insert_ending(struct node *head){
    struct node* temp = NULL;
    struct node* temp1 = NULL;

    temp1 = (struct node*)malloc(sizeof(struct node));
    temp1 -> data = 5;
    temp1 -> link = NULL;

    temp = head;
    while(temp != NULL){
       temp = temp -> link;
    }temp -> link = temp1;
}

void PrintList(struct node *head){
    while( head != NULL ){
        printf(" %d -->", head -> data);
        head = head -> link;
    }
    printf("\n");
}

int main(){
    struct node* head = NULL;
    head = (struct node*)malloc(sizeof(struct node));
    head -> data = 1;
    head -> link = NULL;

    create(head);
    PrintList(head);

    insert_ending(head);
    PrintList(head);
    return 0;
}

I'm getting a segmentation fault. The output is as follows.

1 --> 2 --> 3 --> Segmentation fault (core dumped)

Answer 1

in your insert function you need to change to :

 temp = head;
    while(temp -> link != NULL){
       temp = temp -> link;
    }
    temp -> link = temp1;

the reason is that when you loop with while until temp == null, you can't afterward do: temp -> link because temp is allready null.

Answer 2

In the function 'insert_ending' in the while loop you want to change the condition from:

while ( temp != NULL ) 

to:

while ( temp->link != NULL )

because now once the loop is finished, temp is NULL and then you try to dereference it (a NULL pointer) and get an error.