Segmentation fault while implementing linked list in C

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)

if you use a sentry node all special cases regarding first node, last node, empty list etc go away — sp2danny
– sp2danny, Commented May 17, 2015 at 12:54

israel altar · Accepted Answer · 2015-05-17 12:46:17Z

3

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.

answered May 17, 2015 at 12:46

israel altar

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4 Comments

TheChetan Over a year ago

When do I use (temp -> link != NULL) and when to use ( temp != NULL)

israel altar Over a year ago

you are using it in the insert_ending function, instead of the for while loop there.

TheChetan Over a year ago

Not in this example, in a general case, where do I use it?

israel altar Over a year ago

There is no rule for that, you need to pay attention when you need to itterate until the last node of the list (of Tree and etc.) or to itterate to the one before. If you are new in programming, I suggest that any time you have to deal with such situations, paint the scenario on some paper and you'll see it by yourself (It worked for me :-) )

bim_bam · Accepted Answer · 2015-05-17 12:51:32Z

3

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.

answered May 17, 2015 at 12:51

bim_bam

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