Thread: Learning about linked lists, getting "Segmentation fault (core dumped)" error

Here's the code i'm working on. As far as I can tell the problem concerns passing in 'head' to 'insertEntry', or maybe it's just passing 'head' to any function - I'm totally at a loss as to how to fix it. I haven't gotten around to filling in the 'removeEntry' function yet.

Code:

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


struct entry{
    int val;
    struct entry* next;
};


void insertEntry(struct entry* this, struct entry* after);


void removeEntry(struct entry* after);


void printList(struct entry* head);


int main(){
    
    struct entry* head;
    struct entry* temp_ptr;
    
    int i;
    int val;
    
    // initialize the beginning of the list
    head = NULL;
    
    // print the list
    printList(head);
    
    // insert 3 items
    for( i=0; i<3; i++ ){
        printf("Enter a value: ");
        scanf(" %d", &val);
        
        //allocate and initialize a new node
        temp_ptr = (struct entry*) malloc(sizeof(struct entry));
        temp_ptr->val = val;
        
        //insert the new node into the list
        insertEntry(temp_ptr, head);
    }
    
    // print the elements
    printList(head);
    
    // remove 3 items
//    removeEntry(head);
//    removeEntry(head);
//    removeEntry(head);
    
    //print the list
    printList(head);
    
    
    return 0;
}




void insertEntry(struct entry* this, struct entry* after){
    this->next = after->next;
    after->next = this;
}


void removeEntry(struct entry* after){


}




void printList(struct entry* head){
    struct entry* curr = head->next;
    
    printf("List is: ");
    
    if(!curr){
        printf("EMPTY\n");
        return;
    }
    
    while(curr){
        printf("%d ",curr->val);
        
        curr = curr->next;
    }
    printf("/n");
    
}

You start by setting an empty pointer to NULL, then you pass that to printList. The first thing printList does is to dereference head (your NULL pointer) which will not work. You are trying to print a non-existing list.

Hmm.. well going off your suggestion I changed

Code:

void printList(struct entry* head){    

struct entry* curr = head->next;    
    printf("List is: ");

to just 'struct entry* curr = head;' and that seems to work, but I don't know what I did. So... by setting 'head = NULL' that sets its address to 0? And then the printList tries to... I don't know

Well, this doesn't exactly work. How would i go about initializing head to work with my program?

OHH! I finally get it! Thank you Subsonics, that link really helped.

I made the initialization into a malloc so that it wouldn't be an empty pointer anymore.