Question

I’ve written an implementation of a doubly linked list with a .h prototype here and everything runs fine until I start entering values in the terminal. I get a segmentation fault after entering the second value, but, if I just use 1 value it executes normally. I’ve gone through it several times, but, I can’t find my mistake. Could you guys help me find why I’m getting the error?

Here’s the .h file:

#include <stdio.h>
typedef struct node Node;

struct node
{
    int d;
    Node *link;
}*head,*current,*prev;

int num_nodes;

void linked_list_init(int data);
void linked_list_sort();
void linked_list_print();

And here’s the .c file:

#include "link.h"
#include <stdio.h>
#include <malloc.h>
#include <stdlib.h>

void main(){

    int n,e,i;
    printf("How many numbers do you want to sort: ");
    scanf("%d",&e);
    for(i=0;i<e;i++){
        printf("Enter number: ");
        scanf("%d",&n);
        linked_list_init(n);
    }
    linked_list_sort();
    printf("The sorted numbers are: ");
    linked_list_print();
}

void linked_list_init(int data){
    Node *prev=0,*next=0;

    current=(Node*)malloc(sizeof(Node));    
    if(head==0)
    {
        head=current;
        current->d=data;
        current->link=0;
        prev=current;

    }
    else{
        current->d=data;
        current->link=0;
        prev->link=current;
        prev=current;
    }
    }

void linked_list_sort(){
    int i,j;
    Node *prev=0,*next=0;

    current=head;
    prev=head;
    next=head->link;

    for(i=0;i<num_nodes-1;i++)
    {
        for(j=0;j<num_nodes-i-1;j++)
        {
            if(current->d>next->d)
            {
                current->link=next->link;
                next->link=current;
                if(current==head)
                {
                    head=next;prev=next;
                }
                else
                {
                    prev->link=next;prev=next;
                }
                if(next!=0) //check whether final node is reached
                    next=current->link;

            }
            else //move each node pointer by one position
            {
                prev=current;
                current=next;
                next=current->link;
            }

        }
        //next iteration
        current=head;
        prev=head;
        next=current->link;
    }

}

void linked_list_print(){
    current=head;
    while(current!=0){
        printf("%d ",current->d);
        current=current->link;
    }
}

Answer 1

The problem is that you’re masking the global variables with local declarations in your function. That means that the variable prev in the functions are not the same as the global variable prev.

Aside from that, you should never place variable definitions in header files, as those will clash with each other if the header file is included in multiple files.

There is also another small bug, in that you don’t increase the counter when inserting a new node into the list.