Question

thanx k-ballo, u solved my prevoius problem but then i landed into another one!!
i created nodes and then tried to display them, but every time i called append_node(), the double pointer **head_ptr(used to hold the address of the very first pointer *head, which in turn, holds the address of the very first node of the linked list), was holding a NULL value as if previous calls to append_node(&head, value) didnt add any node to *head.
so whenever i display the list, it reamins empty!! :

#include <stdio.h>

struct __node
{   int data;
    struct __node *next;
};

typedef struct __node node;

int append_node(node **head_ptr, int value)  //double pointer head_ptr to simulate call-by-reference
{   node *temp, *q;

    temp = (node *) malloc(sizeof(node));
    if(!temp)
    {   printf("\ninsufficient memory!!");
        return -1;
    }

    q = *head_ptr; //as *head_ptr is address of a pointer (which is *head), so any changes made after this line in q should also be reflected in main().. (i guess so!)

    temp->data = value;
    temp->next = NULL;

    if(q == NULL)
    {    q = temp;
         printf("\nq is empty");
         return 0;
    }

    while( q->next != NULL)
    {   q = q->next;
    }

    printf("\nq is not empty");
    q->next = temp;

    return 0;
}

int disp_list(node **head_ptr)
{   node *q;
    int i=1;

    q = *head_ptr;
    if(q != NULL)
    {   while( q != NULL )
        {   printf("|%d-%d|--->", i++, q->data);
            q = q->next;
        }
    }
    else
    {   printf("\nlist is empty!!");
    }

    return 0;
}

int main()
{   node *head=NULL;
    int value, res, i=0;

    while(i<3)
    {   printf("\nenter the data to be inserted into the node: ");
        scanf("%d", &value);
        res = append_node( &head, value);
            i++;    
    }

    printf("\nprinting all the nodes...\n") ;
    res = disp_list(&head);
    printf("\n---------------\nexiting...\n\n\n");  
    return 0;
}

i know i could have returned *q from append_node() and reassigned it to *head or declared *head as global.. but i want to *head to be manipulated by so-called-pass-by-reference method only. (theres no pass-by-reference actually in c!) my compiler is: gcc version 4.5.2 (Ubuntu/Linaro 4.5.2-8ubuntu4)

plz help..i m not an expert so please use easier terminology!! :p

Answer 1

Your initial call to append_node is passing a pointer to a pointer to node which is uninitialized (let’s pressume its null, though it will probably be just garbage). Then you do

q = *head_ptr;
//above statement causes a segment fault error..
// that statement should be fine, we will get the value of main's head, which we pressume to be null

// now we will try to dereference null by accesing its next element
while( q->next != NULL)
{   q = q->next;
}