linked list remains empty even after adding nodes? [closed] - c

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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;
}

Answer 2

There are two major problems with this program:
head is not initialized and contains garbage.
You don't handle the case of an empty list in append_node().
Both problems will lead to segmentation fault errors.