Link lists..

i'm declaring a data structure for link list of integers in A.h

#ifndef A_H (can anyone please explain how ifndef works as well..i
just seem to see it in almost every program)

#define A_H

typedef struct node nodestruct

{

int data;

struct node *next;

}node;

extern node * head /*indicates head node of the list , declaring as
extern so that it can be used in A.c */



Now i want to use this link list so..A.c


#include "A.h"

node * head; /*definition, is this sufficient for link list ?, can i
initialize head to NULL here itself */

LL()/* computes the link list */
{

node *p, *prev; /*prev is previous node */

head = NULL;

head->next = NULL;

for(i=0;i<10;i++) /*Creating list of 10 elements */

{

p = (node *) malloc(sizeof(node));

p->next = NULL;

scanf("%d",&(p->data));

if(head ==NULL)

head = p;

else
{
prev->next = p;

prev = p;
}

}


main()

{

struct node *p;

LL();

p = head;

while(p!= NULL)
{
printf("%d\n",p->data);

p = p->next;

}

}


.

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