Link list: 3 steps of listAdd operation

 1. Allocate memory for the new element

                        - can be done in the add routine  

 2. Fill the structure data/prev/next fields for the new element

 3. Connect in the linklist as follows:

     Check listHead == 0 --> 1st elm --> both head and tail equal to the new elm
                                != 0  --> not 1st elm --> put in after tail. then this new elm become tail.

#include <stdio.h>

#include <stdlib.h> // for malloc

// minimum needed as follows:

typedef struct node {

  int data;

  struct node *next;

  struct node *prev;

} NodeType;

NodeType *listHead=0, *listTail=0; // this is convenient

int listAdd(int newData) {

    NodeType *newNode;

    newNode = (NodeType *) malloc(sizeof(NodeType));

   // TBD check NULL and return if so

    newNode->data = newData;

    newNode->next = 0;

    newNode->prev = 0;

    if(listHead == 0) {

       listHead = newNode;

       listTail = newNode;

       printf("1st node added (%d)\n", listHead->data);

       return 0;

    }

    listTail->next = newNode;

    newNode->prev = listTail;

    // do not forget to set new listTail

    listTail = listTail->next;

    return 0;

}