Malloc memory layout

Question

Hi I have the below code to create the linked list

#include<stdio.h>
#include<stdlib.h>
struct node{
        unsigned int data1;
        unsigned int  data2;
        struct node *ptr;
}obj;
void enterData()                                                        // Here the EnterDAta fnnction uses the obj object to enter the data and note that this
{                                                                       // obj is used agauin and again in the every node of the list to enter the data
        printf("\n Enter the data1 ");
        scanf("%u",&obj.data1);
        printf("\n Enter the data2 ");
        scanf("%u",&obj.data2);
}
void append(struct node **start)                                        // This is used to append the dara un the list or also used to add the first element in the list
{
        enterData();
        struct node *next_node=*start;
        if(next_node==NULL)
        {
                printf("\nAdding first element in the list ......\n");
                next_node=malloc(sizeof(struct node));
                printf("\n The memory location of next_node is %p",&next_node);     
                if(next_node==NULL)
                {
                        printf("\n Out of Memory");
                }
                else{
                        next_node->data1=obj.data1;
                        printf("\n The memory location of next_node->data1 is %p",&next_node->data1);
                        next_node->data2=obj.data2;
                        printf("\n The memory location of next_node->data2 is %p",&next_node->data2);
                        next_node->ptr=NULL;
                        *start=next_node;                                               //This line of code here is modifying the header pointer see the magic of the pointer :)
                }
                printf("\n The first element added successfully");
        }
        else
        {
                printf("\n Appending the data ......\n");
                struct node *temp=next_node;
                next_node=malloc(sizeof(struct node));
                if(next_node==NULL)
                        printf("\n Out of Memory");
                else
                {
                        next_node->data1=obj.data1;
                        next_node->data2=obj.data2;
                        next_node->ptr=NULL;
                        while(temp->ptr!=NULL)
                                temp=temp->ptr;
                }
                temp->ptr=next_node;
                temp=NULL;
                printf("\n Data appended Successfully!!! ");

        }
next_node=NULL;
}
int main()
{
struct node *head=NULL;
append(&head);
return 0;
}

In the above code say if I get the memory address of next_node as 1000 then the memory address which I will get for the next_node->data1 is 1000 and the memory address of the next_node->data2 is 1004

But if in the above append function if just tweak some changes in the code like this

void append(struct node **start)                                        // This is used to append the dara un the list or also used to add the first element in the list
{
        enterData();
        struct node *next_node=*start;
        if(next_node==NULL)
        {
                printf("\nAdding first element in the list ......\n");
                next_node=malloc(sizeof(struct node));
                if(next_node==NULL)
                {
                        printf("\n Out of Memory");
                }
                else{
                        next_node->data2=obj.data2;
            printf("\n The memory address of next_node->data2 is %p ",&next_node->data2);
                        next_node->data1=obj.data1;
            printf("\n The memory address of next_node->data1 is %p ",&next_node->data1);
                        next_node->ptr=NULL;
                        *start=next_node;                                               //This line of code here is modifying the header pointer see the magic of the pointer :)
                }
                printf("\n The first element added successfully");
        }
        else
        {
                printf("\n Appending the data ......\n");
                struct node *temp=next_node;
                next_node=malloc(sizeof(struct node));
                printf("\n The memory address of next_node is %p ",&next_node);
                if(next_node==NULL)
                        printf("\n Out of Memory");
                else
                {
                        next_node->data1=obj.data1;
                        next_node->data2=obj.data2;
                        next_node->ptr=NULL;
                        while(temp->ptr!=NULL)
                                temp=temp->ptr;
                }
                temp->ptr=next_node;
                temp=NULL;
                printf("\n Data appended Successfully!!! ");

        }

Now if the address of next_node is 2000 then I get the memory address of next_node->data1 as 2004 and for data2 is 2008 but shouldn't it be the other way as we are first storing the data2 in the memory location using the next_node pointer ?

Answer 1

The relative addresses of the members of your nodes are a function of the layout of a struct node, not of the order in which you access them. If you swap the data1 and data2 members in the declaration of struct node then you will see data2 appearing at the lower address in every instance, but with the current declaration, data1 will appear first in every instance.