Why the address of node is not assigned to the self referrential pointer of structure?

Question

#include<stdio.h>
#include<stdlib.h>
#include<malloc.h>

struct node
{
    int id;
    struct node *next;
};
typedef struct node NODE;
int main()
{
    NODE *hello;        
    hello=(NODE *) malloc(sizeof(NODE));
    hello->id=5;
    printf("\nAfter malloc\n");
    printf("address of hello: %d ",hello);
    printf("\n");
    printf("Value of Id is: %d , Value of next is: %u\n",hello->id,hello->next);
    return 0;
}

I tried to execute the above code, I got the following output.

Output:

After Malloc

address of hello: 16949264

Value of Id is:5, value of next is: 0

My question is, why the value of hello is not assigned to next?

`hellos->next` is used uninitialized (undefined behaviour), and use `%p` to print the adress of a variable — David Ranieri, Sep 25 '15 at 07:26
Strictly speaking, `%p` is for printing `void*` data. Try using this: printf("%p", (void*)hello->next);` — MikeCAT, Sep 25 '15 at 07:28
Why would you expect it that way? `hello` is not `hello->next` — Niklas R, Sep 25 '15 at 07:29
Thank you for your response, I have a doubt, when will the memory will be allocated for structure at the time of declaring or when the variable is created for it? — Lokesh Sanapalli, Sep 25 '15 at 07:45
Possible duplicate of [Why the address of structure and next is not same?](http://stackoverflow.com/questions/32780459/why-the-address-of-structure-and-next-is-not-same) — Lokesh Sanapalli, Aug 19 '16 at 12:58

score 0 · Answer 1 · edited May 23 '17 at 12:22

0

why the value of hello is not assigned to next?

Why it should be? You've assigned the memory to hello, so it has a value. next is not allocated memory yet, so it does not show any value. (To be truthful, it is indeterminate).

Also, to print an address, you should be using the %p format specifier with printf() and cast the corresponding argument to (void *).

Also, Please see this discussion on why not to cast the return value of malloc() and family in C..

edited May 23 '17 at 12:22

Community

1
1

answered Sep 25 '15 at 07:26

Sourav Ghosh

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Thank you for your response, I have a doubt, when will the memory will be allocated for structure at the time of declaring or when the variable is created for it? – Lokesh Sanapalli Sep 25 '15 at 07:49
@lokesh1729 the memory for the pointer variable is allocated, the memory to the pointer (to which it will point) needs to be allocated. – Sourav Ghosh Sep 25 '15 at 07:50
@lokesh1729 try printing `(void *)&(hello->next)` and see for yourself. – Sourav Ghosh Sep 25 '15 at 07:51
NODE *hi; printf("\nbefore malloc\n"); printf("\naddress of node is: %p",(void *)hi); printf("\naddress of next is: %p",(void *)hi->next); – Lokesh Sanapalli Sep 25 '15 at 07:56
I got the output like this before malloc address of node is: 0x7ffda77a81b0 address of next is: 0x7ffda77aa470 here why the address of node and next is different, next should store the address of node – Lokesh Sanapalli Sep 25 '15 at 07:59

score 0 · Answer 2 · answered Sep 25 '15 at 07:27

0

hello->next is not hello, so the value of hello is not assigned to hello->next.

hello->next is not hello->value, either, so hello->value is not assigned to hello->next.

answered Sep 25 '15 at 07:27

MikeCAT

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Why the address of node is not assigned to the self referrential pointer of structure?

2 Answers2