Pointer Riddle that is bugging me

Here is a snippet of code that is bugging me. The riddle is to allocate a node for a linked list in a function without returning the return statement.

So far I have this

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


struct node
{
int value;
struct node *next;
} ;

void NewNode(struct node *newnode)
{
struct node *newnodeinstance;
newnodeinstance = malloc(sizeof(node));
newnodeinstance->value = 0;
newnodeinstance->next=NULL;
*newnode = newnodeinstance;
}


void main()
{
struct node *head;
int i=0;
NewNode(head);
head->value=100; // This causes unhandled exception. basically head is still NULL when I return from function.
}


What am I missing !!

-sl

You know, I don't think that you actually allocate memory for head at any point.

I'm not criticizing you, I'm just going to point out that this is a firly obtuse way of doing this, since returning a pointer to the node would be the technically right way. (Yea yea, I know, it's a riddle).

Is this what you're looking for?

Code:

void NewNode(struct node ** newnode) 
{ 
* newnode = malloc(sizeof(node)); 
(* newnodeinstance)->value = 0; 
(* newnodeinstance)->next=NULL; 
} 


void main() 
{ 
struct node *head; // No reason to believe head != NULL...
int i=0; 
NewNode(&head); // We can change head to a usefull value
  // by passing it's address.
head->value=100; // Dunno if it'll work.
}

Just noticed a problem too...

Code:

*newnode = newnodeinstance;

Is using incompatible types (assigning a noode * to a node), which I'm surprised the compiler didn't pick up. Try

Code:

*newnode = *newnodeinstance;

Although quite frankly, I'd still expect your program to crash with that code... just maybe it'll crash earlier.

The only change you need to make is the following:

Don't bother making a new pointer variable, as it's not useful for anything. Change:

struct node *newnodeinstance;
newnodeinstance = malloc(sizeof(node));
newnodeinstance->value = 0;
newnodeinstance->next=NULL;
*newnode = newnodeinstance;

to:

newnode = malloc(sizeof(node));
newnode->value = 0;
newnode->next=NULL;

If you MUST use your original way, the only error is that:
*newnode = newnodeinstance;
is incorrect, use:
newnode = newnodeinstance;

But, it's probably best to forget about newnodeinstance altogether.