Need some help with Linked Lists;

Question

Hi,

I have a simple question in understanding the pointers and struct definitions in the linked list code.

1)

typedef struct node
{
 struct node* next;
 int val;

}node;

here if I use two "node" when i initialize node *head; which node I am referring to?

2) Here I use an int val in the struct. If I use a void* instead of int is there any thing thats going to change ?

3)Also if I pass to a function

reverse(node* head)
{
    node* temp = head; or node* temp = *head;     
    //what is the difference between the two
}

I am sorry if these are silly question I am new to c language.

Thanks & Regards, Brett

Answer 1

<1> in C you need to specify struct node for structs

    struct node 
    {
...
    } node;

the last 'node' is variable of type struct node e.g.

node.val = 1;

and not a type.

if you want to use 'node' as a type you need to write

typedef struct node { .. } node;

<2> if you use void* you will need a mechanism to handle what the pointers point to e.g. if void* points to an integer you need keep the integer either on the stack or the heap.

node n;
int value = 1;
n.val = &value; // pointing to a single integer on stack

int values[]={1,2,3};
n.val = values; // pointing to an array of integers on stack

void* ptr = malloc(sizeof(int));
n.val = ptr;  // pointing to a single (uninit) integer allocated on heap
int* ptrval = (int*)ptr; // setting an int ptr to the same memory loc.
*ptrval = value; // ptrval now points to same as n.val does

<3> reverse(node* head) head is a pointer to your list, *head is the content of what the pointer points to (first node below)

head->[node next]->[node next]->[node next]

EDIT: rephrased and edited. EDITx2: apparently the question got edited and a typedef was added so the question was altered.

Answer 2

*head is the dereference of the pointer : ie the actual place in memory that is pointed to by the pointer head...

Think of head as a coat hanger and *head as the coat itself, if that helps.

ie:

struct * coat c; //this is a coat hanger, not a coat
....
struct coat k = *c;//this is the coat itself, not a coat hanger 

Answer 3

For #1:

In C, struct's have a separate name space. So if you wrote:

struct foo { ... };

You then have to use struct foo to reference the type. If you tried just foo after the above definition, the compiler would give an error as it doesn't know anything about that unqualified name.

A typedef gives a type an alternate name. A typedef name does not need to be qualified, so once you do:

typedef struct foo foo;

You can now use an unqualified foo to reference the type. Since it's just an alternate name, you can now use struct foo and foo interchangeably.

For #2.

It's possible that if you changed val to a void * it could change the size of the entire structure. Whether that makes a difference will depend on how you've written the rest of your code.