Question

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

What is difference between following two functions:

void traverse(struct node *q)
{ }

and

void traverse(struct node **q)
{ }

How can i call above functions from main program?

Answer 1

The first argument list passes a pointer to a struct node. This allows you to change the struct node in the body of the function. For instance:

// this will change the structure, caller will see the changes:
q->data = newValue;

// but this will only change q in the function, caller WON'T see the NULL:
q = NULL;

The second argument list passes a pointer to a pointer to a struct node. This allows you to not only change the struct node in the body of the function, but also to change what the pointer points to. For instance:

// this will change the structure, caller will see the changes:
(*q)->data = newValue;

// This will change the pointer, caller will now see a NULL:
*q = NULL:

As for calling the two versions of the function, from main or otherwise: given a pointer to your structure: struct node *arg;,

• you could call the first version like this: traverse(arg);
• and the second version like this: traverse(&arg);

Alternately, given a structure declared as struct node arg;, you can make a pointer to it:

struct node arg;
struct node *ptrToArg = &arg;

And then:

• you could call the first version like this: traverse(&arg);
• and the second version like this: traverse(&ptrToArg);