Question

Hey everybody ! I am a beginner programmer and need some help with pointers. This is what I am trying to do: I have two pointer arguments to a function(caller), say arg1 and arg2. Now I want to manipulate these pointers inside some other function, say func, such that change reflects in the caller from where the func was called. However right now the change I make in the function gets undone in the calling function. Here is the source code:

func(node* arg1, node* argv2)
{
  node* point3 = (struct node*) malloc(struct node);
  arg2 = arg1;
  arg 1  = point3;
}

caller(node* argv1, node* argv2)
{
  func(arg1, arg2);
}

Now I know that this can be done using pass by reference technique. But for that func becomes func(node** arg, node** arg2) and I dont want to get into double pointers. I was thinking more on the lines of how an array when manipulated or changed in a function changes for all the functions in the program. Please help me out !

Answer 1

There are two ways to do it:
1) Swap pointers, for this you need to use double pointers(**)

func(node** arg1, node** arg2)
{
  node* tmp = *arg2;
  *arg2 = *arg1;
  *arg1  = tmp;
}

2) Swap content, I think you can do it like this:

func(node* arg1, node* arg2)
{
  node* tmp = (struct node*) malloc(sizeof(struct node));
  node* clean = tmp; 
  *tmp = *arg2;
  *arg2 = *arg1;
  *arg1  = *tmp;
  free(clean);
}

Method #1 is more efficient since it will swap pointers instead of whole struct. And as someone mentioned, you should use sizeof() inside malloc.