Thread: Changing the value of something through a double derefence

I was talking to someone earlier about how to change the value of something from a function, and they said what was needed was to use a ** to change something, and was wondering if I could get a walk - through of what happens. I understand a single pointer well enough, but a pointer through a pointer is kind of confusing to me. Here is a simple example.

Code:

#include <stdio.h>
#include <stdlib.h>
#define SIZE  5
int add(int ** TOP, int * stack);

int *stack = NULL;
    int *TOP = NULL;
    stack = (int *)malloc (SIZE * sizeof(int));
    TOP = stack;
    int b = 0; 
    int (*f[])(int ** TOP ,int * stack) = {add, pop};
    
    while (b < 3)
    {
        printf("The address that TOP points to is %d\n", TOP);
        printf("What would you like to do?");
        scanf("%d", &b);
        (*f[b])( &TOP , &stack);
        printf("The address that TOP points to is %d\n", TOP);
        }

int add(int ** TOP, int * stack)
{
      int a = 0;
       
      printf("The address that TOP points to is %d\n", TOP);
      printf("Please enter a number to stack, push 0 to quit this operation.");
      scanf("%d", &a);
      *TOP = a;
      printf("The number put on the stack was %d\n", **TOP);
      **TOP ++;

Why is it that when the program prints the address of TOP in main, it is different than the address of TOP in the function? Is it because it is a different instance of TOP because it is in the function? When I put the number on *TOP, and come out of the function back to main, it then says the adress of TOP is the number entered into *TOP, and am not sure why. And the **TOP ++ at the end I am thinking it increments malloc by 1, therefore bringing the pointer TOP up to point at the next element, or am I completely off base there? Thank you for the help

My problem is I need to be able to change the TOP value in main from the function so I can tell if I am out of bounds with what is set up by malloc.

That is the part that is confusing me. When I pass in **TOP to the function, which value am I actually passing in? The value of the 1st space malloc is in?

I am having problems with the checking of the bounds of malloc. I have an if statement in my function saying if the address of top equals the last part of malloc, to print that the stack is full. When I test it, though, it just keeps going past it. Any idea why?

Code:

#include <stdio.h>
#include <stdlib.h>
#define SIZE  5
int add(int ** top, int * stack);
int pop(int **top, int * stack);




int main()
{
	int *stack = NULL;
	int *top = NULL;
	stack = (int *)malloc (SIZE * sizeof(int));
	top = stack;
    int b = 0; 
	int (*f[])(int ** top ,int * stack) = {add, pop};
	
	while (b < 2)
	{
		printf("The address of stack is %d\n", stack + 5);
		printf("What would you like to do?\n");
		scanf("%d", &b);
		f[b]( &top , &stack);
		printf("The address that TOP points to is %d\n", top);
	}
	
}


int add(int ** top, int * stack)
{
	  int a = 0;
	  if ( *top == stack + 5)
	  {
	  
	    printf("The address of *top is %d ", *top);
	  	printf("The stack is full.");
	  }
	   else
	   
	   {
	   
	  printf("The address that TOP points to is %d\n", *top);
	  printf("Please enter a number to stack, push 0 to quit this operation.");
	  scanf("%d", &a);
	  **top = a;
	  printf("The number put on the stack was %d\n", **top);
	   ++ *top;
	   printf("the address of top is now %d\n", *top);
    	}
	        
	        
	        
		}
	


int pop(int **top, int  * stack)
{
	if (*top == *stack - 1)
		printf("The stack is already empty.\n");
	else
	{
	
	   printf("The number that was just popped was %d", **top);
	    -- *top;
	}
}