c - Can someone explain how pointer to pointer works?

Question

I don't really understand how the pointer to pointer works. Any way to do the same work without using pointer to pointer?

struct customer
{
    char name[20];
    char surname[20];
    int code;
    float money;
};
typedef struct customer customer;
void inserts(customer **tmp)
{
    *tmp = (customer *)malloc(sizeof(customer));
    puts("Give me a customer name, surname code and money");
    scanf("%s %s %d %f", (*tmp)->name, (*tmp)->surname, &(*tmp)->code, &(*tmp)->money);
}

Answer 1

Pointers to Pointers 101

Lets say you have an int variable x: int x;.

• A portion of memory will be assigned to x large enough to hold an int.
• The memory assigned to x has an address in the process memory map.
• To see the address where x is located in memory, use:
  printf("x's memory address = %p ", (void*)&x);
• &x means address of x.
• To see the integer value stored in x, use:
  printf("x=%d ", x);
• x can only be manipulated directly within it's scope of existence. For example, it can be manipulated within the function in which it is declared:
  x = 42;
• Outside its scope of definition, the value of x can be manipulated if its memory address is known.
• If the value of x (ie: 42) is passed to a function(x), that function cannot manipulate the value of x.
• If the address of x is passed to a function(&x), that function can manipulate the value of x.

Now, lets say that you have a pointer variable p that assumes it points to an int: int *p;

• A portion of memory will be assigned to p large enough to hold a memory address.
• The memory assigned to p has an address in the process memory map.
• &p means address of p.
• To see the address where p is located in memory, use:
  printf("p's memory addr = %p ", &p);
• To see the address where p is pointing, use:
  printf("Address where p is pointing: %p ", p);
• To see the alleged integer being pointed to, use:
  printf("int = %d ", *p);
• The value of p is an address in memory; and p can be set to point to any address, whether or not that address actually exists in the process memory map.
• The address of an object is a pointer to that object. Hence, To cause p to point to x:
  p = &x
• Whatever p is pointing at can be referred to by the pointer type (type int for p).
• The amount of memory assigned to p will vary, depending on the architecture of the machine; and how it represents an address.
• p can only be manipulated directly within it's scope of existence. For example, it can be manipulated within the function in which it is declared:
  p = &x; or p = NULL
• Outside its scope of existence, the value of p can be manipulated if its' memory address is known.
• If the value of p (an address) is passed to a function(p), that function cannot manipulate 'p' to change where it points.
• If the address of p is passed to a function(&p), that function can manipulate what p points to.

Now, lets say that you have a pointer to a pointer variable pp that assumes it points to a pointer to an 'int': int **pp; ...or: void inserts(customer **tmp) :

• The address of a pointer is a pointer to a pointer.
• ...

Back to the question

Q: Any way to do the same work without using pointer to pointer?

No. Assume the following:

void inserts(customer **tmp);

...
   {
   customer cust;
   custPtr  custPtr = &cust;

   inserts(&custPtr);
   ... 

The inserts() function requires the address of a pointer in order to manipulate where custPtr points.

If instead:

void inserts2(customer *tmp);

...
   {
   customer cust;
   custPtr  custPtr = &cust;

   inserts2(custPtr);
   ... 

The insert2() would get a copy of the value of custPtr, which is the address of cust. Hence, insert2() could modify the value(s) of cust, but could not change where custPtr is pointing.