Pointers in C

A pointer in C can be declared and initialized in two steps:

int a = 10;
int *p;
p = &a;

The declaration says: "this variable holds a memory address. At that memory address is a value of type integer."

The memory table might look like this:

Mem Addr	Value
40452	10
40453	40452

The initialization and declaration can be done in one step:

int *p = &a;

To get the memory address of a variable, we use &. To get the value at a memory address, we use *.

In C, arguments are passed by value. So, to modify the value of a variable, we need to pass a pointer to that variable:

void add_n(int *p, int n)
{
    *p = *p + n;
}

int main()
{
    int num = 1, n = 1;
    add_n(&num, n);
    return 0;
}

When we call add_n(), we are passing a memory address to the function. The function expects a memory address at which an integer resides. In add_n(), we change the value that is stored at the memory address.

1. What if we want to change what a pointer points to?

Then, we should pass a pointer to the pointer.

void modify_ptr(int **p)
{
    *p = new_memory_address;
}

int a = 10;
int *p = &a;
modify_ptr(&p);

In the above, int **p is a pointer to a pointer to an int. That is, it is the memory address of the pointer p. Then:

p is a variable that holds a memory address, at which is stored a memory address
*p is the value pointed at by p. This value is itself a memory address that points to an integer.
**p is the integer.

The memory table looks like this:

Memory Addr	Value	Value Accessed By
40452	10	**p = 10
40453	40452	*p = 40452
40454	40453	p = 40453