When I first came across pointers in C, I thought it was a pretty tricky topic. Even when I was just doing simple pointer exercises, like working with arrays, I didn't really understand what was going on under the hood. Today, I finally took the plunge and dived into chapter 5 of the C book, which explains in detail what pointers are and how to work with them.

I found this example really helpful in understanding pointers better. It goes like this:

#include <stdio.h>

int main()
{
    int x = 1, y = 2, z[10];    // Don't worry about z[10] for now;
    int *ip;
    ip = &x;
    y = *ip;
    *ip = 0;
    ip = &z[0]; // I think this point was added to show that a pointer can also change its value; don't worry about z[10] for now;

    return 0;
}

Declaring variables and a pointer

Let's dive in and start declaring some variables! We'll go ahead and create variables: x, y. We also create an ip pointer that will point to a variable of type int, which we'll use to help us keep track of things.

int x = 1, y = 2;
int *ip;

I'd like to draw your attention to the way a pointer is declared: int *ip. This just means that ip is a pointer to a variable of the int type. The asterisk * here is super helpful because it shows us that ip is not just a regular variable, but a pointer! This means that it will store the address of a variable of the int type in memory.

It's really important to understand that the * is referring to ip and not int. I'm sure you'll find this example really helpful in understanding this point:

int *w; k = 1; // w – a pointer to a variable of `int` type; k is just a variable of `int` type;

Let's continue parsing the code from the book...

Assigning the address of a variable to a pointer

Next, we assign the address of the x variable to the ip pointer:

ip = &x;

The & operator before x means “address of the variable x”. Thus, now ip stores the address of the variable x. This is a key part of understanding pointers: a pointer itself does not store the value of a variable, it only stores its address in memory.

Pointer dereferencing

The next step is pointer dereferencing:

y = *ip;

Here, the asterisk * before ip means “the value at the address pointed to by the ip pointer”. Since ip points to x, *ip returns the value of x which is 1. It means *ip equal to x or vice versa.

Therefore, the variable y is assigned the value 1.

Changing the value of a variable through a pointer

Now we will change the value of the variable x through a pointer:

*ip = 0;

This line means “assign the value 0 to the variable pointed to by ip”. Since ip points to x, x becomes 0. That is, the * in *ip works both ways. With *ip you can take out a value, likewise with *ip you can write a value. In detail: *ip is x. Lines printf in C code work the same way:

int x = 1;
int *ip = &x;

printf("%d", x);      // 1
printf("%d", *ip);    // 1

Important points

1. The pointer itself stores only the address of the variable, not its value. This is an important distinction to understand. A pointer points to a memory location where the value of a variable is stored, but the pointer itself stores only the address of that location.

2. Pointer dereferencing, that is, when we write “asterisk variable pointer” - *ip, we are accessing the value of the variable pointed to by ip. This is called pointer dereferencing.

3. We can assign the value of the variable through the pointer. That is, dereferencing works both ways.

4. The asterisk - * is used to dereference a pointer and the ampersand - & is used to get the address of a variable.

At first glance, pointers in C may seem complicated, but when you begin to understand their essence, they become a powerful tool for memory management.

This example helped me to better understand how pointers work and how they can be used. I hope my experience will help you learn this topic faster.

I also suggest you to read the post The Asterisk * Operator in C.