Pointers Basics & Memory Addresses

⚙️ C Language 🟢 Lesson 11 of 20 📅 2026 Edition

Pointers are what make C uniquely powerful — and what gives it a reputation for being difficult. A pointer is simply a variable that stores a memory address instead of an ordinary value. Once this idea clicks, pointers become one of the most useful tools in your entire C toolkit.

1What Is a Memory Address?

Every variable in your program lives somewhere in the computer's memory, and that location has a numeric address, just like every house on a street has an address. Normally you never see these addresses — you just refer to variables by name, and C handles the lookup for you behind the scenes.

2Declaring and Using a Pointer

C Language ▶ Run Code

int age = 25;
int *ptr = &age;    // ptr now stores the ADDRESS of age

printf("%d\n", age);    // 25 - the value
printf("%p\n", &age);   // e.g. 0x7ffee3a3 - the address
printf("%p\n", ptr);    // same address - ptr points to age
printf("%d\n", *ptr);   // 25 - dereferencing: 'follow the pointer to its value'

The asterisk * means two different things depending on context: in a declaration (int *ptr) it marks the variable as a pointer; in an expression (*ptr) it dereferences the pointer, meaning "give me the value stored at this address."

3Changing a Value Through a Pointer

C Language ▶ Run Code

int age = 25;
int *ptr = &age;

*ptr = 30;   // changes age itself, through the pointer!
printf("%d\n", age);  // 30

This is the real power of pointers: dereferencing with *ptr = 30; reaches through the pointer and modifies the original variable directly, not just a copy.

4NULL Pointers

A pointer that isn't pointing at any valid variable should be set to NULL rather than left uninitialized, and should always be checked before being dereferenced:

C Language ▶ Run Code

int *ptr = NULL;

if (ptr != NULL) {
    printf("%d\n", *ptr);
} else {
    printf("Pointer is not pointing to anything yet\n");
}

⚠️ Common Mistake: Dereferencing an Uninitialized Pointer

Declaring int *ptr; without immediately setting it to point somewhere valid (or to NULL) leaves it pointing at a random, garbage memory address. Dereferencing it with *ptr before assigning it a real address is undefined behavior and a very common cause of program crashes.

💻 Try It Yourself

Create an int variable, a pointer to it, print the value both directly and through the pointer, then change the value using the pointer and confirm the original variable changed too.

C Language ▶ Run Code

#include <stdio.h>

int main() {
    int score = 75;
    int *ptr = &score;

    printf("Before: %d\n", score);
    *ptr = 95;
    printf("After: %d\n", score);

    return 0;
}

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