C Pointers

What is a pointer?

Every variable you create in C is stored somewhere in memory, and that spot has a numeric address. A pointer simply remembers that address. You can think of a normal variable as a house and a pointer as a piece of paper with the house's address written on it.

Two operators do most of the work. The address-of operator & gives you the address of a variable, and the dereference operator * lets you read or change the value stored at an address.

#include <stdio.h>

int main() {
    int age = 30;
    int *ptr = &age;   // ptr stores the address of age

    printf("Value of age: %d\n", age);
    printf("Address of age: %p\n", &age);
    printf("Value via pointer: %d\n", *ptr);

    return 0;
}

Here int *ptr declares a pointer to an int. We assign it &age so it points at our variable. Writing *ptr then reads back through the pointer to find 30. The %p format specifier is used to print an address.

Changing a value through a pointer

Because a pointer knows where a variable lives, you can update the original variable by writing to the dereferenced pointer. This is the idea behind passing data into functions so they can modify it.

#include <stdio.h>

int main() {
    int score = 50;
    int *p = &score;

    *p = 95;   // write a new value at score's address

    printf("Score is now %d\n", score);  // prints 95
    return 0;
}