Pointers — Addresses and Values in C

C in 100 Seconds, Episode 17

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Pointers are the defining feature of C. A pointer is a variable that holds a memory address. Once you understand pointers, you understand how C really works.

The Two Operators

• & — the address-of operator. Gets the memory address of a variable.
• * — the dereference operator. Gets the value at a memory address.

int x = 42;
int *ptr = &x;

ptr now holds the address of x. The type int * means "pointer to an integer."

Reading a Pointer

printf("Value of x: %d\n", x);        // 42
printf("Address of x: %p\n", &x);     // 0x7ff...
printf("ptr holds: %p\n", ptr);       // same address
printf("Value at ptr: %d\n", *ptr);   // 42

ptr and &x are the same address. *ptr follows that address to get the value — which is x.

Writing Through a Pointer

Dereferencing works for assignment too:

*ptr = 99;
printf("x is now: %d\n", x);  // 99

We didn't touch x directly. We went through the pointer and modified the value at that address. Since ptr points to x, the change shows up in x.

Why Does This Matter?

Pointers are how C does everything that higher-level languages hide from you:

• Pass by reference — modify variables inside functions
• Dynamic memory — malloc returns a pointer
• Data structures — linked lists, trees, graphs all use pointers
• Hardware access — memory-mapped I/O is pointer-based

You can't avoid pointers in C. They're not an advanced feature — they're the foundation.

Full Code

#include <stdio.h>

int main() {
  int x = 42;
  int *ptr = &x;

  printf("Value of x: %d\n", x);
  printf("Address of x: %p\n", &x);
  printf("ptr holds: %p\n", ptr);
  printf("Value at ptr: %d\n", *ptr);

  *ptr = 99;
  printf("x is now: %d\n", x);

  return 0;
}

Compile and Run

gcc pointers.c -o pointers
./pointers

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Next episode: Pointer Arithmetic — walking through memory one element at a time.

Student code: github.com/GoCelesteAI/c-in-100-seconds/episode17