Lesson 09 of 20

6 mins read

Pointers, Memory Addresses & Dereferencing

In Plain English

A pointer in C is a variable storing the physical memory address of another variable. The address-of operator (&) extracts an address, the dereference operator (*) accesses/mutates the target value, and pointer arithmetic (ptr + i) steps through memory in sizeof(Type) chunks.

The Mental Model

"A variable is a house holding furniture (data); a pointer is a piece of paper with the street address of that house written on it."

Deep Dive: How It Works

Address-of (&): Evaluates to the memory address where a variable lives in RAM.

Dereference (*): Accesses the memory cell referenced by the pointer to read or write data.

Pointer Arithmetic: Adding 1 to int* advances the address by 4 bytes.

Syntax Blueprint

int val = 100;
int *ptr = &val;
*ptr += 50; // val is now 150

Store address in pointer variable and dereference with * to mutate in place.

Core Rules to Remember

Physical Address: Pointers store actual 64-bit RAM memory locations.

Direct Mutation: Dereferencing (*ptr) allows remote modification of variables across scopes.

NULL Initialization: Always initialize unassigned pointers to NULL to prevent reading wild memory.

Common Beginner Traps & How to Fix Them

Mistake:Dereferencing an uninitialized or NULL pointer: int *p; *p = 10;.

Why it happens: p contains random garbage stack bits; writing to it causes an OS segmentation fault (SIGSEGV).

How to fix: Always assign pointers to a valid address (&var) or allocate heap memory before dereferencing.

Pro Tip: Use pointer arithmetic (*(arr + i)) to traverse high-performance contiguous buffers without array indexing overhead.

Live Interactive Example

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Pointer Mutation

C11 / GCC 9.2

Output Console

Click "Run Code" to view the rendered output.

How it works: *ptr += 50 modifies the memory location of value directly.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Print Value via Pointer Dereference

Print "Dereferenced: %d\n" accessing value through *ptr.

Sandbox Output

Click "Run & Check" to test your solution.

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