Low-Level Mastery: The Mechanics of Pointer Arithmetic

0. Introduction

Imagine a program that manipulates memory with the precision of a grandmaster moving pieces on a chessboard. This capability is granted to developers through pointer arithmetic. Let’s explore the underlying mechanics of this powerful tool.

1. The Pointer: A Foundation

A pointer is a variable that stores the memory address of an object. Through a pointer, you can perform any operation that you would on the object directly: reading, writing, and lifecycle management.

A key distinction in C++ is the member access operator (->). When dealing with pointers to structs or classes, -> is used to dereference the pointer and access the member in a single step:

struct Car {
    int speed{};
};

Car* ptr = new Car;
ptr->speed = 150; // Syntactic shorthand for (*ptr).speed
std::cout << ptr->speed << std::endl;

delete ptr;

2. Dynamic Arrays and Syntactic Sugar

Pointers allow for the creation of dynamic arrays whose size is determined at runtime. While the subscript operator [] is standard, it is essentially "syntactic sugar" for pointer arithmetic.

Let’s examine a pure C implementation that bypasses the sugar to manipulate memory directly:

#include <stdio.h>
#include <stdlib.h>

int main() {
    // Allocate memory for 10 integers
    int* arr = (int*)malloc(10 * sizeof(int));

    if (arr == NULL) return 1; // Error handling: a must for senior code

    for (int i = 0; i < 10; ++i) {
        *(arr + i) = i; // Direct pointer arithmetic
    }

    free(arr);
    return 0;
}

3. The Scale Factor

Why do we use (arr + i) instead of manually calculating the byte offset like *(arr + i * 4)?
The compiler handles this through type-based scaling. When you increment an int, the address increases by sizeof(int). This abstraction ensures that the developer focuses on logic rather than manual memory offsets.

4. Arrays vs. Pointers: Key Distinctions

While arrays often "decay" into pointers when passed to functions, they are not identical:
• Allocation: Stack-allocated arrays have a fixed size at compile-time. Pointers can manage heap memory of arbitrary size.
• Mutatibility: A pointer is a variable that can be reassigned; an array name acts as a constant pointer to the first element.
• Performance: At the assembly level, arr[i] and *(arr + i) result in the same machine instructions. The difference lies solely in memory management and scope.

5. Real-World Applications and Critical Risks

Pointer arithmetic is the backbone of high-performance engineering:
• Data Processing: Efficiently traversing large datasets, buffers, or image pixels.
• System Architecture: Implementing complex data structures (trees, graphs) and custom memory allocators.
• Security & Red Teaming: Developing exploits and performing memory forensics.

Common Pitfalls:

1. Buffer Overflows: Accessing memory outside of allocated bounds leads to Undefined Behavior.
2. Memory Leaks: Failing to deallocate memory, leading to resource exhaustion.
3. Dangling Pointers: Attempting to dereference memory that has already been freed.

End

The pointer in C/C++ is like fire. In the right hands, it's a powerful and very useful weapon, but fire in the hands of a fool will cause his own house to crumble.

That's all, I hope you learned something new.

Comments

[Paul J. Lucas]

You're mixing up C and C++ in your examples and not stating which is which. You should just pick one language and stick with it, or explain both with clear markings.

While arrays often "decay" into pointers when passed to functions, they are not identical:

Array decay has nothing to do with functions. With the exception of being an argument to sizeof, an array always (not "often") decays to a pointer to its first element:

int a[4];
int *p = a;  // decay: no function here
f( a );      // decay just as above

The fact that neither C nor C++ have array parameters for functions is separate. In passing a to f() above, a decays just as the line above it. The fact that it just so happens to be an argument to a function is irrelevant. The decay happens regardless.

Stack-allocated arrays have a fixed size at compile-time.

That's true only for C++. C99 allows variable length arrays:

void f( unsigned n ) {
  int a[n];  // VLA
  // ...
}

(That said, it's generally best not to use VLAs.)

[Miraly]

Thanks for feedback!