c-struct-binary-mapper

Learn low-level memory concepts by building a C Struct to Binary Mapper that bypasses the OS to decode raw hard drive bytes directly.

Bare-Metal File System Decoder

Have you ever wondered how an operating system actually reads a hard drive?

When Linus Torvalds was first building the Linux kernel, he needed a way to read files from his hard drive. Instead of relying on an existing OS to do it for him, he wrote a low-level driver that looked at the raw 1s and 0s on the disk and decoded the file system from scratch.

This repository is a hands-on learning artifact that recreates that exact experience. We create a "virtual" floppy disk, format it with the ancient FAT12 file system, and use a C struct to bypass the operating system and read the raw binary boot sector directly.

Prerequisites

To run this workshop, you need a Linux or macOS environment with a C compiler and the tools to format a FAT file system.

• gcc (or clang)
• dosfstools (provides the mkfs.fat command)

How to Run It

If you have make installed, you can build the virtual drive, compile the code, and run the decoder all in one command:

make run

If you prefer to do it manually step-by-step:

1. Create and format the virtual hard drive:

make run

#The "Aha!" Moment: How It Works

Normally, programs ask the OS Kernel to read files for them. This program bypasses the Kernel's file explorer entirely.

If you look at the floppy.img file with a hex editor (xxd -l 128 floppy.img), it just looks like a wall of random numbers.

In driver.c, we define a rigid struct BootSector that perfectly matches the mathematical blueprint of a FAT12 disk. When we use fread(), C takes that struct and stamps it down over the raw binary chaos like a stencil. The chaotic bytes instantly snap into readable C variables (like bytes_per_sector or volume_label), proving that files and folders are just illusions painted over raw memory offsets!