This topic is one of the most important bridges between:

Your Program
      ↓
System Calls
      ↓
Linux Kernel
      ↓
Hardware

And later, when we build the Apex compiler, understanding this bridge will help you understand what machine code eventually does.

What Exactly Is A System Call?

A system call (syscall) is a request from a user program to the operating system kernel to perform a privileged operation. The kernel is the only component allowed to directly interact with hardware, manage memory, create processes, access files, etc. (Man7)

For example:

use std::fs::File;

fn main() {
    let file = File::open("hello.txt");
}

Looks innocent.

But eventually Linux must:

• Find the file
• Check permissions
• Read disk metadata
• Return a file descriptor

Your Rust code cannot do this directly.

Instead it asks Linux through a syscall.

Real Example

Suppose:

println!("Hello");