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Your program has no access to hardware resources. For example, it cannot read a file, send a network request, or even print a message on the screen.
You might think “that can't be right.” You've written code that has done these things before.
That's because your programming language is using system calls (syscall) under the hood.
The operating system (OS) has two modes:
user mode: no access to the hardware
kernel mode: access to everything
Your program runs in user mode.
When we need to access the hardware, we switch to kernel mode by using a system call and ask the OS to perform the actions on our behalf.
Quick Example:
Let's say you want to create a variable and print it.
#include <stdio.h>
int main() {
char *message = "Hello, World!";
// syscall
printf("%s\n", message);
return 0;
}
First, it creates the variable, no problem.
But printing a message requires access to the terminal, which our program doesn't have.
This is where it needs to use a syscall to tell the OS to print the message.
Each OS provides a set of APIs for syscalls.
For printing a message to the Linux terminal, there's an API called write().
The flow goes like this:
printf() calls the write() syscall
write() loads a number to a specific CPU register (eg rax)
The number depends on the OS's syscall table (architecture-specific). x86 is different from ARM-based
OS activates kernel mode, executes sys_write, and controls the terminal to print that message.
A system call is the middleman between your program and hardware. It’s used in all kinds of actions, including reading files, sending network requests, and managing processes.
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