System Calls and APIs in Operating Systems

When a user runs a program on a computer, it doesn’t directly control hardware or access system resources like memory, files, or devices. Instead, it relies on the Operating System (OS) to do that securely and efficiently. The bridge between user programs and the OS kernel is built using System Calls and APIs (Application Programming Interfaces).

These two components are essential to understand how programs interact with the OS, and how operating systems offer controlled access to low-level operations.

 What Are System Calls?

A System Call is a way for programs to request services from the OS kernel. These are built-in functions provided by the OS that allow user programs to perform actions like reading files, writing data, creating processes, and more.

Because direct access to hardware and system resources would be unsafe, system calls act as a controlled gateway. Whenever an application needs something that involves the system’s core functions, it issues a system call.

Examples of operations using system calls:

• Creating or deleting files.
• Allocating memory.
• Sending and receiving data over a network.
• Starting or ending a process.
• Accessing input/output devices.

System calls are usually hidden behind library functions. For example, when you write printf() in C, it eventually calls a system-level function to write to the screen.

 Types of System Calls

System calls are generally grouped into categories based on the service they provide:

• Process Control – Create, end, or manage processes (e.g., fork(), exit()).
• File Management – Read, write, create, or delete files (e.g., open(), read(), write()).
• Device Management – Request or release devices (e.g., ioctl()).
• Information Maintenance – Get system time, process info (e.g., getpid()).
• Communication – Exchange data between processes (e.g., pipes, sockets).

 What Are APIs?

An Application Programming Interface (API) is a set of predefined functions and protocols that programmers use to interact with the OS or other software. Unlike system calls, APIs are more user-friendly and abstract—designed to help developers build applications without dealing with low-level OS complexities.

Think of an API as a toolkit provided by the OS or software, where developers can access functions that internally handle system calls.

Key Characteristics:

• Acts as an interface between user applications and system services.
• Hides complexity by offering ready-to-use functions.
• Widely used in software libraries and frameworks.

Example:
In Windows, a function like CreateFile() is part of the Windows API. It doesn’t directly touch the hardware; it uses system calls under the hood to get the job done.

 System Calls vs. APIs

System Calls and APIs are two sides of the same coin. System calls are the actual means by which the OS provides services to programs, while APIs offer an easier, structured way to access those services. Together, they ensure that applications can run securely, efficiently, and without needing to manage complex hardware operations directly.