System Calls And APIs In Operating Systems | System Calls And APIs

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  Category:  OPERATING SYSTEM | 2nd May 2025, Friday

System Calls And APIs In Operating Systems

Operating Systems Serve As An Interface Between Hardware And User Applications. This Interaction Is Made Possible Through Well-defined Mechanisms—System Calls And Application Programming Interfaces (APIs). These Two Concepts Are Fundamental In OS Design, Enabling Processes To Request Services From The Kernel And Helping Developers Interact With System Resources In A Structured Manner. This Article Explains Both In Depth, Along With Their Differences, Types, And Real-world Examples.

1. What Are System Calls?

A system Call Is A Way For User-level Applications To Request Services From The Operating System's Kernel. Since Direct Access To Hardware Is Restricted In User Mode (for Security And Stability), Applications Must Use System Calls To Perform Low-level Operations Like Reading Files, Allocating Memory, Or Communicating Over A Network.

Why Use System Calls?

User Programs Do Not Have The Privilege To Interact With Hardware Directly. The Kernel, Operating In supervisor Mode, Manages All Hardware Resources. To Bridge This Gap, System Calls Act As A Controlled Entry Point Into Kernel-mode Operations.

How System Calls Work

1. The Application Issues A System Call Using A Standard Library Function.

2. The OS Transfers Control From user Mode To kernel Mode Using A Software Interrupt Or Trap.

3. The Kernel Performs The Requested Service.

4. Control Returns To The User Process With The Result.

Typical System Call Categories

1. Process Control

   • Create, Terminate, Or Manage Processes.

   • Example: fork(), exec(), exit()

2. File Management

   • Create, Open, Read, Write, And Delete Files.

   • Example: open(), read(), write(), close()

3. Device Management

   • Request And Release Devices, Perform I/O.

   • Example: ioctl(), read(), write()

4. Information Maintenance

   • Retrieve Or Set System Data And Time, Memory Usage.

   • Example: getpid(), alarm()

5. Communication

   • Between Processes Or Over A Network.

   • Example: pipe(), send(), recv(), socket()

2. What Are APIs (Application Programming Interfaces)?

An API Is A Set Of Functions, Protocols, And Tools That Allow Applications To Communicate With Software Components, Including The Operating System. It Provides A High-level Interface For Application Developers To Interact With System Functionality Without Worrying About The Low-level Implementation Details.

Relationship Between APIs And System Calls

• System Calls Are Low-level Functions Executed By The OS Kernel.

• APIs Are High-level Abstractions Provided By Programming Libraries.

• Most APIs Eventually Translate Into One Or More System Calls.

Why Use APIs Instead Of System Calls Directly?

1. Ease Of Use: APIs Abstract Complex Kernel-level Operations.

2. Portability: APIs Can Standardize Access To System Services Across Platforms.

3. Security: APIs Can Include Validation And Security Checks Before Calling The Kernel.

4. Maintainability: Changes In The OS Can Be Handled Within The API Layer, With Minimal Impact On The Application.

3. Example: File Operations

Let’s Examine How APIs And System Calls Work Together In A Real-world Scenario.

Example In C (POSIX System)

#include 
#include 
#include 

Int Main() {
    Int Fd = Open("file.txt", O_RDONLY);  // API Function
    Char Buffer[100];
    Read(fd, Buffer, 100);                // API Function
    Close(fd);                            // API Function
    Return 0;
}

• Here, open(), read(), And close() Are API Functions Provided By The Standard C Library (glibc).

• These Functions Internally Issue system Calls Like sys_open, sys_read, And sys_close.

4. Types Of System Calls In Detail

1. Process Control

Used For Process Creation And Termination.

• fork(): Creates A New Process.

• exec(): Replaces Current Process Image With A New Program.

• exit(): Terminates A Process.

• wait(): Waits For A Child Process To Finish.

2. File Management

Used To Manipulate Files And Directories.

• open(): Opens A File.

• read(): Reads From A File.

• write(): Writes To A File.

• close(): Closes A File.

• unlink(): Deletes A File.

3. Device Management

Used For Input/output Operations.

• ioctl(): Sends Control Command To A Device.

• read(), write(): Also Used For I/O Devices Like Keyboards Or USB.

4. Information Maintenance

• getpid(): Gets Process ID.

• gettimeofday(): Gets System Time.

• alarm(): Sets A Timer For The Process.

5. Communication

Facilitates Inter-process Communication (IPC).

• pipe(): Establishes A Pipe Between Processes.

• shmget(), shmat(): Shared Memory Functions.

• msgget(), msgsnd(): Message Queue Functions.

5. Types Of APIs

a. WinAPI (Windows API)

• Used For Developing Applications On Microsoft Windows.

• Offers Access To File Systems, GUIs, Processes, Threads, And Networking.

b. POSIX API (Portable Operating System Interface)

• A Standard API For Unix-like Operating Systems.

• Promotes Compatibility Across Different Systems.

c. Java API

• Abstracts System Calls Within The Java Virtual Machine (JVM).

• Offers File Handling, Networking, Threads, And More.

d. Linux System API (glibc)

• Provides Functions Like fopen, fread, fprintf, Which Internally Call System Calls.

6. Key Differences: System Calls Vs APIs

Feature	System Call	API
Definition	Interface To OS Kernel Functions	Interface For Applications To Use OS
Level	Low-level	High-level Abstraction
Ease Of Use	Complex	Easier, User-friendly
Security	Direct Access To Kernel	Indirect, Safer Access Via Validation
Portability	OS-dependent	More Portable
Examples	write(), fork()	printf(), fopen()
Responsibility	Managed By OS Developers	Used By Application Developers

7. Security And Performance Considerations

• Security: System Calls Can Be A Source Of Security Vulnerabilities If Not Properly Handled. Hence, APIs Provide Validation And Buffer Checks Before Invoking System Calls.

• Performance: System Calls Involve Context Switching Between User And Kernel Mode, Which Adds Overhead. Optimizing This Interaction Is Critical In High-performance Computing.

8. Conclusion

System Calls And APIs Form The Backbone Of Interaction Between Applications And The Operating System. While system Calls Provide The actual Mechanism For Requesting Services From The OS, APIs Offer An Abstract, User-friendly Interface That Enhances Developer Productivity And Application Portability. Understanding Both Is Crucial For Anyone Developing Low-level Applications, Operating Systems, Or System Utilities. As Computing Evolves, The Boundary Between System Calls And APIs Continues To Be Refined For Better Performance, Security, And Compatibility.

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System Calls And APIs In Operating Systems, System Calls In OS