🧠 What is an Application Programming Interface (API)?
An API is a set of functions, protocols, or tools that allows different software components to communicate with each other.
In simple terms:
An API is like a menu in a restaurant — it tells you what you can order (what functions/services are available) without showing how the kitchen (code behind) works.
🔍 Core Idea
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Software A wants to use a service/functionality from Software B.
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Instead of directly accessing B’s internal logic, A makes API calls — a clean, documented, and controlled way to interact.
🔧 Where You See APIs
| Domain | Example |
|---|---|
| 🧠 Operating Systems | write(), open(), malloc() — C standard library & system calls |
| 🌐 Web Development | GET /users — REST APIs |
| ☁️ Cloud Services | AWS SDKs, Google Maps API, Stripe API |
| 📱 Mobile Apps | Android/iOS APIs to access camera, GPS, etc. |
| 🎮 Game Engines | Unity or Unreal APIs to control game objects |
🛠️ Types of APIs
| Type | Description |
|---|---|
| 📚 Library APIs | Functions provided by a programming library (e.g., math.h, NumPy) |
| 🔌 OS APIs | Interface to OS features (e.g., Windows API, POSIX) |
| 🌐 Web APIs | Communication between client ↔ server over HTTP/HTTPS |
| 🧰 Hardware APIs | Communicate with hardware via drivers |
| 📦 Internal APIs | Used between internal components of a system (microservices, modules) |
🧪 Real-World Example
Web API (REST):
GET https://api.weather.com/v3/weather/forecast?city=Pune
You send a request, and the API returns a JSON response:
{
"city": "Pune",
"forecast": "Partly cloudy",
"temp": "28°C"
}You don’t know or care how weather data is fetched—you just use the API.
📜 Benefits of Using APIs
| Benefit | Why It Matters |
|---|---|
| 🔒 Abstraction | You don’t need to know the implementation |
| 🛠️ Reusability | Shared logic can be reused via APIs |
| 🧪 Testing | Easier to test components in isolation |
| 🏗️ Modularity | Makes software clean and loosely coupled |
| 🌍 Interoperability | Allows different systems/languages to work together |
⚖️ API vs System Call
| Feature | API | System Call |
|---|---|---|
| 🧠 Abstraction Level | Higher-level | Lower-level (closer to OS) |
| 🔁 Role | Interface exposed to developer | Interface to OS kernel |
| 🧱 Example | fopen() | open() syscall inside fopen() |
| 📚 Usage | Used in applications, SDKs | Used in C stdlib, OS internals |
🧠 Interview-Ready Definition:
An Application Programming Interface (API) is a defined set of rules, protocols, or tools that allows one software component to interact with another. APIs abstract implementation details and enable modular, reusable, and scalable software design.