📚 Introducing Functions and Libraries in C

🎯 Learning Objectives

By the end of this lesson, you will:

Understand the purpose and syntax of functions in C
Prepare for modular design by introducing the concept of libraries
Generalize the delay using a parameterized function

1️⃣ Conceptual Warm-Up: Why Functions?

Start with a relatable analogy: “Imagine writing the same instructions over and over to boil water. Wouldn’t it be easier to say ‘boilWater()’ and let that do the work?”

Explain that functions:

Reduce repetition
Improve readability
Enable modular design
Make debugging easier

2️⃣ What Is a Function?

A function is a reusable block of code that performs a specific task. It can take inputs (parameters), perform operations, and optionally return a result. Example:

int add(int a, int b) 
{
    return a + b;
}

3️⃣ Function Prototypes: The Blueprint

A function prototype tells the compiler:

The function’s name
Its return type
The types of its parameters It’s like declaring, “This function exists—trust me, I’ll define it later.” Example:

int add(int a, int b); // Prototype

This allows you to call add() before its actual definition appears in the code.

4️⃣ Function Prototypes: The Blueprint

Type	Location	Visibility	Purpose
Local	Inside `.c` file	Private to that file	Used for internal helper functions
Public	Inside `.h` file	Visible to other files	Used to expose functionality to other modules

🔒 Local Prototype Example

// delay.c
static void calibrateDelay(void);  // Only used inside delay.c

void delay1ms(void) {
    calibrateDelay();
}

static keyword restricts visibility to the current file
Keeps internal logic hidden from other modules

🌐 Public Prototype Example

// delay.h
void delay1ms(void);
void delayMs(int ms);

These are visible to any .c file that includes delay.h
Promotes modularity and reuse

5️⃣ Why Use Header Files?

Header files (.h) act as interfaces:

They define what functions are available
They hide implementation details
They allow multiple .c files to share functionality cleanly Example usage:

#include "delay.h"

int main(void) {
    delayMs(500);  // Uses public function from delay.h
}

6️⃣ `#include <library.h>` vs `#include "library.h"`

🔍 `#include <library.h>` — System or Standard Headers

Used for: Standard libraries or system headers (e.g., , )
Search path: The compiler looks in system directories predefined by your development environment
Example:

#include <stdio.h> // Brings in standard I/O functions like printf

This tells the compiler: “Look in your built-in include paths for this header.”

🛠️ `#include "library.h"` — User-Defined or Local Headers

Used for: Your own header files or project-specific libraries
Search path: The compiler first looks in the current directory (where the source file is), then in system directories
Example:

#include "gpio.h" // Brings in your custom GPIO library

This tells the compiler: “Start by looking in the same folder as this source file. If not found, check system paths.”

🧪 Why It Matters

Type	Location	Visibility	Purpose
Local	Inside `.c` file	Private to that file	Used for internal helper functions
Public	Inside `.h` file	Visible to other files	Used to expose functionality to other modules