Chapter 3: Your First C Program

Introduction

The best way to learn C programming is by writing and running your first program. Traditionally, the first program prints "Hello, World!".

This simple exercise teaches you the core fundamentals:

Program structure
The main() function
Output statements
Compilation process
Running a program

First C Program

hello.c

#include <stdio.h>  // Includes standard I/O library (for printf)

int main()                // Main function: program execution begins here
{                         // Opening brace: starts the function body
    printf("Hello, World!"); // Prints "Hello, World!" to the screen
    return 0;             // Returns 0 to indicate successful completion
}                         // Closing brace: ends the function body

Understanding Each Line

Line 1: `#include <stdio.h>`

This includes the Standard Input Output library. It gives access to essential functions like:

printf() - to display text
scanf() - to get user input

Line 2: `int main()`

This is the main function. Every C program starts execution from main(). The int means the function returns an integer value to the operating system upon completion.

Line 3 and 6: `{ }`

These curly braces define the block of the function. Everything inside these braces belongs to main().

Line 4: `printf("Hello, World!");`

The printf() function prints whatever is inside the double quotes to the screen.

Line 5: `return 0;`

This tells the operating system that the program ended successfully.

How to Compile and Run

1. Compile: Use the GCC compiler to build your executable: gcc hello.c -o hello
2. Run: Execute the compiled program based on your operating system:
- Linux/macOS: ./hello
- Windows: hello.exe

Console Output

Hello, World!

Flow of Execution

                        Start
                    

                        main()
                    

                        printf()
                    

                        return 0
                    

                        End Program
                    

Embedded Focus

In the desktop world, your first C program prints "Hello, World!" to a console. In the embedded world, most microcontrollers do not have a computer screen! For an embedded engineer, the traditional "Hello, World!" is Blinking an LED by toggling a General Purpose Input/Output (GPIO) pin. To print messages for debugging, the printf() function is redirected (or "retargeted") to send characters over a serial interface like UART to your computer's terminal emulator.

Interview Question

Why is main() important in C?

Because program execution starts from main(). Without it, the compiler wouldn't know where the program begins.

← Chapter 2: How C Programs Work Chapter 4: Variables & Data Types →