Lesson 2: Hello World

Write your first C program, Hello World. A clear walkthrough of the compile and run flow.

The Simplest Program — Hello World

The classic first program is Hello World. Despite being tiny, it packs in every essential piece of a C program.

#include<stdio.h>

int main(void){
  printf("Hello World!\n");
  return 0;
}

Output

Hello World!

Click any line to see what it does.

#include<stdio.h>
int main(void){
  printf("Hello World!\n");
  return 0;
}

↑ Click any line above to see an explanation.

Change the text inside printf("...") and print your own message.

my_hello.c

Output

Click "Run" to execute...

💡 A few things to try

Check your understanding of this lesson!

start()

main()

init()

In C, main() is the program's entry point — it's where execution starts.

Hello (with newline)

Hello (no newline)

Error

printf doesn't add a newline on its own — you have to include \n. For a newline, write printf("Hello\n");.

Runs the program

Imports the standard I/O library

Declares a variable

stdio.h is the standard I/O header — you need it to use functions like printf and scanf.

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