C Get Started

To run a C program, two steps are needed. First a compiler translates the C source code, which people can read, into a program the computer can run. Then you run that program. On your own computer you would install a compiler such as GCC or Clang; in this tutorial the online editor does both steps for you.

Your first program

#include <stdio.h>

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

Hello, World!

Let us read it line by line.

• #include <stdio.h> brings in the standard input and output library. printf, which prints text, is declared there. Without this line the compiler does not know printf.
• int main(void) starts the main function. Every C program has exactly one main, and the program starts running at its first line. int means main gives back a whole number when it ends; void means it takes no inputs.
• The curly braces { and } mark the start and the end of main's body.
• printf("Hello, World!\n"); prints the text between the double quotes. \n is a newline: it moves the cursor to the next line.
• return 0; ends main and tells the system the program finished without problems.
• Every statement ends with a semicolon ;.

Compiling and running in the terminal

When you press Run in the editor, it types two commands into the terminal for you:

gcc -Wall main.c -o main && ./main

gcc -Wall main.c -o main compiles main.c into a program called main; -Wall asks for warnings about suspicious code, which helps you find mistakes early. ./main runs the program. The && means "run the second command only if the first one worked", so a program with errors is not run.

When something goes wrong

Mistakes are normal. If you forget the semicolon after printf(...), the compiler stops and prints the file name, the line, the column and what it expected, for example expected ';' after expression. The editor also adds a short hint. Fix the line and run again.

A program can print several lines with several printf calls, or with several \n in one call:

#include <stdio.h>

int main(void) {
    printf("Learning C\n");
    printf("is fun.\n");
    printf("One\nTwo\nThree\n");
    return 0;
}

Learning C
is fun.
One
Two
Three

Practice exercises

1. Print your name on one line and your age on the next, with two printf calls.
2. Print the same two lines with a single printf call.
3. Print a small box made of * characters over three lines.
4. Remove #include <stdio.h>, run the program and read the message. Then put the line back.