Step 1 of 5

Your first program

A program is a list of instructions the computer follows from top to bottom. C is a compiled language: you write the instructions as text (the source code), a tool called the compiler checks them and translates them into machine code the processor can run, and then the program runs. Every time you press Check, a real C compiler does exactly that in your browser.

The shape of every C program

#include <stdio.h>

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

Hi!

Line by line:

• #include <stdio.h> pulls in the standard input/output library. It tells the compiler what printf is. Without it the compiler doesn't know the name and reports an error.
• int main(void) starts a function called main. Every C program begins running at main, no matter how big it is. int says it gives back a whole number when it finishes, and (void) says it takes no inputs.
• The curly braces { } hold the function's body: the instructions that run, in order.
• printf("Hi!\n"); calls the printf function to print text. The text goes inside double quotes. \n is a newline: it ends the line, like pressing Enter.
• return 0; ends main and reports "success" (0) to the operating system. A non-zero number would mean something went wrong.

Common mistakes

• Spelling and capitals matter. Printf or print are different names, and the compiler will say something like call to undeclared function 'print'.
• Forgetting #include <stdio.h> gives a similar error about printf.
• Forgetting the ; at the end of a statement. You'll practice reading that error in step 5.

Your turn: fill in the blank so the program prints Hello, World!.

Next: Printing your own text