Your First C Program

Why this matters

If you’ve written Python, JavaScript, or Java before, you already know what a variable is, what a loop is, what a function is. What’s actually new about C, at first, isn’t any of that – it’s the ceremony: the ritual of #include, int main(void), semicolons, curly braces. This lesson is only about that ceremony. There’s no new programming logic here at all – just what each piece of text in the smallest possible C program is for, so that starting with the next lesson, none of it is a mystery you’re working around.

Running the example

The panel on the right already has a complete, working C program in it. Press Run. You should see:

Hello, learncpp!

Every lesson in this course works this way: there’s always a real, runnable program on the right. Before reading any further, try changing the text between the quotes on the printf line to something else, and press Run again to see your change take effect. That’s the whole loop you’ll use throughout this course: change code, run it, read the output.

Reading the program, one line at a time

#include <stdio.h>

C doesn’t come with printing built in the way Python’s print or JavaScript’s console.log are just always available. Instead, functions like the one that prints text (printf, covered below) live in separate files called headers, and you have to explicitly pull one in before you can use anything from it. #include <stdio.h> means “bring in the declarations from the standard input/output header” – it’s what makes printf available at all. You’ll see an #include line like this at the top of nearly every C file in this course, and you’ll learn more about what it’s actually doing under the hood in Module 3. For now: no #include <stdio.h>, no printf.

int main(void) {
    ...
}

In Python, a script just runs top to bottom – there’s no required starting point you have to name. C is stricter: every C program must define exactly one function called main, and that function is where the program starts running. Nothing outside main executes just by existing.

Breaking that line down piece by piece:

main is the function’s name. It has to be exactly this name – it’s how the system that launches your program knows where to start.
(void) means “this function takes no inputs.” You’ll write functions that do take inputs starting in the next module; main here takes none.
int before main means this function will hand back an integer when it finishes – that’s what the return 0; on the last line inside it is doing. By convention, returning 0 means “the program finished successfully”; a nonzero value signals some kind of failure. You don’t need to think hard about this yet – just know that return 0; at the end of main is boilerplate you’ll write in almost every program in this course.
The { and } mark where main’s body starts and ends. Everything the function actually does lives between them.

printf("Hello, learncpp!\n");

printf (“print formatted”) is the function, made available by that #include line, that writes text to the output. The text you want printed goes in double quotes. \n inside the quotes is not two separate characters – it’s a single escape sequence meaning “newline,” the same idea as pressing Enter. Leave it out and whatever prints next would run onto the same line.

And the semicolon at the end: in C, a semicolon marks the end of a statement – one complete instruction. Every statement in this course ends with one. Python and JavaScript can often infer where a line ends from the line break alone; C never guesses – it requires the semicolon to be told explicitly. Forgetting one is the single most common first mistake in C, and it’s exactly what the very first exercise in Module 3 has you diagnose from a real compiler error.

“Compiling” in one paragraph

You’ve been clicking Run this whole time, so you’ve already been doing this without needing the theory: the text you write is not itself a program a computer can execute directly. A separate program, the compiler, translates your C source code into an executable – and only then does that executable actually run. If your code has a mistake the compiler can’t make sense of, it refuses to produce an executable at all and tells you why instead – that’s what a compile error is. This two-step process (compile, then run) is different from Python, where you mostly just run the source file directly – but it’s exactly what’s happening, invisibly, every time you press Run in this course. Module 3 goes into this in real depth; for now, just know the two steps exist and are separate.

What comments are for

You’ll see lines like this in example code throughout the course:

// this is a comment