Your First Program

It is time. We have talked about hardware, history, and thinking. Now let's write code.

The traditional first program in any language is one that prints the words "Hello, world!". This tradition started in 1974, in an internal memo by Brian Kernighan at Bell Labs, and it has remained unbroken ever since.

Hello, world

Here is the entire program. Click Run.

You should see Hello, world! in the output panel. Eight words of code; a complete, real C program.

Now change the message to your own name and run it again. Then add a second printf line below the first to print a second message. The compiler does not care what you put in the string, as long as the quotes match.

Reading the program line by line

Let's dissect it. Every character is there for a reason.

#include <stdio.h>

Lines starting with # are not C statements; they are preprocessor directives. They run before the real compilation step. #include <stdio.h> means:

"Paste the contents of the standard header file stdio.h here."

stdio.h (short for standard input/output header) is part of the C standard library. It tells the compiler what printf is — its type, the kinds of arguments it accepts, and so on. Without this line, the compiler would not know what printf means.

int main(void) { ... }

Every C program needs an entry point, and that entry point is a function named main. The operating system, after loading your program into memory, jumps to main and starts executing.

Let's break the signature apart:

• int — the return type. main returns an integer. By convention, 0 means "success" and any non-zero value means "something went wrong".
• main — the special name the C runtime looks for.
• (void) — the parameter list. void is an explicit way to say "this function takes no arguments". (You may sometimes see int main(int argc, char *argv[]) instead — that version of main accepts command-line arguments, which we'll meet later.)
• { ... } — the function body, a block of code that runs when main is called.

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

printf is a function from the standard library. It prints formatted output. Here we're passing it a single argument — the string "Hello, world!\n".

The \n at the end is an escape sequence. It represents a single character: a newline. Without it, the next thing printed would appear on the same line.

The semicolon ; at the end is mandatory. C uses semicolons to mark the end of a statement. Forgetting one is the #1 source of "what did I do wrong?" for beginners. It's also the first thing the compiler will yell at you about.

return 0;

This returns the value 0 from main, telling the operating system the program finished successfully. Reach the closing } without an explicit return and (since C99) main returns 0 automatically, but explicit is better than implicit.

What just happened, in pictures

In our browser sandbox, every step still happens — the compiler is just bundled into the same web page you are reading, and the "terminal" is a panel to the right of the editor.

Two small experiments

The fastest way to learn syntax is to break it on purpose. Try each of these in turn in the code block above; run; observe what the compiler or runtime tells you; then fix it.

Experiment 1 — remove the semicolon

printf("Hello, world!\n")

You'll see an error like expected ';' before 'return'. The compiler keeps reading until it sees something that cannot follow what came before. That mismatch is what produces the error.

Experiment 2 — remove the \n

printf("Hello, world!");

Run it. The text still prints, but there's no newline at the end. In the browser sandbox this is mostly cosmetic; in a real terminal, the shell prompt would appear on the same line as your output. Subtle bugs like this are why we use \n.

Experiment 3 — print two things

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

Same output as before. Multiple printf calls just keep adding to the same line until you print a \n.

A slightly larger first program

Let's print a few numbers as well. Two new ideas appear:

1. The %d placeholder in a printf format string, replaced by the value of an integer argument.
2. A variable declaration: int year = 1972;.

The printf function fills %d with the next integer argument in order. The second call also shows that the argument can be a whole expression like 2025 - year, evaluated before printing.

Comments

C ignores anything between /* */ and anything from // to the end of the line. Use comments to explain why, not what.

Your first challenge