Comments in C

Code tells the computer what to do. Comments tell people why. C comments are notes you write inside your code that the compiler completely ignores. They help you remember your own thinking weeks later, and they help teammates understand your code. In this lesson, you'll learn both kinds of comments in C, how to temporarily switch off code, a trap that catches many beginners, and what makes a comment genuinely useful.

What is a comment in C?

A comment is text in a C source file that the compiler ignores. During the preprocessing stage you met in How C Programs Are Compiled, every comment is replaced by a single space, so comments never reach the compiler proper and never make your program bigger or slower.

C has two styles of comments:

Single-line comments with //

Two forward slashes // start a comment that runs to the end of the line. You can put one on its own line or after a statement:

#include <stdio.h>

int main(void) {
    // Show the current room temperature
    printf("Room: 19 C\n");
    printf("Target: 22 C\n"); // the user turned the heat up
    return 0;
}

Room: 19 C
Target: 22 C

Neither comment appears in the output. The code before // on the same line still runs normally.

Multi-line comments with /* */

A comment that starts with /* continues, across as many lines as you like, until the first */:

#include <stdio.h>

/* This program prints a short
   playlist summary for the
   home screen of a music app. */
int main(void) {
    printf("Playlist: Morning Run\n");
    printf("Tracks: 12\n");
    return 0;
}

Playlist: Morning Run
Tracks: 12

Because a /* */ comment has a clear end, you can also put one in the middle of a line, such as return 0; /* success */.

Can you nest comments in C?

No. A /* */ comment always ends at the first */, even if there's another /* inside it. This is one of the classic C traps:

/* Turned off for now:
   printf("Total\n"); /* old total line */
*/

The comment ends right after old total line. The last line, */, is then treated as code, and GCC reports an error such as expected ... before '/' token. With -Wall, GCC also warns "/*" within comment, which is a big hint. Single-line // comments inside a /* */ comment are fine, though, because they're simply part of the comment text.

How to comment out code

"Commenting out" means turning a line of code into a comment so it doesn't run, without deleting it. It's handy when you're testing or hunting for a bug:

#include <stdio.h>

int main(void) {
    printf("Track 1: Sunrise Drive\n");
    // printf("Track 2: Rainy Window\n");
    printf("Track 3: City Lights\n");
    return 0;
}

Track 1: Sunrise Drive
Track 3: City Lights

Track 2 is skipped because its whole line is now a comment. Remove the // to bring it back.

Slashes inside quotes are not comments

Comment markers only count in code, not inside text in double quotes. That's lucky, because web addresses contain //:

#include <stdio.h>

int main(void) {
    printf("Website: https://example.com\n");
    printf("/* this is printed too */\n");
    return 0;
}

Website: https://example.com
/* this is printed too */

How to write a file header comment

Many teams start every C file with a header comment that says what the file is for. A common style lines up a * at the start of each line:

/*
 * File:    parking.c
 * Purpose: Shows how many spaces are free on each level.
 * Author:  Sam Lee
 * Date:    2026-10-26
 */
#include <stdio.h>

int main(void) {
    printf("Level 1: 14 free\n");
    printf("Level 2: 3 free\n");
    return 0;
}

Level 1: 14 free
Level 2: 3 free

The extra stars are just decoration inside one big /* */ comment. They make the block easy to spot when scrolling.

What makes a good comment?

A good comment explains something the code can't say by itself: the reason behind a choice, a warning, or a unit of measurement. A bad comment repeats the code word for word.

/* Bad: just repeats the code */
printf("Seats: 180\n"); // print seats

/* Good: explains what the code alone can't */
printf("Seats: 180\n"); // the Airbus A320 layout this airline uses

While you're learning, it's fine to write extra comments that explain each new idea to yourself. As you get more experienced, let clear names and simple code do most of the talking.

Common mistakes with C comments

1. Forgetting to close a /* comment

If you write /* and never write */, the rest of the file becomes one giant comment. GCC reports error: unterminated comment, plus other errors because your code has "disappeared". Many editors show the swallowed code in comment color, which is a quick way to spot it.

2. Nesting /* */ comments

As shown above, the first */ ends the comment. Use // on each line (or your editor's shortcut) when you need to comment out code that already contains comments.

3. Using # for comments, like in Python

# Print the welcome message
printf("Welcome!\n");

In C, # starts a preprocessor directive, not a comment. GCC replies with error: invalid preprocessing directive #Print. Use // instead.

4. Leaving comments that lie

A comment saying // 10% discount above code that now gives 15% is worse than no comment at all. When you change code, update its comments too.

Practice exercise

#include <stdio.h>

int main(void) {
    printf("Debug: loaded\n");
    printf("Player: Nova\n");
    printf("High score: 4200\n");
    return 0;
}

Debug: loaded
Player: Nova
High score: 4200

/*
 * File:    scores.c
 * Purpose: Shows the current player and the weekly high score.
 */
#include <stdio.h>

int main(void) {
    // printf("Debug: loaded\n");
    printf("Player: Nova\n");
    // The high score resets every Monday
    printf("High score: 4200\n");
    return 0;
}

Player: Nova
High score: 4200

What's next?

That completes the Getting Started chapter. You can write, compile, structure and comment a C program. Next, start storing data with C Variables.