C — Comments

Single-line comments

Use // for a comment on one line:

#include <stdio.h>

int main() {
    int x = 10; // This is an assignment
    printf("%d\n", x); // Print the value
    return 0;
}

Single-line comments are the most common form. Use them to explain why a line exists rather than what it does.

Multi-line comments

Wrap longer comments between /* and */:

#include <stdio.h>

int main() {
    /*
      This block explains the algorithm
      we are about to implement. Multi-line
      comments are useful for longer notes.
    */
    int result = 42;
    printf("%d\n", result);
    return 0;
}

Multi-line comments can span any number of lines. They are not nested — opening a second /* inside an active block causes an error:

/*
  Valid comment
  /* Invalid nested comment — compiler error */
*/

Inline comments

Place a comment at the end of a code line:

#include <stdio.h>

int main() {
    int radius = 5;
    double area = 3.14159 * radius * radius; // Calculate circle area
    printf("Area: %.2f\n", area);
    return 0;
}

Commenting out code

Use comments to temporarily disable code during debugging:

#include <stdio.h>

int main() {
    int x = 10;
    int y = 20;

    // printf("Debug: x=%d, y=%d\n", x, y);  // Disabled for testing
    printf("Sum: %d\n", x + y);

    /* This entire block is commented out:
    for (int i = 0; i < 10; i++) {
        printf("%d ", i);
    }
    */

    return 0;
}

Documentation comments

Use consistent comment styles for API documentation:

/*
 * calculate_sum - Add two integers together.
 * @a: First operand.
 * @b: Second operand.
 *
 * Return: The sum of @a and @b.
 *
 * This is a Linux kernel style documentation comment.
 * Tools like KernelDoc can parse these into documentation.
 */
int calculate_sum(int a, int b) {
    return a + b;
}

When to comment

Follow these guidelines:

#include <stdio.h>

// GOOD: Explain WHY, not WHAT
int mask = value & 0xFF;  // Extract low byte for legacy compatibility

// GOOD: Mark TODOs and FIXMEs
// TODO: Replace with dynamic allocation after sprint 3
// FIXME: This breaks when input is NULL
char buffer[256];

// BAD: Redundant comment
int count = 0; // Initialize count to 0

// GOOD: Explain complex logic
double average = (double)sum / count; // Cast to avoid integer division

Best practices

Practice	Example
Keep comments near code	Comment above the line it explains
Update comments with code	Stale comments mislead readers
Use TODO/FIXME markers	// TODO: ... is searchable
Don't comment obvious code	i++; // increment i is noise
Use function-level docs	Document every public function

Mini Practice

Write C code that:

1. Uses // comments to explain variable declarations
2. Uses /* */ to write a multi-line description of a function
3. Comments out a line of code and explains why it's disabled
4. Documents a function with parameter descriptions

Up Next

In the next lesson, you'll learn about Variables — declaring and using variables in C.