C Comments

Try it on GfG Practice

The comments in C are human-readable notes in the source code of a C program used to make the program easier to read and understand. They are not a part of the executable program by the compiler or an interpreter.

Example:

#include <stdio.h>
int main() {
  
  	// This is a comment, the below
  	// statement will not be executed
    // printf("Hi from comment");
    printf("Hello");
    return 0;
}

Hello

Types of Comments in C

In C, there are two types of comments in C language:

• Single-line Comments
• Multi-line Comments

Single-line Comments

Single-line comments are used to comment out a single line of code or a part of it. The single line comments in C start with two forward slashes (//), and everything after the slashes on that line is considered a comment. They are also called C++ Style comments as they were first introduced in C++ and later adopted in C also.

Syntax

//  This is a single line comment

Example:

#include <stdio.h>

int main() {
  
    // This is a single-line comment explaining the variable x
    int x = 5;  
  
    // Output the value of x
    printf("Value of x: %d\n", x);  
    return 0;
}

Value of x: 5

In this C program, the comments provide explanations about the code. The compiler ignores the comments and does not execute them.

We can also create a comment that displays at the end of a line of code using a single-line comment. But generally, it's better to practice putting the comment before the line of code.

#include <stdio.h>

int main() {
    // single line comment here
  
    printf("Hi"); // After line comment here
    return 0;
}

Hi

Multi-line Comments

Multi-line comments in C are used write comments that span more than one line. They are generally used for longer descriptions or for commenting out multiple lines of code. In C language, these comments begin with /* and end with */. Everything between these markers is treated as a comment.

Syntax:

/* This is a multi-line comment
   which can span multiple lines */

Example:

#include <stdio.h>

int main() {
  
     /*
    This comment contains some code which
    will not be executed.
    printf("Code enclosed in Comment");
    */
    printf("Welcome to GeeksforGeeks");
    return 0;
}

Welcome to GeeksforGeeks

Nesting Comments in C

In C language, comments cannot be nested. That means you cannot place a multi-line comment inside another multi-line comment in C language. If you try to do so, the compiler will treat the closing */ of the inner comment as the end of the entire multi-line comment. and the rest of the part after the inner closing (*/) will not be commented.

#include <stdio.h>

int main() {
    /* This is the start of a multi-line comment
       /* This is an inner multi-line comment */
       This line will cause an error because the compiler
       considers the above '*/' as the end of the comment block.
    */
    printf("This program will not compile.\n");
    return 0;
}

Output

./Solution.c: In function 'main':
./Solution.c:6:8: error: unknown type name 'This'
        This line will cause an error because the compiler
        ^
./Solution.c:6:18: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'will'
        This line will cause an error because the compiler
                  ^

If nested comments are needed, it’s best to use single-line comments or comment out individual parts.

#include <stdio.h>

int main() {
    // /* This block of code is commented out
    // int x = 10; 
    // printf("Value of x: %d\n", x); 
    // */

    printf("Program runs without errors.\n");
    return 0;
}

Program runs without errors.

Best Practices for Writing Comments in C

Following are some best practices to write comments in your C program:

• Write comments that are easy to understand.
• Do not comment on every line unnecessarily and avoid writing obvious comments.
• Update comments regularly.
• Focus on explaining the intent behind the code rather than restating the obvious.