Comments

Comments serve as a sort of in-code documentation. When inserted into a program, they are effectively ignored by the compiler; they are solely intended to be used as notes by the humans that read source code.

Syntax


`/` comment `/`	(1)

`//` comment	(2)	(since C99)

1) Often known as "C-style" or "multi-line" comments.

2) Often known as "C++-style" or "single-line" comments.

All comments are removed from the program at translation phase 3 by replacing each comment with a single whitespace character.

C-style

C-style comments are usually used to comment large blocks of text or small fragments of code; however, they can be used to comment single lines. To insert text as a C-style comment, simply surround the text with /* and */. C-style comments tell the compiler to ignore all content between /* and */. Although it is not part of the C standard, /** and */ are often used to indicate documentation blocks; this is legal because the second asterisk is simply treated as part of the comment.

Except within a character constant, a string literal, or a comment, the characters /* introduce a comment. The contents of such a comment are examined only to identify multibyte characters and to find the characters */ that terminate the comment. C-style comments cannot be nested.

C++-style

C++-style comments are usually used to comment single lines of text or code; however, they can be placed together to form multi-line comments. To insert text as a C++-style comment, simply precede the text with // and follow the text with the new line character. C++-style comments tell the compiler to ignore all content between // and a new line.

Except within a character constant, a string literal, or a comment, the characters // introduce a comment that includes all multibyte characters up to, but not including, the next new-line character. The contents of such a comment are examined only to identify multibyte characters and to find the new-line character that terminates the comment. C++-style comments can be nested:

//  y = f(x);   // invoke algorithm

A C-style comment may appear within a C++-style comment:

//  y = f(x);   /* invoke algorithm */

A C++-style comment may appear within a C-style comment; this is a mechanism for excluding a small block of source code:

/*
    y = f(x);   // invoke algorithms
    z = g(x);
*/

(since C99)

Notes

Because comments are removed before the preprocessor stage, a macro cannot be used to form a comment and an unterminated C-style comment doesn't spill over from an #include'd file.

/* An attempt to use a macro to form a comment. */
/* But, a space replaces characters "//".       */
#ifndef DEBUG
    #define PRINTF //
#else
    #define PRINTF printf
#endif
...  
PRINTF("Error in file%s at line%i\n", __FILE__, __LINE__);

Besides commenting out, other mechanisms used for source code exclusion are:

#if 0
    puts("this will not be compiled");
    /* no conflict with C-style comments */
    // no conflict with C++-style comments
#endif

and

if(0) {
    puts("this will be compiled but not be executed");
    /* no conflict with C-style comments */
    // no conflict with C++-style comments
}

The introduction of // comments in C99 was a breaking change in some rare circumstances:

a = b //*divisor:*/ c
+ d; /* C89 compiles a = b / c + d;
        C99 compiles a = b + d; */

Example

Run this code

#include <stdio.h>
/*
C-style comments can contain
multiple lines.
*/

/* Or, just one line. */

// C++-style comments can comment one line.

// Or, they can
// be strung together.

int main(void)
{
  // The below code won't be run
  // puts("Hello");

  // The below code will be run
  puts("Hello");
}

Output:

Hello

Language
Headers
Type support
Program utilities
Variadic function support
Error handling
Dynamic memory management
Date and time utilities
Strings library
Algorithms
Numerics
Input/output support
Localization support
Atomic operations (C11)
Thread support (C11)
Technical Specifications

comments
ASCII
Translation phases
identifier
scope
lifetime
lookup and name spaces
type
objects and alignment
arithmetic types
the main() function
the as-if rule
undefined behavior
memory model and data races