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\n	(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:

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

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

1	`// 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

/*
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
  // return 1;
  
  // The below code will be run
  return 0;
}

References

C11 standard (ISO/IEC 9899:2011):
- 6.4.9 Comments (p: 75)
C99 standard (ISO/IEC 9899:1999):
- 6.4.9 Comments (p: 66)
C89/C90 standard (ISO/IEC 9899:1990):
- 3.1.9 Comments