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

(1) /* comment */
(2) // comment (since C99)

Often known as “C-style” or “multi-line” comments.
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

#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("World");

  // A note regarding backslash + newline.
  // Despite belonging to translation phase 2 (vs phase 3 for comments),
  // '\' still determines which portion of the source code is considered
  // as 'comments':
  // This comment will be promoted to the next line \
  puts("Won't be run"); // may issue a warning "multi-line comment"
  puts("Hello, again");
}

Output:

World
Hello, again

References

C17 standard (ISO/IEC 9899:2018):
- 6.4.9 Comments (p: 54)
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