# Comments

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< c

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:

    //  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

    /*
    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

### See Also

C++ documentation for Comments

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