---
description: "Learn more about: C Comments"
title: "C Comments"
ms.date: "06/25/2018"
helpviewer_keywords: ["code comments, C code", "comments, documenting code", "comments, C code", "/* */ comment delimiters", "comments"]
ms.assetid: 0f5f2825-e673-49e7-8669-94e2f5294989
---
# C Comments

A "comment" is a sequence of characters beginning with a forward slash/asterisk combination (<strong>/\*</strong>) that is treated as a single white-space character by the compiler and is otherwise ignored. A comment can include any combination of characters from the representable character set, including newline characters, but excluding the "end comment" delimiter (<strong>\*/</strong>). Comments can occupy more than one line but can't be nested.

Comments can appear anywhere a white-space character is allowed. Since the compiler treats a comment as a single white-space character, you can't include comments within tokens. The compiler ignores the characters in the comment.

Use comments to document your code. This example is a comment accepted by the compiler:

```C
/* Comments can contain keywords such as
   for and while without generating errors. */
```

Comments can appear on the same line as a code statement:

```C
printf( "Hello\n" );  /* Comments can go here */
```

You can choose to precede functions or program modules with a descriptive comment block:

```C
/* MATHERR.C illustrates writing an error routine
* for math functions.
*/
```

Since comments can't contain nested comments, this example causes an error:

```C
/* Comment out this routine for testing

   /* Open file */
    fh = _open( "myfile.c", _O_RDONLY );
    .
    .
    .
*/
```

The error occurs because the compiler recognizes the first `*/`, after the words `Open file`, as the end of the comment. It tries to process the remaining text and produces an error when it finds the `*/` outside a comment.

While you can use comments to render certain lines of code inactive for test purposes, the preprocessor directives `#if` and `#endif` and conditional compilation are a useful alternative for this task. For more information, see [Preprocessor Directives](../preprocessor/preprocessor-directives.md) in the *Preprocessor Reference*.

**Microsoft Specific**

The Microsoft compiler also supports single-line comments preceded by two forward slashes (**`//`**). These comments can't extend to a second line.

```C
// This is a valid comment
```

Comments beginning with two forward slashes (**`//`**) are terminated by the next newline character that isn't preceded by an escape character. In the next example, the newline character is preceded by a backslash (**`\`**), creating an "escape sequence." This escape sequence causes the compiler to treat the next line as part of the previous line. (For more information, see [Escape Sequences](../c-language/escape-sequences.md).)

```C
// my comment \
    i++;
```

Therefore, the `i++;` statement is commented out.

The default for Microsoft C is that the Microsoft extensions are enabled. Use /Za to disable these extensions.

**END Microsoft Specific**

## See also

[C Tokens](../c-language/c-tokens.md)