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| 3 | <!-- Copyright (C) 2022 Richard Stallman and Free Software Foundation, Inc.
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| 5 | (The work of Trevis Rothwell and Nelson Beebe has been assigned or
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| 8 | Permission is granted to copy, distribute and/or modify this document
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| 20 | <head>
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| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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| 22 | <title>Comments (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Comments (GNU C Language Manual)">
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| 25 | <meta name="keywords" content="Comments (GNU C Language Manual)">
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| 29 | <link href="index.html" rel="start" title="Top">
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| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Lexical-Syntax.html" rel="up" title="Lexical Syntax">
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| 33 | <link href="Identifiers.html" rel="next" title="Identifiers">
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| 34 | <link href="Whitespace.html" rel="prev" title="Whitespace">
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| 37 | a.summary-letter {text-decoration: none}
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| 53 |
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| 54 |
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| 55 | </head>
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| 56 |
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| 57 | <body lang="en">
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| 58 | <span id="Comments"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Identifiers.html" accesskey="n" rel="next">Identifiers</a>, Previous: <a href="Whitespace.html" accesskey="p" rel="prev">Whitespace</a>, Up: <a href="Lexical-Syntax.html" accesskey="u" rel="up">Lexical Syntax</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Comments-1"></span><h3 class="section">5.4 Comments</h3>
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| 64 | <span id="index-comments"></span>
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| 65 |
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| 66 | <p>A comment encapsulates text that has no effect on the program’s
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| 67 | execution or meaning.
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| 68 | </p>
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| 69 | <p>The purpose of comments is to explain the code to people that read it.
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| 70 | Writing good comments for your code is tremendously important—they
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| 71 | should provide background information that helps programmers
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| 72 | understand the reasons why the code is written the way it is. You,
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| 73 | returning to the code six months from now, will need the help of these
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| 74 | comments to remember why you wrote it this way.
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| 75 | </p>
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| 76 | <p>Outdated comments that become incorrect are counterproductive, so part
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| 77 | of the software developer’s responsibility is to update comments as
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| 78 | needed to correspond with changes to the program code.
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| 79 | </p>
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| 80 | <p>C allows two kinds of comment syntax, the traditional style and the
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| 81 | C<tt>++</tt> style. A traditional C comment starts with ‘<samp>/*</samp>’ and ends
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| 82 | with ‘<samp>*/</samp>’. For instance,
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| 83 | </p>
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| 84 | <div class="example">
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| 85 | <pre class="example">/* <span class="roman">This is a comment in traditional C syntax.</span> */
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| 86 | </pre></div>
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| 87 |
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| 88 | <p>A traditional comment can contain ‘<samp>/*</samp>’, but these delimiters do
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| 89 | not nest as pairs. The first ‘<samp>*/</samp>’ ends the comment regardless of
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| 90 | whether it contains ‘<samp>/*</samp>’ sequences.
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| 91 | </p>
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| 92 | <div class="example">
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| 93 | <pre class="example">/* <span class="roman">This</span> /* <span class="roman">is a comment</span> */ But this is not! */
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| 94 | </pre></div>
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| 95 |
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| 96 | <p>A <em>line comment</em> starts with ‘<samp>//</samp>’ and ends at the end of the line.
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| 97 | For instance,
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| 98 | </p>
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| 99 | <div class="example">
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| 100 | <pre class="example">// <span class="roman">This is a comment in C<tt>++</tt> style.</span>
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| 101 | </pre></div>
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| 102 |
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| 103 | <p>Line comments do nest, in effect, because ‘<samp>//</samp>’ inside a line
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| 104 | comment is part of that comment:
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| 105 | </p>
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| 106 | <div class="example">
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| 107 | <pre class="example">// <span class="roman">this whole line is</span> // <span class="roman">one comment</span>
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| 108 | This is code, not comment.
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| 109 | </pre></div>
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| 110 |
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| 111 | <p>It is safe to put line comments inside block comments, or vice versa.
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| 112 | </p>
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| 113 | <div class="example">
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| 114 | <pre class="example">/* <span class="roman">traditional comment</span>
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| 115 | // <span class="roman">contains line comment</span>
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| 116 | <span class="roman">more traditional comment</span>
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| 117 | */ text here is not a comment
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| 118 |
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| 119 | // <span class="roman">line comment</span> /* <span class="roman">contains traditional comment</span> */
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| 120 | </pre></div>
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| 121 |
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| 122 | <p>But beware of commenting out one end of a traditional comment with a line
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| 123 | comment. The delimiter ‘<samp>/*</samp>’ doesn’t start a comment if it occurs
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| 124 | inside an already-started comment.
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| 125 | </p>
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| 126 | <div class="example">
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| 127 | <pre class="example"> // <span class="roman">line comment</span> /* <span class="roman">That would ordinarily begin a block comment.</span>
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| 128 | Oops! The line comment has ended;
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| 129 | this isn't a comment any more. */
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| 130 | </pre></div>
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| 131 |
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| 132 | <p>Comments are not recognized within string constants. <tt>"/* blah */"<!-- /@w --></tt> is the string constant ‘<samp>/* blah */<!-- /@w --></samp>’, not an empty
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| 133 | string.
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| 134 | </p>
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| 135 | <p>In this manual we show the text in comments in a variable-width font,
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| 136 | for readability, but this font distinction does not exist in source
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| 137 | files.
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| 138 | </p>
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| 139 | <p>A comment is syntactically equivalent to whitespace, so it always
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| 140 | separates tokens. Thus,
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| 141 | </p>
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| 142 | <div class="example">
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| 143 | <pre class="example"> int/* <span class="roman">comment</span> */foo;
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| 144 | <span class="roman">is equivalent to</span>
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| 145 | int foo;
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| 146 | </pre></div>
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| 147 |
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| 148 | <p>but clean code always uses real whitespace to separate the comment
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| 149 | visually from surrounding code.
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| 150 | </p>
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| 151 | <hr>
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| 152 | <div class="header">
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| 153 | <p>
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| 154 | Next: <a href="Identifiers.html" accesskey="n" rel="next">Identifiers</a>, Previous: <a href="Whitespace.html" accesskey="p" rel="prev">Whitespace</a>, Up: <a href="Lexical-Syntax.html" accesskey="u" rel="up">Lexical Syntax</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
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| 155 | </div>
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| 156 |
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| 157 |
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| 158 |
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| 159 | </body>
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| 160 | </html>
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