| 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|---|
| 2 | <html>
|
|---|
| 3 | <!-- Copyright (C) 2022 Richard Stallman and Free Software Foundation, Inc.
|
|---|
| 4 |
|
|---|
| 5 | (The work of Trevis Rothwell and Nelson Beebe has been assigned or
|
|---|
| 6 | licensed to the FSF.)
|
|---|
| 7 |
|
|---|
| 8 | Permission is granted to copy, distribute and/or modify this document
|
|---|
| 9 | under the terms of the GNU Free Documentation License, Version 1.3 or
|
|---|
| 10 | any later version published by the Free Software Foundation; with the
|
|---|
| 11 | Invariant Sections being "GNU General Public License," with the
|
|---|
| 12 | Front-Cover Texts being "A GNU Manual," and with the Back-Cover
|
|---|
| 13 | Texts as in (a) below. A copy of the license is included in the
|
|---|
| 14 | section entitled "GNU Free Documentation License."
|
|---|
| 15 |
|
|---|
| 16 | (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
|
|---|
| 17 | modify this GNU manual. Buying copies from the FSF supports it in
|
|---|
| 18 | developing GNU and promoting software freedom." -->
|
|---|
| 19 | <!-- Created by GNU Texinfo 6.7, http://www.gnu.org/software/texinfo/ -->
|
|---|
| 20 | <head>
|
|---|
| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|---|
| 22 | <title>Stringification (GNU C Language Manual)</title>
|
|---|
| 23 |
|
|---|
| 24 | <meta name="description" content="Stringification (GNU C Language Manual)">
|
|---|
| 25 | <meta name="keywords" content="Stringification (GNU C Language Manual)">
|
|---|
| 26 | <meta name="resource-type" content="document">
|
|---|
| 27 | <meta name="distribution" content="global">
|
|---|
| 28 | <meta name="Generator" content="makeinfo">
|
|---|
| 29 | <link href="index.html" rel="start" title="Top">
|
|---|
| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
|
|---|
| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|---|
| 32 | <link href="Macros.html" rel="up" title="Macros">
|
|---|
| 33 | <link href="Concatenation.html" rel="next" title="Concatenation">
|
|---|
| 34 | <link href="Macro-Arguments.html" rel="prev" title="Macro Arguments">
|
|---|
| 35 | <style type="text/css">
|
|---|
| 36 | <!--
|
|---|
| 37 | a.summary-letter {text-decoration: none}
|
|---|
| 38 | blockquote.indentedblock {margin-right: 0em}
|
|---|
| 39 | div.display {margin-left: 3.2em}
|
|---|
| 40 | div.example {margin-left: 3.2em}
|
|---|
| 41 | div.lisp {margin-left: 3.2em}
|
|---|
| 42 | kbd {font-style: oblique}
|
|---|
| 43 | pre.display {font-family: inherit}
|
|---|
| 44 | pre.format {font-family: inherit}
|
|---|
| 45 | pre.menu-comment {font-family: serif}
|
|---|
| 46 | pre.menu-preformatted {font-family: serif}
|
|---|
| 47 | span.nolinebreak {white-space: nowrap}
|
|---|
| 48 | span.roman {font-family: initial; font-weight: normal}
|
|---|
| 49 | span.sansserif {font-family: sans-serif; font-weight: normal}
|
|---|
| 50 | ul.no-bullet {list-style: none}
|
|---|
| 51 | -->
|
|---|
| 52 | </style>
|
|---|
| 53 |
|
|---|
| 54 |
|
|---|
| 55 | </head>
|
|---|
| 56 |
|
|---|
| 57 | <body lang="en">
|
|---|
| 58 | <span id="Stringification"></span><div class="header">
|
|---|
| 59 | <p>
|
|---|
| 60 | Next: <a href="Concatenation.html" accesskey="n" rel="next">Concatenation</a>, Previous: <a href="Macro-Arguments.html" accesskey="p" rel="prev">Macro Arguments</a>, Up: <a href="Macros.html" accesskey="u" rel="up">Macros</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>
|
|---|
| 61 | </div>
|
|---|
| 62 | <hr>
|
|---|
| 63 | <span id="Stringification-1"></span><h4 class="subsection">26.5.4 Stringification</h4>
|
|---|
| 64 | <span id="index-stringification"></span>
|
|---|
| 65 | <span id="index-_0023-operator"></span>
|
|---|
| 66 |
|
|---|
| 67 | <p>Sometimes you may want to convert a macro argument into a string
|
|---|
| 68 | constant. Parameters are not replaced inside string constants, but
|
|---|
| 69 | you can use the <code>#</code> preprocessing operator instead. When a macro
|
|---|
| 70 | parameter is used with a leading <code>#</code>, preprocessing replaces it
|
|---|
| 71 | with the literal text of the actual argument, converted to a string
|
|---|
| 72 | constant. Unlike normal parameter replacement, the argument is not
|
|---|
| 73 | macro-expanded first. This is called <em>stringification</em>.
|
|---|
| 74 | </p>
|
|---|
| 75 | <p>There is no way to combine an argument with surrounding text and
|
|---|
| 76 | stringify it all together. But you can write a series of string
|
|---|
| 77 | constants and stringified arguments. After preprocessing replaces the
|
|---|
| 78 | stringified arguments with string constants, the consecutive string
|
|---|
| 79 | constants will be concatenated into one long string constant
|
|---|
| 80 | (see <a href="String-Constants.html">String Constants</a>).
|
|---|
| 81 | </p>
|
|---|
| 82 | <p>Here is an example that uses stringification and concatenation of
|
|---|
| 83 | string constants:
|
|---|
| 84 | </p>
|
|---|
| 85 | <div class="example">
|
|---|
| 86 | <pre class="example">#define WARN_IF(EXP) \
|
|---|
| 87 | do { if (EXP) \
|
|---|
| 88 | fprintf (stderr, "Warning: " #EXP "\n"); } \
|
|---|
| 89 | while (0)
|
|---|
| 90 |
|
|---|
| 91 | WARN_IF (x == 0);
|
|---|
| 92 | →
|
|---|
| 93 | do { if (x == 0)
|
|---|
| 94 | fprintf (stderr, "Warning: " "x == 0" "\n"); }
|
|---|
| 95 | while (0);
|
|---|
| 96 | </pre></div>
|
|---|
| 97 |
|
|---|
| 98 | <p>The argument for <code>EXP</code> is substituted once, as is, into the
|
|---|
| 99 | <code>if</code> statement, and once, stringified, into the argument to
|
|---|
| 100 | <code>fprintf</code>. If <code>x</code> were a macro, it would be expanded in the
|
|---|
| 101 | <code>if</code> statement but not in the string.
|
|---|
| 102 | </p>
|
|---|
| 103 | <p>The <code>do</code> and <code>while (0)</code> are a kludge to make it possible to
|
|---|
| 104 | write <code>WARN_IF (<var>arg</var>);</code>. The resemblance of <code>WARN_IF</code>
|
|---|
| 105 | to a function makes that a natural way to write it.
|
|---|
| 106 | See <a href="Swallowing-the-Semicolon.html">Swallowing the Semicolon</a>.
|
|---|
| 107 | </p>
|
|---|
| 108 | <p>Stringification in C involves more than putting double-quote
|
|---|
| 109 | characters around the fragment. It also backslash-escapes the quotes
|
|---|
| 110 | surrounding embedded string constants, and all backslashes within
|
|---|
| 111 | string and character constants, in order to get a valid C string
|
|---|
| 112 | constant with the proper contents. Thus, stringifying <code>p = "foo\n";<!-- /@w --></code> results in <tt>"p = \"foo\\n\";"<!-- /@w --></tt>. However, backslashes
|
|---|
| 113 | that are not inside string or character constants are not duplicated:
|
|---|
| 114 | ‘<samp>\n</samp>’ by itself stringifies to <tt>"\n"</tt>.
|
|---|
| 115 | </p>
|
|---|
| 116 | <p>All leading and trailing whitespace in text being stringified is
|
|---|
| 117 | ignored. Any sequence of whitespace in the middle of the text is
|
|---|
| 118 | converted to a single space in the stringified result. Comments are
|
|---|
| 119 | replaced by whitespace long before stringification happens, so they
|
|---|
| 120 | never appear in stringified text.
|
|---|
| 121 | </p>
|
|---|
| 122 | <p>There is no way to convert a macro argument into a character constant.
|
|---|
| 123 | </p>
|
|---|
| 124 | <p>To stringify the result of expansion of a macro argument, you have to
|
|---|
| 125 | use two levels of macros, like this:
|
|---|
| 126 | </p>
|
|---|
| 127 | <div class="example">
|
|---|
| 128 | <pre class="example">#define xstr(S) str(S)
|
|---|
| 129 | #define str(s) #s
|
|---|
| 130 | #define foo 4
|
|---|
| 131 | str (foo)
|
|---|
| 132 | → "foo"
|
|---|
| 133 | xstr (foo)
|
|---|
| 134 | → xstr (4)
|
|---|
| 135 | → str (4)
|
|---|
| 136 | → "4"
|
|---|
| 137 | </pre></div>
|
|---|
| 138 |
|
|---|
| 139 | <p><code>s</code> is stringified when it is used in <code>str</code>, so it is not
|
|---|
| 140 | macro-expanded first. But <code>S</code> is an ordinary argument to
|
|---|
| 141 | <code>xstr</code>, so it is completely macro-expanded before <code>xstr</code>
|
|---|
| 142 | itself is expanded (see <a href="Argument-Prescan.html">Argument Prescan</a>). Therefore, by the time
|
|---|
| 143 | <code>str</code> gets to its argument text, that text already been
|
|---|
| 144 | macro-expanded.
|
|---|
| 145 | </p>
|
|---|
| 146 | <hr>
|
|---|
| 147 | <div class="header">
|
|---|
| 148 | <p>
|
|---|
| 149 | Next: <a href="Concatenation.html" accesskey="n" rel="next">Concatenation</a>, Previous: <a href="Macro-Arguments.html" accesskey="p" rel="prev">Macro Arguments</a>, Up: <a href="Macros.html" accesskey="u" rel="up">Macros</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>
|
|---|
| 150 | </div>
|
|---|
| 151 |
|
|---|
| 152 |
|
|---|
| 153 |
|
|---|
| 154 | </body>
|
|---|
| 155 | </html>
|
|---|
