| 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>Macro Arguments (GNU C Language Manual)</title>
|
|---|
| 23 |
|
|---|
| 24 | <meta name="description" content="Macro Arguments (GNU C Language Manual)">
|
|---|
| 25 | <meta name="keywords" content="Macro Arguments (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="Stringification.html" rel="next" title="Stringification">
|
|---|
| 34 | <link href="Function_002dlike-Macros.html" rel="prev" title="Function-like Macros">
|
|---|
| 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="Macro-Arguments"></span><div class="header">
|
|---|
| 59 | <p>
|
|---|
| 60 | Next: <a href="Stringification.html" accesskey="n" rel="next">Stringification</a>, Previous: <a href="Function_002dlike-Macros.html" accesskey="p" rel="prev">Function-like Macros</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="Macro-Arguments-1"></span><h4 class="subsection">26.5.3 Macro Arguments</h4>
|
|---|
| 64 | <span id="index-arguments"></span>
|
|---|
| 65 | <span id="index-macros-with-arguments"></span>
|
|---|
| 66 | <span id="index-arguments-in-macro-definitions"></span>
|
|---|
| 67 |
|
|---|
| 68 | <p>Function-like macros can take <em>arguments</em>, just like true functions.
|
|---|
| 69 | To define a macro that uses arguments, you insert <em>parameters</em>
|
|---|
| 70 | between the pair of parentheses in the macro definition that make the
|
|---|
| 71 | macro function-like. The parameters must be valid C identifiers,
|
|---|
| 72 | separated by commas and optionally whitespace.
|
|---|
| 73 | </p>
|
|---|
| 74 | <p>To invoke a macro that takes arguments, you write the name of the macro
|
|---|
| 75 | followed by a list of <em>actual arguments</em> in parentheses, separated
|
|---|
| 76 | by commas. The invocation of the macro need not be restricted to a
|
|---|
| 77 | single logical line—it can cross as many lines in the source file as
|
|---|
| 78 | you wish. The number of arguments you give must match the number of
|
|---|
| 79 | parameters in the macro definition. When the macro is expanded, each
|
|---|
| 80 | use of a parameter in its body is replaced by the tokens of the
|
|---|
| 81 | corresponding argument. (The macro body is not required to use all of the
|
|---|
| 82 | parameters.)
|
|---|
| 83 | </p>
|
|---|
| 84 | <p>As an example, here is a macro that computes the minimum of two numeric
|
|---|
| 85 | values, as it is defined in many C programs, and some uses.
|
|---|
| 86 | </p>
|
|---|
| 87 | <div class="example">
|
|---|
| 88 | <pre class="example">#define min(X, Y) ((X) < (Y) ? (X) : (Y))
|
|---|
| 89 | x = min(a, b); → x = ((a) < (b) ? (a) : (b));
|
|---|
| 90 | y = min(1, 2); → y = ((1) < (2) ? (1) : (2));
|
|---|
| 91 | z = min(a+28, *p); → z = ((a+28) < (*p) ? (a+28) : (*p));
|
|---|
| 92 | </pre></div>
|
|---|
| 93 |
|
|---|
| 94 | <p>In this small example you can already see several of the dangers of
|
|---|
| 95 | macro arguments. See <a href="Macro-Pitfalls.html">Macro Pitfalls</a>, for detailed explanations.
|
|---|
| 96 | </p>
|
|---|
| 97 | <p>Leading and trailing whitespace in each argument is dropped, and all
|
|---|
| 98 | whitespace between the tokens of an argument is reduced to a single
|
|---|
| 99 | space. Parentheses within each argument must balance; a comma within
|
|---|
| 100 | such parentheses does not end the argument. However, there is no
|
|---|
| 101 | requirement for square brackets or braces to balance, and they do not
|
|---|
| 102 | prevent a comma from separating arguments. Thus,
|
|---|
| 103 | </p>
|
|---|
| 104 | <div class="example">
|
|---|
| 105 | <pre class="example">macro (array[x = y, x + 1])
|
|---|
| 106 | </pre></div>
|
|---|
| 107 |
|
|---|
| 108 | <p>passes two arguments to <code>macro</code>: <code>array[x = y</code> and <code>x +
|
|---|
| 109 | 1]</code>. If you want to supply <code>array[x = y, x + 1]</code> as an argument,
|
|---|
| 110 | you can write it as <code>array[(x = y, x + 1)]</code>, which is equivalent C
|
|---|
| 111 | code. However, putting an assignment inside an array subscript
|
|---|
| 112 | is to be avoided anyway.
|
|---|
| 113 | </p>
|
|---|
| 114 | <p>All arguments to a macro are completely macro-expanded before they are
|
|---|
| 115 | substituted into the macro body. After substitution, the complete text
|
|---|
| 116 | is scanned again for macros to expand, including the arguments. This rule
|
|---|
| 117 | may seem strange, but it is carefully designed so you need not worry
|
|---|
| 118 | about whether any function call is actually a macro invocation. You can
|
|---|
| 119 | run into trouble if you try to be too clever, though. See <a href="Argument-Prescan.html">Argument Prescan</a>, for detailed discussion.
|
|---|
| 120 | </p>
|
|---|
| 121 | <p>For example, <code>min (min (a, b), c)</code> is first expanded to
|
|---|
| 122 | </p>
|
|---|
| 123 | <div class="example">
|
|---|
| 124 | <pre class="example"> min (((a) < (b) ? (a) : (b)), (c))
|
|---|
| 125 | </pre></div>
|
|---|
| 126 |
|
|---|
| 127 | <p>and then to
|
|---|
| 128 | </p>
|
|---|
| 129 | <div class="example">
|
|---|
| 130 | <pre class="example">((((a) < (b) ? (a) : (b))) < (c)
|
|---|
| 131 | ? (((a) < (b) ? (a) : (b)))
|
|---|
| 132 | : (c))
|
|---|
| 133 | </pre></div>
|
|---|
| 134 |
|
|---|
| 135 | <p>(The line breaks shown here for clarity are not actually generated.)
|
|---|
| 136 | </p>
|
|---|
| 137 | <span id="index-empty-macro-arguments"></span>
|
|---|
| 138 | <p>You can leave macro arguments empty without error, but many macros
|
|---|
| 139 | will then expand to invalid code. You cannot leave out arguments
|
|---|
| 140 | entirely; if a macro takes two arguments, there must be exactly one
|
|---|
| 141 | comma at the top level of its argument list. Here are some silly
|
|---|
| 142 | examples using <code>min</code>:
|
|---|
| 143 | </p>
|
|---|
| 144 | <div class="example">
|
|---|
| 145 | <pre class="example">min(, b) → (( ) < (b) ? ( ) : (b))
|
|---|
| 146 | min(a, ) → ((a ) < ( ) ? (a ) : ( ))
|
|---|
| 147 | min(,) → (( ) < ( ) ? ( ) : ( ))
|
|---|
| 148 | min((,),) → (((,)) < ( ) ? ((,)) : ( ))
|
|---|
| 149 |
|
|---|
| 150 | min() error→ macro "min" requires 2 arguments, but only 1 given
|
|---|
| 151 | min(,,) error→ macro "min" passed 3 arguments, but takes just 2
|
|---|
| 152 | </pre></div>
|
|---|
| 153 |
|
|---|
| 154 | <p>Whitespace is not a preprocessing token, so if a macro <code>foo</code> takes
|
|---|
| 155 | one argument, <code>foo ()<!-- /@w --></code> and <code>foo ( )<!-- /@w --></code> both supply it an
|
|---|
| 156 | empty argument.
|
|---|
| 157 | </p>
|
|---|
| 158 |
|
|---|
| 159 | <p>Macro parameters appearing inside string literals are not replaced by
|
|---|
| 160 | their corresponding actual arguments.
|
|---|
| 161 | </p>
|
|---|
| 162 | <div class="example">
|
|---|
| 163 | <pre class="example">#define foo(x) x, "x"
|
|---|
| 164 | foo(bar) → bar, "x"
|
|---|
| 165 | </pre></div>
|
|---|
| 166 |
|
|---|
| 167 | <p>See the next subsection for how to insert macro arguments
|
|---|
| 168 | into a string literal.
|
|---|
| 169 | </p>
|
|---|
| 170 | <p>The token following the macro call and the last token of the macro
|
|---|
| 171 | expansion do not become one token even if it looks like they could:
|
|---|
| 172 | </p>
|
|---|
| 173 | <div class="example">
|
|---|
| 174 | <pre class="example">#define foo() abc
|
|---|
| 175 | foo()def → abc def
|
|---|
| 176 | </pre></div>
|
|---|
| 177 |
|
|---|
| 178 | <hr>
|
|---|
| 179 | <div class="header">
|
|---|
| 180 | <p>
|
|---|
| 181 | Next: <a href="Stringification.html" accesskey="n" rel="next">Stringification</a>, Previous: <a href="Function_002dlike-Macros.html" accesskey="p" rel="prev">Function-like Macros</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>
|
|---|
| 182 | </div>
|
|---|
| 183 |
|
|---|
| 184 |
|
|---|
| 185 |
|
|---|
| 186 | </body>
|
|---|
| 187 | </html>
|
|---|
