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149 lines
7.1 KiB
HTML
149 lines
7.1 KiB
HTML
4 years ago
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<html lang="en">
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<head>
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<title>Macro Arguments - The C Preprocessor</title>
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<meta http-equiv="Content-Type" content="text/html">
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<meta name="description" content="The C Preprocessor">
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<meta name="generator" content="makeinfo 4.13">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Macros.html#Macros" title="Macros">
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<link rel="prev" href="Function_002dlike-Macros.html#Function_002dlike-Macros" title="Function-like Macros">
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<link rel="next" href="Stringification.html#Stringification" title="Stringification">
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<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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<!--
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Copyright (C) 1987-2015 Free Software Foundation, Inc.
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Permission is granted to copy, distribute and/or modify this document
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under the terms of the GNU Free Documentation License, Version 1.3 or
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any later version published by the Free Software Foundation. A copy of
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the license is included in the
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section entitled ``GNU Free Documentation License''.
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This manual contains no Invariant Sections. The Front-Cover Texts are
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(a) (see below), and the Back-Cover Texts are (b) (see below).
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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funds for GNU development.
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--></style>
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</head>
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<body>
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<div class="node">
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<a name="Macro-Arguments"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Stringification.html#Stringification">Stringification</a>,
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Previous: <a rel="previous" accesskey="p" href="Function_002dlike-Macros.html#Function_002dlike-Macros">Function-like Macros</a>,
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Up: <a rel="up" accesskey="u" href="Macros.html#Macros">Macros</a>
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<hr>
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</div>
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<h3 class="section">3.3 Macro Arguments</h3>
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<p><a name="index-arguments-46"></a><a name="index-macros-with-arguments-47"></a><a name="index-arguments-in-macro-definitions-48"></a>
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Function-like macros can take <dfn>arguments</dfn>, just like true functions.
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To define a macro that uses arguments, you insert <dfn>parameters</dfn>
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between the pair of parentheses in the macro definition that make the
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macro function-like. The parameters must be valid C identifiers,
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separated by commas and optionally whitespace.
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<p>To invoke a macro that takes arguments, you write the name of the macro
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followed by a list of <dfn>actual arguments</dfn> in parentheses, separated
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by commas. The invocation of the macro need not be restricted to a
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single logical line—it can cross as many lines in the source file as
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you wish. The number of arguments you give must match the number of
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parameters in the macro definition. When the macro is expanded, each
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use of a parameter in its body is replaced by the tokens of the
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corresponding argument. (You need not use all of the parameters in the
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macro body.)
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<p>As an example, here is a macro that computes the minimum of two numeric
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values, as it is defined in many C programs, and some uses.
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<pre class="smallexample"> #define min(X, Y) ((X) < (Y) ? (X) : (Y))
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x = min(a, b); ==> x = ((a) < (b) ? (a) : (b));
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y = min(1, 2); ==> y = ((1) < (2) ? (1) : (2));
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z = min(a + 28, *p); ==> z = ((a + 28) < (*p) ? (a + 28) : (*p));
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</pre>
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<p class="noindent">(In this small example you can already see several of the dangers of
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macro arguments. See <a href="Macro-Pitfalls.html#Macro-Pitfalls">Macro Pitfalls</a>, for detailed explanations.)
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<p>Leading and trailing whitespace in each argument is dropped, and all
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whitespace between the tokens of an argument is reduced to a single
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space. Parentheses within each argument must balance; a comma within
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such parentheses does not end the argument. However, there is no
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requirement for square brackets or braces to balance, and they do not
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prevent a comma from separating arguments. Thus,
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<pre class="smallexample"> macro (array[x = y, x + 1])
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</pre>
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<p class="noindent">passes two arguments to <code>macro</code>: <code>array[x = y</code> and <code>x +
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1]</code>. If you want to supply <code>array[x = y, x + 1]</code> as an argument,
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you can write it as <code>array[(x = y, x + 1)]</code>, which is equivalent C
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code.
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<p>All arguments to a macro are completely macro-expanded before they are
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substituted into the macro body. After substitution, the complete text
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is scanned again for macros to expand, including the arguments. This rule
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may seem strange, but it is carefully designed so you need not worry
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about whether any function call is actually a macro invocation. You can
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run into trouble if you try to be too clever, though. See <a href="Argument-Prescan.html#Argument-Prescan">Argument Prescan</a>, for detailed discussion.
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<p>For example, <code>min (min (a, b), c)</code> is first expanded to
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<pre class="smallexample"> min (((a) < (b) ? (a) : (b)), (c))
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</pre>
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<p class="noindent">and then to
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<pre class="smallexample"> ((((a) < (b) ? (a) : (b))) < (c)
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? (((a) < (b) ? (a) : (b)))
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: (c))
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</pre>
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<p class="noindent">(Line breaks shown here for clarity would not actually be generated.)
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<p><a name="index-empty-macro-arguments-49"></a>You can leave macro arguments empty; this is not an error to the
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preprocessor (but many macros will then expand to invalid code).
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You cannot leave out arguments entirely; if a macro takes two arguments,
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there must be exactly one comma at the top level of its argument list.
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Here are some silly examples using <code>min</code>:
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<pre class="smallexample"> min(, b) ==> (( ) < (b) ? ( ) : (b))
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min(a, ) ==> ((a ) < ( ) ? (a ) : ( ))
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min(,) ==> (( ) < ( ) ? ( ) : ( ))
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min((,),) ==> (((,)) < ( ) ? ((,)) : ( ))
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min() error--> macro "min" requires 2 arguments, but only 1 given
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min(,,) error--> macro "min" passed 3 arguments, but takes just 2
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</pre>
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<p>Whitespace is not a preprocessing token, so if a macro <code>foo</code> takes
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one argument, <code>foo ()<!-- /@w --></code> and <code>foo ( )<!-- /@w --></code> both supply it an
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empty argument. Previous GNU preprocessor implementations and
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documentation were incorrect on this point, insisting that a
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function-like macro that takes a single argument be passed a space if an
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empty argument was required.
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<p>Macro parameters appearing inside string literals are not replaced by
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their corresponding actual arguments.
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<pre class="smallexample"> #define foo(x) x, "x"
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foo(bar) ==> bar, "x"
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</pre>
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</body></html>
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