| <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> |
| <html> |
| <!-- Copyright (C) 1987-2015 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.3 or |
| any later version published by the Free Software Foundation. A copy of |
| the license is included in the |
| section entitled "GNU Free Documentation License". |
| |
| This manual contains no Invariant Sections. The Front-Cover Texts are |
| (a) (see below), and the Back-Cover Texts are (b) (see below). |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| You have freedom to copy and modify this GNU Manual, like GNU |
| software. Copies published by the Free Software Foundation raise |
| funds for GNU development. --> |
| <!-- Created by GNU Texinfo 5.2, http://www.gnu.org/software/texinfo/ --> |
| <head> |
| <title>The C Preprocessor: Object-like Macros</title> |
| |
| <meta name="description" content="The C Preprocessor: Object-like Macros"> |
| <meta name="keywords" content="The C Preprocessor: Object-like Macros"> |
| <meta name="resource-type" content="document"> |
| <meta name="distribution" content="global"> |
| <meta name="Generator" content="makeinfo"> |
| <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> |
| <link href="index.html#Top" rel="start" title="Top"> |
| <link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives"> |
| <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> |
| <link href="Macros.html#Macros" rel="up" title="Macros"> |
| <link href="Function_002dlike-Macros.html#Function_002dlike-Macros" rel="next" title="Function-like Macros"> |
| <link href="Macros.html#Macros" rel="prev" title="Macros"> |
| <style type="text/css"> |
| <!-- |
| a.summary-letter {text-decoration: none} |
| blockquote.smallquotation {font-size: smaller} |
| div.display {margin-left: 3.2em} |
| div.example {margin-left: 3.2em} |
| div.indentedblock {margin-left: 3.2em} |
| div.lisp {margin-left: 3.2em} |
| div.smalldisplay {margin-left: 3.2em} |
| div.smallexample {margin-left: 3.2em} |
| div.smallindentedblock {margin-left: 3.2em; font-size: smaller} |
| div.smalllisp {margin-left: 3.2em} |
| kbd {font-style:oblique} |
| pre.display {font-family: inherit} |
| pre.format {font-family: inherit} |
| pre.menu-comment {font-family: serif} |
| pre.menu-preformatted {font-family: serif} |
| pre.smalldisplay {font-family: inherit; font-size: smaller} |
| pre.smallexample {font-size: smaller} |
| pre.smallformat {font-family: inherit; font-size: smaller} |
| pre.smalllisp {font-size: smaller} |
| span.nocodebreak {white-space:nowrap} |
| span.nolinebreak {white-space:nowrap} |
| span.roman {font-family:serif; font-weight:normal} |
| span.sansserif {font-family:sans-serif; font-weight:normal} |
| ul.no-bullet {list-style: none} |
| --> |
| </style> |
| |
| |
| </head> |
| |
| <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> |
| <a name="Object_002dlike-Macros"></a> |
| <div class="header"> |
| <p> |
| Next: <a href="Function_002dlike-Macros.html#Function_002dlike-Macros" accesskey="n" rel="next">Function-like Macros</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
| </div> |
| <hr> |
| <a name="Object_002dlike-Macros-1"></a> |
| <h3 class="section">3.1 Object-like Macros</h3> |
| <a name="index-object_002dlike-macro"></a> |
| <a name="index-symbolic-constants"></a> |
| <a name="index-manifest-constants"></a> |
| |
| <p>An <em>object-like macro</em> is a simple identifier which will be replaced |
| by a code fragment. It is called object-like because it looks like a |
| data object in code that uses it. They are most commonly used to give |
| symbolic names to numeric constants. |
| </p> |
| <a name="index-_0023define"></a> |
| <p>You create macros with the ‘<samp>#define</samp>’ directive. ‘<samp>#define</samp>’ is |
| followed by the name of the macro and then the token sequence it should |
| be an abbreviation for, which is variously referred to as the macro’s |
| <em>body</em>, <em>expansion</em> or <em>replacement list</em>. For example, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define BUFFER_SIZE 1024 |
| </pre></div> |
| |
| <p>defines a macro named <code>BUFFER_SIZE</code> as an abbreviation for the |
| token <code>1024</code>. If somewhere after this ‘<samp>#define</samp>’ directive |
| there comes a C statement of the form |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">foo = (char *) malloc (BUFFER_SIZE); |
| </pre></div> |
| |
| <p>then the C preprocessor will recognize and <em>expand</em> the macro |
| <code>BUFFER_SIZE</code>. The C compiler will see the same tokens as it would |
| if you had written |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">foo = (char *) malloc (1024); |
| </pre></div> |
| |
| <p>By convention, macro names are written in uppercase. Programs are |
| easier to read when it is possible to tell at a glance which names are |
| macros. |
| </p> |
| <p>The macro’s body ends at the end of the ‘<samp>#define</samp>’ line. You may |
| continue the definition onto multiple lines, if necessary, using |
| backslash-newline. When the macro is expanded, however, it will all |
| come out on one line. For example, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define NUMBERS 1, \ |
| 2, \ |
| 3 |
| int x[] = { NUMBERS }; |
| → int x[] = { 1, 2, 3 }; |
| </pre></div> |
| |
| <p>The most common visible consequence of this is surprising line numbers |
| in error messages. |
| </p> |
| <p>There is no restriction on what can go in a macro body provided it |
| decomposes into valid preprocessing tokens. Parentheses need not |
| balance, and the body need not resemble valid C code. (If it does not, |
| you may get error messages from the C compiler when you use the macro.) |
| </p> |
| <p>The C preprocessor scans your program sequentially. Macro definitions |
| take effect at the place you write them. Therefore, the following input |
| to the C preprocessor |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">foo = X; |
| #define X 4 |
| bar = X; |
| </pre></div> |
| |
| <p>produces |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">foo = X; |
| bar = 4; |
| </pre></div> |
| |
| <p>When the preprocessor expands a macro name, the macro’s expansion |
| replaces the macro invocation, then the expansion is examined for more |
| macros to expand. For example, |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define TABLESIZE BUFSIZE |
| #define BUFSIZE 1024 |
| TABLESIZE |
| → BUFSIZE |
| → 1024 |
| </pre></div> |
| |
| <p><code>TABLESIZE</code> is expanded first to produce <code>BUFSIZE</code>, then that |
| macro is expanded to produce the final result, <code>1024</code>. |
| </p> |
| <p>Notice that <code>BUFSIZE</code> was not defined when <code>TABLESIZE</code> was |
| defined. The ‘<samp>#define</samp>’ for <code>TABLESIZE</code> uses exactly the |
| expansion you specify—in this case, <code>BUFSIZE</code>—and does not |
| check to see whether it too contains macro names. Only when you |
| <em>use</em> <code>TABLESIZE</code> is the result of its expansion scanned for |
| more macro names. |
| </p> |
| <p>This makes a difference if you change the definition of <code>BUFSIZE</code> |
| at some point in the source file. <code>TABLESIZE</code>, defined as shown, |
| will always expand using the definition of <code>BUFSIZE</code> that is |
| currently in effect: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample">#define BUFSIZE 1020 |
| #define TABLESIZE BUFSIZE |
| #undef BUFSIZE |
| #define BUFSIZE 37 |
| </pre></div> |
| |
| <p>Now <code>TABLESIZE</code> expands (in two stages) to <code>37</code>. |
| </p> |
| <p>If the expansion of a macro contains its own name, either directly or |
| via intermediate macros, it is not expanded again when the expansion is |
| examined for more macros. This prevents infinite recursion. |
| See <a href="Self_002dReferential-Macros.html#Self_002dReferential-Macros">Self-Referential Macros</a>, for the precise details. |
| </p> |
| <hr> |
| <div class="header"> |
| <p> |
| Next: <a href="Function_002dlike-Macros.html#Function_002dlike-Macros" accesskey="n" rel="next">Function-like Macros</a>, Up: <a href="Macros.html#Macros" accesskey="u" rel="up">Macros</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Directives.html#Index-of-Directives" title="Index" rel="index">Index</a>]</p> |
| </div> |
| |
| |
| |
| </body> |
| </html> |
