164 lines
7.5 KiB
HTML
164 lines
7.5 KiB
HTML
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html>
|
||
|
|
<!-- Copyright (C) 1987-2016 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: Ifdef</title>
|
||
|
|
|
||
|
|
<meta name="description" content="The C Preprocessor: Ifdef">
|
||
|
|
<meta name="keywords" content="The C Preprocessor: Ifdef">
|
||
|
|
<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="Conditional-Syntax.html#Conditional-Syntax" rel="up" title="Conditional Syntax">
|
||
|
|
<link href="If.html#If" rel="next" title="If">
|
||
|
|
<link href="Conditional-Syntax.html#Conditional-Syntax" rel="prev" title="Conditional Syntax">
|
||
|
|
<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="Ifdef"></a>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="If.html#If" accesskey="n" rel="next">If</a>, Up: <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="u" rel="up">Conditional Syntax</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="Ifdef-1"></a>
|
||
|
|
<h4 class="subsection">4.2.1 Ifdef</h4>
|
||
|
|
<a name="index-_0023ifdef"></a>
|
||
|
|
<a name="index-_0023endif"></a>
|
||
|
|
|
||
|
|
<p>The simplest sort of conditional is
|
||
|
|
</p>
|
||
|
|
<div class="smallexample">
|
||
|
|
<pre class="smallexample">#ifdef <var>MACRO</var>
|
||
|
|
|
||
|
|
<var>controlled text</var>
|
||
|
|
|
||
|
|
#endif /* <var>MACRO</var> */
|
||
|
|
</pre></div>
|
||
|
|
|
||
|
|
<a name="index-conditional-group"></a>
|
||
|
|
<p>This block is called a <em>conditional group</em>. <var>controlled text</var>
|
||
|
|
will be included in the output of the preprocessor if and only if
|
||
|
|
<var>MACRO</var> is defined. We say that the conditional <em>succeeds</em> if
|
||
|
|
<var>MACRO</var> is defined, <em>fails</em> if it is not.
|
||
|
|
</p>
|
||
|
|
<p>The <var>controlled text</var> inside of a conditional can include
|
||
|
|
preprocessing directives. They are executed only if the conditional
|
||
|
|
succeeds. You can nest conditional groups inside other conditional
|
||
|
|
groups, but they must be completely nested. In other words,
|
||
|
|
‘<samp>#endif</samp>’ always matches the nearest ‘<samp>#ifdef</samp>’ (or
|
||
|
|
‘<samp>#ifndef</samp>’, or ‘<samp>#if</samp>’). Also, you cannot start a conditional
|
||
|
|
group in one file and end it in another.
|
||
|
|
</p>
|
||
|
|
<p>Even if a conditional fails, the <var>controlled text</var> inside it is
|
||
|
|
still run through initial transformations and tokenization. Therefore,
|
||
|
|
it must all be lexically valid C. Normally the only way this matters is
|
||
|
|
that all comments and string literals inside a failing conditional group
|
||
|
|
must still be properly ended.
|
||
|
|
</p>
|
||
|
|
<p>The comment following the ‘<samp>#endif</samp>’ is not required, but it is a
|
||
|
|
good practice if there is a lot of <var>controlled text</var>, because it
|
||
|
|
helps people match the ‘<samp>#endif</samp>’ to the corresponding ‘<samp>#ifdef</samp>’.
|
||
|
|
Older programs sometimes put <var>MACRO</var> directly after the
|
||
|
|
‘<samp>#endif</samp>’ without enclosing it in a comment. This is invalid code
|
||
|
|
according to the C standard. CPP accepts it with a warning. It
|
||
|
|
never affects which ‘<samp>#ifndef</samp>’ the ‘<samp>#endif</samp>’ matches.
|
||
|
|
</p>
|
||
|
|
<a name="index-_0023ifndef"></a>
|
||
|
|
<p>Sometimes you wish to use some code if a macro is <em>not</em> defined.
|
||
|
|
You can do this by writing ‘<samp>#ifndef</samp>’ instead of ‘<samp>#ifdef</samp>’.
|
||
|
|
One common use of ‘<samp>#ifndef</samp>’ is to include code only the first
|
||
|
|
time a header file is included. See <a href="Once_002dOnly-Headers.html#Once_002dOnly-Headers">Once-Only Headers</a>.
|
||
|
|
</p>
|
||
|
|
<p>Macro definitions can vary between compilations for several reasons.
|
||
|
|
Here are some samples.
|
||
|
|
</p>
|
||
|
|
<ul>
|
||
|
|
<li> Some macros are predefined on each kind of machine
|
||
|
|
(see <a href="System_002dspecific-Predefined-Macros.html#System_002dspecific-Predefined-Macros">System-specific Predefined Macros</a>). This allows you to provide
|
||
|
|
code specially tuned for a particular machine.
|
||
|
|
|
||
|
|
</li><li> System header files define more macros, associated with the features
|
||
|
|
they implement. You can test these macros with conditionals to avoid
|
||
|
|
using a system feature on a machine where it is not implemented.
|
||
|
|
|
||
|
|
</li><li> Macros can be defined or undefined with the <samp>-D</samp> and <samp>-U</samp>
|
||
|
|
command-line options when you compile the program. You can arrange to
|
||
|
|
compile the same source file into two different programs by choosing a
|
||
|
|
macro name to specify which program you want, writing conditionals to
|
||
|
|
test whether or how this macro is defined, and then controlling the
|
||
|
|
state of the macro with command-line options, perhaps set in the
|
||
|
|
Makefile. See <a href="Invocation.html#Invocation">Invocation</a>.
|
||
|
|
|
||
|
|
</li><li> Your program might have a special header file (often called
|
||
|
|
<samp>config.h</samp>) that is adjusted when the program is compiled. It can
|
||
|
|
define or not define macros depending on the features of the system and
|
||
|
|
the desired capabilities of the program. The adjustment can be
|
||
|
|
automated by a tool such as <code>autoconf</code>, or done by hand.
|
||
|
|
</li></ul>
|
||
|
|
|
||
|
|
<hr>
|
||
|
|
<div class="header">
|
||
|
|
<p>
|
||
|
|
Next: <a href="If.html#If" accesskey="n" rel="next">If</a>, Up: <a href="Conditional-Syntax.html#Conditional-Syntax" accesskey="u" rel="up">Conditional Syntax</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>
|