164 lines
7.5 KiB
HTML
164 lines
7.5 KiB
HTML
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any later version published by the Free Software Foundation. A copy of
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section entitled "GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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<head>
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<title>The C Preprocessor: Ifdef</title>
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<meta name="description" content="The C Preprocessor: Ifdef">
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<link href="Index-of-Directives.html#Index-of-Directives" rel="index" title="Index of Directives">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Conditional-Syntax.html#Conditional-Syntax" rel="up" title="Conditional Syntax">
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<link href="If.html#If" rel="next" title="If">
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<link href="Conditional-Syntax.html#Conditional-Syntax" rel="prev" title="Conditional Syntax">
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</head>
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<body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
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<a name="Ifdef"></a>
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<div class="header">
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<p>
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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>
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</div>
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<hr>
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<a name="Ifdef-1"></a>
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<h4 class="subsection">4.2.1 Ifdef</h4>
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<a name="index-_0023ifdef"></a>
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<a name="index-_0023endif"></a>
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<p>The simplest sort of conditional is
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</p>
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<div class="smallexample">
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<pre class="smallexample">#ifdef <var>MACRO</var>
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<var>controlled text</var>
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#endif /* <var>MACRO</var> */
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</pre></div>
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<a name="index-conditional-group"></a>
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<p>This block is called a <em>conditional group</em>. <var>controlled text</var>
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will be included in the output of the preprocessor if and only if
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<var>MACRO</var> is defined. We say that the conditional <em>succeeds</em> if
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<var>MACRO</var> is defined, <em>fails</em> if it is not.
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</p>
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<p>The <var>controlled text</var> inside of a conditional can include
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preprocessing directives. They are executed only if the conditional
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succeeds. You can nest conditional groups inside other conditional
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groups, but they must be completely nested. In other words,
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‘<samp>#endif</samp>’ always matches the nearest ‘<samp>#ifdef</samp>’ (or
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‘<samp>#ifndef</samp>’, or ‘<samp>#if</samp>’). Also, you cannot start a conditional
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group in one file and end it in another.
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</p>
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<p>Even if a conditional fails, the <var>controlled text</var> inside it is
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still run through initial transformations and tokenization. Therefore,
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it must all be lexically valid C. Normally the only way this matters is
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that all comments and string literals inside a failing conditional group
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must still be properly ended.
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</p>
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<p>The comment following the ‘<samp>#endif</samp>’ is not required, but it is a
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good practice if there is a lot of <var>controlled text</var>, because it
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helps people match the ‘<samp>#endif</samp>’ to the corresponding ‘<samp>#ifdef</samp>’.
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Older programs sometimes put <var>MACRO</var> directly after the
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‘<samp>#endif</samp>’ without enclosing it in a comment. This is invalid code
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according to the C standard. CPP accepts it with a warning. It
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never affects which ‘<samp>#ifndef</samp>’ the ‘<samp>#endif</samp>’ matches.
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</p>
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<a name="index-_0023ifndef"></a>
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<p>Sometimes you wish to use some code if a macro is <em>not</em> defined.
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You can do this by writing ‘<samp>#ifndef</samp>’ instead of ‘<samp>#ifdef</samp>’.
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One common use of ‘<samp>#ifndef</samp>’ is to include code only the first
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time a header file is included. See <a href="Once_002dOnly-Headers.html#Once_002dOnly-Headers">Once-Only Headers</a>.
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</p>
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<p>Macro definitions can vary between compilations for several reasons.
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Here are some samples.
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</p>
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<ul>
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<li> Some macros are predefined on each kind of machine
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(see <a href="System_002dspecific-Predefined-Macros.html#System_002dspecific-Predefined-Macros">System-specific Predefined Macros</a>). This allows you to provide
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code specially tuned for a particular machine.
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</li><li> System header files define more macros, associated with the features
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they implement. You can test these macros with conditionals to avoid
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using a system feature on a machine where it is not implemented.
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</li><li> Macros can be defined or undefined with the <samp>-D</samp> and <samp>-U</samp>
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command-line options when you compile the program. You can arrange to
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compile the same source file into two different programs by choosing a
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macro name to specify which program you want, writing conditionals to
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test whether or how this macro is defined, and then controlling the
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state of the macro with command-line options, perhaps set in the
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Makefile. See <a href="Invocation.html#Invocation">Invocation</a>.
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</li><li> Your program might have a special header file (often called
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<samp>config.h</samp>) that is adjusted when the program is compiled. It can
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define or not define macros depending on the features of the system and
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the desired capabilities of the program. The adjustment can be
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automated by a tool such as <code>autoconf</code>, or done by hand.
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</li></ul>
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<hr>
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<div class="header">
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<p>
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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>
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</div>
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</body>
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</html>
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