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<h4 class="subsection">4.2.1 Ifdef</h4>
<p><a name="index-g_t_0023ifdef-85"></a><a name="index-g_t_0023endif-86"></a>
The simplest sort of conditional is
<pre class="smallexample"> #ifdef <var>MACRO</var>
<var>controlled text</var>
#endif /* <var>MACRO</var> */
</pre>
<p><a name="index-conditional-group-87"></a>This block is called a <dfn>conditional group</dfn>. <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 <dfn>succeeds</dfn> if
<var>MACRO</var> is defined, <dfn>fails</dfn> if it is not.
<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><span class="samp">#endif</span></samp>’ always matches the nearest ‘<samp><span class="samp">#ifdef</span></samp>’ (or
‘<samp><span class="samp">#ifndef</span></samp>’, or ‘<samp><span class="samp">#if</span></samp>’). Also, you cannot start a conditional
group in one file and end it in another.
<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>The comment following the ‘<samp><span class="samp">#endif</span></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><span class="samp">#endif</span></samp>’ to the corresponding ‘<samp><span class="samp">#ifdef</span></samp>’.
Older programs sometimes put <var>MACRO</var> directly after the
‘<samp><span class="samp">#endif</span></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><span class="samp">#ifndef</span></samp>’ the ‘<samp><span class="samp">#endif</span></samp>’ matches.
<p><a name="index-g_t_0023ifndef-88"></a>Sometimes you wish to use some code if a macro is <em>not</em> defined.
You can do this by writing ‘<samp><span class="samp">#ifndef</span></samp>’ instead of ‘<samp><span class="samp">#ifdef</span></samp>’.
One common use of ‘<samp><span class="samp">#ifndef</span></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>Macro definitions can vary between compilations for several reasons.
Here are some samples.
<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>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>Macros can be defined or undefined with the <samp><span class="option">-D</span></samp> and <samp><span class="option">-U</span></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>Your program might have a special header file (often called
<samp><span class="file">config.h</span></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 <samp><span class="command">autoconf</span></samp>, or done by hand.
</ul>
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