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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><strong class="selflink"><span class="t-lines"><span>#if</span><span>#ifdef</span><span>#ifndef</span><span>#else</span><span>#elif</span><span>#endif</span></span></strong></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="replace.html" title="c/preprocessor/replace"><span class="t-lines"><span>#define</span><span>#undef</span><span>#,## operators</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="include.html" title="c/preprocessor/include"><span class="t-lines"><span>#include</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="error.html" title="c/preprocessor/error"><span class="t-lines"><span>#error</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="impl.html" title="c/preprocessor/impl"><span class="t-lines"><span>#pragma</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="line.html" title="c/preprocessor/line"><span class="t-lines"><span>#line</span></span></a></div></div></td></tr>
</table></div><div><span class="editsection noprint plainlinks" title="Edit this template"><a rel="nofollow" class="external text" href="http://en.cppreference.com/mwiki/index.php?title=Template:c/preprocessor/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>The preprocessor supports conditional compilation of parts of a source file. This behavior is controlled by <code>#if</code>, <code>#else</code>, <code>#elif</code>, <code>#ifdef</code>, <code>#ifndef</code> and <code>#endif</code> directives.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="conditional.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="conditional.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="conditional.html#Conditional_evaluation"><span class="tocnumber">3</span> <span class="toctext">Conditional evaluation</span></a>
<ul>
<li class="toclevel-2 tocsection-4"><a href="conditional.html#.23if.2C_.23elif"><span class="tocnumber">3.1</span> <span class="toctext">#if, #elif</span></a></li>
<li class="toclevel-2 tocsection-5"><a href="conditional.html#.23ifdef.2C_.23ifndef"><span class="tocnumber">3.2</span> <span class="toctext">#ifdef, #ifndef</span></a></li>
</ul>
</li>
<li class="toclevel-1 tocsection-6"><a href="conditional.html#Example"><span class="tocnumber">4</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-7"><a href="conditional.html#References"><span class="tocnumber">5</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-8"><a href="conditional.html#See_also"><span class="tocnumber">6</span> <span class="toctext">See also</span></a></li>
</ul>
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/preprocessor/conditional&action=edit&section=1" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span></h3>
<table class="t-sdsc-begin">
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#if</b></code> <span class="t-spar">expression</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#ifdef</b></code> <span class="t-spar">expression</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#ifndef</b></code> <span class="t-spar">expression</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#elif</b></code> <span class="t-spar">expression</span>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#else</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td class="t-sdsc-nopad"> <code><b>#endif</b></code>
</td>
<td class="t-sdsc-nopad">
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/preprocessor/conditional&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The conditional preprocessing block starts with <code>#if</code>, <code>#ifdef</code> or <code>#ifndef</code> directive, then optionally includes any number of <code>#elif</code> directives, then optionally includes at most one <code>#else</code> directive and is terminated with the <code>#endif</code> directive. Any inner conditional preprocessing blocks are processed separately.
</p><p>Each of <code>#if</code>, <code>#elif</code>, <code>#else</code>, <code>#ifdef</code> and <code>#ifndef</code> directives control a code block until the first <code>#elif</code>, <code>#else</code>, <code>#endif</code> directive not belonging to any inner conditional preprocessing blocks.
</p><p><code>#if</code>, <code>#ifdef</code> and <code>#ifndef</code> directives test the specified condition (see below), and if it evaluates to true, compiles the controlled code block. In that case subsequent <code>#else</code> and <code>#elif</code> directives are ignored. Otherwise, if the specified condition evaluates false, the controlled code block is skipped and the subsequent <code>#else</code> or <code>#elif</code> directive (if any) is processed. In the former case, the code block controlled by the <code>#else</code> directive is unconditionally compiled. In the latter case, the <code>#elif</code> directive acts as if it were a <code>#if</code> directive: checks for condition, compiles or skips the controlled code block based on the result, and in the latter case processes subsequent <code>#elif</code> and <code>#else</code> directives. The conditional preprocessing block is terminated by the <code>#endif</code> directive.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/preprocessor/conditional&action=edit&section=3" title="Edit section: Conditional evaluation">edit</a>]</span> <span class="mw-headline" id="Conditional_evaluation">Conditional evaluation</span></h3>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/preprocessor/conditional&action=edit&section=4" title="Edit section: #if, #elif">edit</a>]</span> <span class="mw-headline" id=".23if.2C_.23elif"><code>#if, #elif</code></span></h4>
<p>The <span class="t-spar">expression</span> is a constant expression, using only <a href="../../cpp/language.1.html#Literals" title="cpp/language">literals</a> and identifiers, defined using <a href="replace.html" title="c/preprocessor/replace"><tt>
#define</tt></a> directive. Any identifier, which is not literal, non defined using <a href="replace.html" title="c/preprocessor/replace"><tt>
#define</tt></a> directive, evaluates to <i>0</i>.
</p><p>The expression may contain unary operators in form <code>defined</code> <span class="t-spar">identifier</span> or <code>defined (</code><span class="t-spar">identifier</span><code>)</code> which return <span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">1</span></span></span> if the <span class="t-spar">identifier</span> was defined using <a href="replace.html" title="c/preprocessor/replace"><tt>
#define</tt></a> directive and <i>0</i> otherwise. If the <span class="t-spar">expression</span> evaluates to nonzero value, the controlled code block is included and skipped otherwise. If any used identifier is not a constant, it is replaced with <span class="t-c"><span class="mw-geshi c source-c"><span class="nu0">0</span></span></span>.
</p>
<h4><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/preprocessor/conditional&action=edit&section=5" title="Edit section: #ifdef, #ifndef">edit</a>]</span> <span class="mw-headline" id=".23ifdef.2C_.23ifndef"><code>#ifdef, #ifndef</code></span></h4>
<p>Checks if the identifier was defined using <a href="replace.html" title="c/preprocessor/replace"><tt>
#define</tt></a> directive.
</p><p><code>#ifdef</code> <span class="t-spar">identifier</span> is essentially equivalent to <code>#if defined(</code> <span class="t-spar">identifier</span><code>)</code>.
</p><p><code>#ifndef</code> <span class="t-spar">identifier</span> is essentially equivalent to <code>#if !defined(</code> <span class="t-spar">identifier</span><code>)</code>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/preprocessor/conditional&action=edit&section=6" title="Edit section: Example">edit</a>]</span> <span class="mw-headline" id="Example">Example</span></h3>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="co2">#define ABCD 2</span>
<span class="co2">#include <stdio.h></span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
 
<span class="co2">#ifdef ABCD</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"1: yes<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#else</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"1: no<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#endif</span>
 
<span class="co2">#ifndef ABCD</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"2: no1<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#elif ABCD == 2</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"2: yes<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#else</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"2: no2<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#endif</span>
 
<span class="co2">#if !defined(DCBA) && (ABCD < 2*4-3)</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"3: yes<span class="es1">\n</span>"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="co2">#endif</span>
<span class="br0">}</span></pre></div></div>
<p>Output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">1: yes
2: yes
3: yes</pre></div></div>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/preprocessor/conditional&action=edit&section=7" title="Edit section: References">edit</a>]</span> <span class="mw-headline" id="References">References</span></h3>
<div class="t-ref-std-11">
<ul><li> C11 standard (ISO/IEC 9899:2011):
</li></ul>
<dl><dd><ul><li> 6.10.1 Conditional inclusion (p: 162-164)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999):
</li></ul>
<dl><dd><ul><li> 6.10.1 Conditional inclusion (p: 147-149)
</li></ul>
</dd></dl>
<div class="t-ref-std-c89">
<ul><li> C89/C90 standard (ISO/IEC 9899:1990):
</li></ul>
<dl><dd><ul><li> 3.8.1 Conditional inclusion
</li></ul>
</dd></dl>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/preprocessor/conditional&action=edit&section=8" title="Edit section: See also">edit</a>]</span> <span class="mw-headline" id="See_also">See also</span></h3>
<table class="t-dsc-begin">
<tr class="t-dsc">
<td colspan="2"> <div class="t-dsc-see"><span><a href="../../cpp/preprocessor/conditional.html" title="cpp/preprocessor/conditional">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>Conditional inclusion</span></span></div>
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