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<p>A union is a type consisting of a sequence of members whose storage overlaps (as opposed to struct, which is a type consisting of a sequence of members whose storage is allocated in an ordered sequence). The value of at most one of the members can be stored in a union at any one time.
</p><p>The <a href="declarations.html" title="c/language/declarations">type specifier</a> for a union is identical to the <a href="struct.html" title="c/language/struct"> <code>struct</code></a> type specifier except for the keyword used:
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="union.html#Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="union.html#Explanation"><span class="tocnumber">2</span> <span class="toctext">Explanation</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="union.html#Keywords"><span class="tocnumber">3</span> <span class="toctext">Keywords</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="union.html#Notes"><span class="tocnumber">4</span> <span class="toctext">Notes</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="union.html#Example"><span class="tocnumber">5</span> <span class="toctext">Example</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="union.html#References"><span class="tocnumber">6</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-7"><a href="union.html#See_also"><span class="tocnumber">7</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/language/union&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> <code><b>union</b></code> <span class="t-spar">name</span><span class="t-mark">(optional)</span> <code><b>{ </b></code> <span class="t-spar">struct-declaration-list</span> <code><b>} </b></code>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <code><b>union</b></code> <span class="t-spar">name</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>
<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<table class="t-par-begin">
<tr class="t-par">
<td> <span class="t-spar">name</span>
</td>
<td> -
</td>
<td> the name of the union that's being defined
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">struct-declaration-list</span>
</td>
<td> -
</td>
<td> any number of variable declarations, bit field declarations, and static assert declarations. Members of incomplete type and members of function type are not allowed.
</td></tr></table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/union&action=edit&section=2" title="Edit section: Explanation">edit</a>]</span> <span class="mw-headline" id="Explanation">Explanation</span></h3>
<p>The union is only as big as necessary to hold its largest member (additional unnamed trailing padding may also be added). The other members are allocated in the same bytes as part of that largest member.
</p><p>A pointer to a union can be cast to a pointer to each of its members (if a union has bit field members, the pointer to a union can be cast to the pointer to the bit field's underlying type). Likewise, a pointer to any member of a union can be cast to a pointer to the enclosing union.
</p><p>If the member used to access the contents of a union is not the same as the member last used to store a value, the object representation of the value that was stored is reinterpreted as an object representation of the new type (this is known as <i>type punning</i>). If the size of the new type is larger than the size of the last-written type, the contents of the excess bytes are unspecified (and may be a trap representation)
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-c11"><td>
<p>Similar to struct, an unnamed member of a union whose type is a union without <span class="t-spar">name</span> is known as <i>anonymous union</i>. Every member of an anonymous union is considered to be a member of the enclosing struct or union. This applies recursively if the enclosing struct or union is also anonymous.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw1">struct</span> v <span class="br0">{</span>
<span class="kw1">union</span> <span class="br0">{</span> <span class="co1">// anonymous union</span>
<span class="kw1">struct</span> <span class="br0">{</span> <span class="kw4">int</span> i, j<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span> <span class="co1">// anonymous structure</span>
<span class="kw1">struct</span> <span class="br0">{</span> <span class="kw4">long</span> k, l<span class="sy4">;</span> <span class="br0">}</span> w<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="kw4">int</span> m<span class="sy4">;</span>
<span class="br0">}</span> v1<span class="sy4">;</span>
 
v1.<span class="me1">i</span> <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="co1">// valid</span>
v1.<span class="me1">k</span> <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span> <span class="co1">// invalid: inner structure is not anonymous</span>
v1.<span class="me1">w</span>.<span class="me1">k</span> <span class="sy1">=</span> <span class="nu0">5</span><span class="sy4">;</span> <span class="co1">// valid</span></pre></div></div>
<p>Similar to struct, the behavior of the program is undefined if union is defined without any named members (including those obtained via anonymous nested structs or unions).
</p>
</td>
<td><span class="t-mark-rev t-since-c11">(since C11)</span></td></tr>
</table>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/union&action=edit&section=3" title="Edit section: Keywords">edit</a>]</span> <span class="mw-headline" id="Keywords">Keywords</span></h3>
<p><a href="../keyword/union.html" title="c/keyword/union"><tt>union</tt></a>
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/union&action=edit&section=4" title="Edit section: Notes">edit</a>]</span> <span class="mw-headline" id="Notes">Notes</span></h3>
<p>See <a href="struct_initialization.html" title="c/language/struct initialization">struct initialization</a> for the rules about initialization of structs and unions.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/union&action=edit&section=5" 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">#include <stdio.h></span>
<span class="co2">#include <stdint.h></span>
<span class="co2">#include <assert.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="kw1">union</span> S <span class="br0">{</span>
<a href="../types/integer.html"><span class="kw122">uint32_t</span></a> u32<span class="sy4">;</span>
<a href="../types/integer.html"><span class="kw121">uint16_t</span></a> u16<span class="br0">[</span><span class="nu0">2</span><span class="br0">]</span><span class="sy4">;</span>
<a href="../types/integer.html"><span class="kw120">uint8_t</span></a> u8<span class="sy4">;</span>
<span class="br0">}</span> s <span class="sy1">=</span> <span class="br0">{</span><span class="nu12">0x12345678</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// s.u32 is now the active member</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"Union S has size %zu and holds %x<span class="es1">\n</span>"</span>, <span class="kw4">sizeof</span> s, s.<span class="me1">u32</span><span class="br0">)</span><span class="sy4">;</span>
s.<span class="me1">u16</span><span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span> <span class="sy1">=</span> <span class="nu12">0x0011</span><span class="sy4">;</span> <span class="co1">// s.u16 is now the active member</span>
<span class="co1">// reading from s.u32 or from s.u8 reinterprets the object representation</span>
<span class="co1">// printf("s.u8 is now %x\n", s.u8); // unspecified, typically 11 or 00</span>
<span class="co1">// printf("s.u32 is now %x\n", s.u32); // unspecified, typically 12340011 or 00115678</span>
 
<span class="co1">// pointers to all members of a union compare equal to themselves and the union</span>
<a href="../error/assert.html"><span class="kw407">assert</span></a><span class="br0">(</span><span class="br0">(</span><a href="../types/integer.html"><span class="kw120">uint8_t</span></a><span class="sy2">*</span><span class="br0">)</span><span class="sy3">&</span>s <span class="sy1">==</span> <span class="sy3">&</span>s.<span class="me1">u8</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="co1">// this union has 3 bytes of trailing padding</span>
<span class="kw1">union</span> pad <span class="br0">{</span>
<span class="kw4">char</span> c<span class="br0">[</span><span class="nu0">5</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// occupies 5 bytes</span>
<span class="kw4">float</span> f<span class="sy4">;</span> <span class="co1">// occupies 4 bytes, imposes alignment 4</span>
<span class="br0">}</span> p <span class="sy1">=</span> <span class="br0">{</span>.<span class="me1">f</span> <span class="sy1">=</span> <span class="nu16">1.23</span><span class="br0">}</span><span class="sy4">;</span> <span class="co1">// the size is 8 to satisfy float's alignment</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"size of union of char[5] and float is %zu<span class="es1">\n</span>"</span>, <span class="kw4">sizeof</span> p<span class="br0">)</span><span class="sy4">;</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">Union S has size 4 and holds 12345678
size of union of char[5] and float is 8</pre></div></div>
</div>
<p>Each member is allocated as if it were the only member of the union, which is why <code>s.c</code> in the example above aliases the first byte of <code>s.s[0]</code>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/union&action=edit&section=6" 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> 6.7.2.1 Structure and union specifiers (p: 112-117)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999):
</li></ul>
<dl><dd><ul><li> 6.7.2.1 Structure and union specifiers (p: 101-104)
</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> 3.5.2.1 Structure and union specifiers
</li></ul>
</dd></dl>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/union&action=edit&section=7" 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/language/union.html" title="cpp/language/union">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>Union declaration</span></span></div>
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