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<p>C programs create, destroy, access, and manipulate objects.
</p><p>An object, in C, is region of <a href="memory_model.html" title="c/language/memory model">data storage</a> in the execution environment, the contents of which can represent <i>values</i> (a value is the meaning of the contents of an object, when interpreted as having a specific <a href="compatible_type.html" title="c/language/type">type</a>).
</p><p>Every object has
</p>
<ul><li> size (can be determined with <a href="sizeof.html" title="c/language/sizeof">sizeof</a>)
</li><li> alignment requirement<span class="t-rev-inl t-since-c11"><span> (can be determined by <a href="alignof.html" title="c/language/ Alignof">alignof</a>)</span> <span><span class="t-mark-rev t-since-c11">(since C11)</span></span></span>
</li><li> <a href="storage_duration.html" title="c/language/storage duration">storage duration</a> (automatic, static, allocated, thread-local)
</li><li> <a href="lifetime.html" title="c/language/lifetime">lifetime</a> (equal to storage duration or temporary)
</li><li> effective type (see below)
</li><li> value (which may be indeterminate)
</li><li> optionally, an <a href="identifier.html" title="c/language/identifier">identifier</a> that denotes this object
</li></ul>
<p>Objects are created by <a href="declarations.html" title="c/language/declarations">declarations</a>, <a href="../memory.html" title="c/memory">allocation functions</a>, <a href="string_literal.html" title="c/language/string literal">string literals</a>, <a href="compound_literal.html" title="c/language/compound literal">compound literals</a>, and by non-lvalue expressions that return <a href="lifetime.html" title="c/language/lifetime">structures or unions with array members</a>.
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<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="object.html#Object_representation"><span class="tocnumber">1</span> <span class="toctext">Object representation</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="object.html#Effective_type"><span class="tocnumber">2</span> <span class="toctext">Effective type</span></a></li>
<li class="toclevel-1 tocsection-3"><a href="object.html#Strict_aliasing"><span class="tocnumber">3</span> <span class="toctext">Strict aliasing</span></a></li>
<li class="toclevel-1 tocsection-4"><a href="object.html#Alignment"><span class="tocnumber">4</span> <span class="toctext">Alignment</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="object.html#References"><span class="tocnumber">5</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="object.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/language/object&action=edit&section=1" title="Edit section: Object representation">edit</a>]</span> <span class="mw-headline" id="Object_representation">Object representation</span></h3>
<p>Except for <a href="bit_field.html" title="c/language/bit field">bit fields</a>, objects are composed of contiguous sequences of one or more bytes, each consisting of <span class="t-lc"><a href="../types/limits.html" title="c/types/limits">CHAR_BIT</a></span> bits, and can be copied with <span class="t-lc"><a href="../string/byte/memcpy.html" title="c/string/byte/memcpy">memcpy</a></span> into an object of type <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">unsigned</span> <span class="kw4">char</span><span class="br0">[</span>n<span class="br0">]</span></span></span>, where <code>n</code> is the size of the object. The contents of the resulting array are known as <i>object representation</i>.
</p><p>If two objects have the same object representation, they compare equal (except if they are floating-point NaNs). The opposite is not true: two objects that compare equal may have different object representations because not every bit of the object representation needs to participate in the value. Such bits may be used for padding to satisfy alignment requirement, for parity checks, to indicate trap representations, etc.
</p><p>If an object representation does not represent any value of the object type, it is known as <i>trap representation</i>. Accessing a trap representation in any way other than reading it through an lvalue expression of character type is undefined behavior. The value of a structure or union is never a trap representation even if any particular member is one.
</p><p>For the objects of type char, signed char, and unsigned char, every bit of the object representation is required to participate in the value representation and each possible bit pattern represents a distinct value (no padding, trap bits, or multiple representations allowed).
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/object&action=edit&section=2" title="Edit section: Effective type">edit</a>]</span> <span class="mw-headline" id="Effective_type">Effective type</span></h3>
<p>Every object has an <i>effective type</i>, which determines which <a href="value_category.html" title="c/language/value category">lvalue</a> accesses are valid and which violate the strict aliasing rules.
</p><p>If the object was created by a <a href="declarations.html" title="c/language/declarations">declaration</a>, the declared type of that object is the object's <i>effective type</i>.
</p><p>If the object was created by an <a href="../memory.html" title="c/memory">allocation function</a> (including <span class="t-lc"><a href="../memory/realloc.html" title="c/memory/realloc">realloc</a></span>), it has no declared type. Such object acquires an effective type as follows:
</p>
<ul><li> The first write to that object through an lvalue that has a type other than character type, at which time the type of that lvalue becomes this object's <i>effective type</i> for that write and all subsequent reads.
</li><li> <span class="t-lc"><a href="../string/byte/memcpy.html" title="c/string/byte/memcpy">memcpy</a></span> or <span class="t-lc"><a href="../string/byte/memmove.html" title="c/string/byte/memmove">memmove</a></span> copy another object into that object, at which time the effective type of the source object (if it had one) becomes the effective type of this object for that write and all subsequent reads.
</li><li> Any other access to the object with no declared type, the effective type is the type of the lvalue used for the access.
</li></ul>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/object&action=edit&section=3" title="Edit section: Strict aliasing">edit</a>]</span> <span class="mw-headline" id="Strict_aliasing">Strict aliasing</span></h3>
<p>Given an object with <i>effective type</i> T1, using an lvalue expression (typically, dereferencing a pointer) of a different type T2 is undefined behavior, unless:
</p>
<ul><li> T2 and T1 are <a href="compatible_type.html#Compatible_types" title="c/language/type">compatible types</a>.
</li><li> T2 is cvr-qualified version of a type that is <a href="compatible_type.html#Compatible_types" title="c/language/type">compatible</a> with T1.
</li><li> T2 is a signed or unsigned version of a type that is <a href="compatible_type.html#Compatible_types" title="c/language/type">compatible</a> with T1.
</li><li> T2 is an aggregate type or union type type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union).
</li><li> T2 is a character type (char, signed char, or unsigned char).
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw4">int</span> i <span class="sy1">=</span> <span class="nu0">7</span><span class="sy4">;</span>
<span class="kw4">char</span><span class="sy2">*</span> pc <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">char</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="sy3">&</span>i<span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">if</span><span class="br0">(</span>pc<span class="br0">[</span><span class="nu0">0</span><span class="br0">]</span> <span class="sy1">==</span> <span class="st0">'\x7'</span><span class="br0">)</span> <span class="co1">// aliasing through char is ok</span>
<a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"This system is little-endian"</span><span class="br0">)</span><span class="sy4">;</span>
<span class="kw1">else</span>
<a href="../io/puts.html"><span class="kw836">puts</span></a><span class="br0">(</span><span class="st0">"This system is big-endian"</span><span class="br0">)</span><span class="sy4">;</span>
 
<span class="kw4">float</span><span class="sy2">*</span> pf <span class="sy1">=</span> <span class="br0">(</span><span class="kw4">float</span><span class="sy2">*</span><span class="br0">)</span><span class="br0">(</span><span class="sy3">&</span>i<span class="br0">)</span><span class="sy4">;</span>
<span class="kw4">float</span> d <span class="sy1">=</span> <span class="sy2">*</span>pf<span class="sy4">;</span> <span class="co1">// UB: float lvalue *p cannot be used to access int</span></pre></div></div>
<p>Note that type-punning may also be performed through the inactive member of a <a href="union.html" title="c/language/union">union</a>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/object&action=edit&section=4" title="Edit section: Alignment">edit</a>]</span> <span class="mw-headline" id="Alignment">Alignment</span></h3>
<p>Every complete <a href="types.html#Type_groups" title="c/language/types" class="mw-redirect">object type</a> has a property called <i>alignment requirement</i>, which is an integer value of type <span class="t-lc"><a href="../types/size_t.html" title="c/types/size t">size_t</a></span> representing the number of bytes between successive addresses at which objects of this type can be allocated. The valid alignment values are non-negative integral powers of two.
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-c11"><td>
<p>The alignment requirement of a type can be queried with <a href="alignof.html" title="c/language/ Alignof">alignof</a>.
</p>
</td>
<td><span class="t-mark-rev t-since-c11">(since C11)</span></td></tr></table>
<p>In order to satisfy alignment requirements of all members of a struct, padding may be inserted after some of its members.
</p>
<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 <stdalign.h></span>
 
<span class="co1">// objects of struct S can be allocated at any address</span>
<span class="co1">// because both S.a and S.b can be allocated at any address</span>
<span class="kw1">struct</span> S <span class="br0">{</span>
<span class="kw4">char</span> a<span class="sy4">;</span> <span class="co1">// size: 1, alignment: 1</span>
<span class="kw4">char</span> b<span class="sy4">;</span> <span class="co1">// size: 1, alignment: 1</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// size: 2, alignment: 1</span>
 
<span class="co1">// objects of struct X must be allocated at 4-byte boundaries</span>
<span class="co1">// because X.n must be allocated at 4-byte boundaries</span>
<span class="co1">// because int's alignment requirement is (usually) 4</span>
<span class="kw1">struct</span> X <span class="br0">{</span>
<span class="kw4">int</span> n<span class="sy4">;</span> <span class="co1">// size: 4, alignment: 4</span>
<span class="kw4">char</span> c<span class="sy4">;</span> <span class="co1">// size: 1, alignment: 1</span>
<span class="co1">// three bytes padding</span>
<span class="br0">}</span><span class="sy4">;</span> <span class="co1">// size: 8, alignment: 4</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="kw4">void</span><span class="br0">)</span>
<span class="br0">{</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"sizeof(struct S) = %zu<span class="es1">\n</span>"</span>, <span class="kw4">sizeof</span><span class="br0">(</span><span class="kw1">struct</span> S<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"alignof(struct S) = %zu<span class="es1">\n</span>"</span>, alignof<span class="br0">(</span><span class="kw1">struct</span> S<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"sizeof(struct X) = %zu<span class="es1">\n</span>"</span>, <span class="kw4">sizeof</span><span class="br0">(</span><span class="kw1">struct</span> X<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<a href="../io/fprintf.html"><span class="kw851">printf</span></a><span class="br0">(</span><span class="st0">"alignof(struct X) = %zu<span class="es1">\n</span>"</span>, alignof<span class="br0">(</span><span class="kw1">struct</span> X<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p>Possible output:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="text source-text"><pre class="de1">sizeof(struct S) = 2
alignof(struct S) = 1
sizeof(struct X) = 8
alignof(struct X) = 4</pre></div></div>
</div>
<p>Each object type imposes its alignment requirement on every object of that type. The strictest (largest) fundamental alignment of any type is the alignment of <span class="t-lc"><a href="../types/max_align_t.html" title="c/types/max align t">max_align_t</a></span>. The weakest (smallest) alignment is the alignment of the types <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">char</span></span></span>, <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">signed</span> <span class="kw4">char</span></span></span>, and <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span>, and equals 1.
</p>
<table class="t-rev-begin">
<tr class="t-rev t-since-c11"><td>If an object's alignment is made stricter (larger) than <span class="t-lc"><a href="../types/max_align_t.html" title="c/types/max align t">max_align_t</a></span> using <a href="alignas.html" title="c/language/ Alignas">alignas</a>, it has <i>extended alignment requirement</i>. A struct or union type whose member has extended alignment is an <i>over-aligned type</i>. It is implementation-defined if over-aligned types are supported, and their support may be different in each kind of <a href="storage_duration.html" title="c/language/storage duration">storage duration</a>.
</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/object&action=edit&section=5" 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> 3.15 object (p: 6)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.2.6 Representations of types (p: 44-46)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.5/6-7 Expressions (p: 77)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.2.8 Alignment of objects (p: 48-49)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999):
</li></ul>
<dl><dd><ul><li> 3.2 alignment (p: 3)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 3.14 object (p: 5)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.2.6 Representations of types (p: 37-39)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 6.5/6-7 Expressions (p: 67-68)
</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> 1.6 Definitions of terms
</li></ul>
</dd></dl>
</div>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/object&action=edit&section=6" 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/objects.html" title="cpp/language/object">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>Object</span></span></div>
</td></tr>
</table></div>
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