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<p>Defines the semantics of computer memory storage for the purpose of the C abstract machine.
</p><p>The data storage (memory) available to a C program is one or more contiguous sequences of <i>bytes</i>. Each byte in memory has a unique <i>address</i>.
</p>
<table id="toc" class="toc"><tr><td><div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1"><a href="memory_model.html#Byte"><span class="tocnumber">1</span> <span class="toctext">Byte</span></a></li>
<li class="toclevel-1 tocsection-2"><a href="memory_model.html#Memory_location"><span class="tocnumber">2</span> <span class="toctext">Memory location</span></a></li>
<li class="toclevel-1"><a href="memory_model.html#Threads_and_data_races"><span class="tocnumber">3</span> <span class="toctext">Threads and data races</span></a></li>
<li class="toclevel-1"><a href="memory_model.html#Memory_order"><span class="tocnumber">4</span> <span class="toctext">Memory order</span></a></li>
<li class="toclevel-1 tocsection-5"><a href="memory_model.html#References"><span class="tocnumber">5</span> <span class="toctext">References</span></a></li>
<li class="toclevel-1 tocsection-6"><a href="memory_model.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/memory_model&action=edit&section=1" title="Edit section: Byte">edit</a>]</span> <span class="mw-headline" id="Byte">Byte</span></h3>
<p>A <i>byte</i> is the smallest addressable unit of memory. It is defined as a contiguous sequence of bits, large enough to hold any member of the <i>basic execution character set</i> (<a href="translation_phases.html" title="c/language/translation phases">the 96 characters</a> that are required to be single-byte). C supports bytes of sizes 8 bits and greater.
</p><p>The <a href="types.html" title="c/language/types" class="mw-redirect">types</a> <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">unsigned</span> <span class="kw4">char</span></span></span>, and <span class="t-c"><span class="mw-geshi c source-c"><span class="kw4">signed</span> <span class="kw4">char</span></span></span> use one byte for both storage and <a href="object.html" title="c/language/object">value representation</a>. The number of bits in a byte is accessible as <span class="t-lc"><a href="../types/limits.html" title="c/types/limits">CHAR_BIT</a></span>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=c/language/memory_model&action=edit&section=2" title="Edit section: Memory location">edit</a>]</span> <span class="mw-headline" id="Memory_location">Memory location</span></h3>
<p>A <i>memory location</i> is
</p>
<ul><li> an object of <a href="compatible_type.html#Type_groups" title="c/language/type">scalar type</a> (arithmetic type, pointer type, enumeration type)
</li><li> or the largest contiguous sequence of <a href="bit_field.html" title="c/language/bit field">bit fields</a> of non-zero length
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="c source-c"><pre class="de1"><span class="kw1">struct</span> S <span class="br0">{</span>
<span class="kw4">char</span> a<span class="sy4">;</span> <span class="co1">// memory location #1</span>
<span class="kw4">int</span> b <span class="sy4">:</span> <span class="nu0">5</span><span class="sy4">;</span> <span class="co1">// memory location #2</span>
<span class="kw4">int</span> c <span class="sy4">:</span> <span class="nu0">11</span>, <span class="co1">// memory location #2 (continued)</span>
<span class="sy4">:</span> <span class="nu0">0</span>,
d <span class="sy4">:</span> <span class="nu0">8</span><span class="sy4">;</span> <span class="co1">// memory location #3</span>
<span class="kw1">struct</span> <span class="br0">{</span>
<span class="kw4">int</span> ee <span class="sy4">:</span> <span class="nu0">8</span><span class="sy4">;</span> <span class="co1">// memory location #4</span>
<span class="br0">}</span> e<span class="sy4">;</span>
<span class="br0">}</span> obj<span class="sy4">;</span> <span class="co1">// The object 'obj' consists of 4 separate memory locations</span></pre></div></div>
<table class="t-rev-begin">
<tr class="t-rev t-since-c11"><td>
<h3> <span class="mw-headline" id="Threads_and_data_races">Threads and data races</span></h3>
<p>A thread of execution is a flow of control within a program that begins with the invocation of a top-level function by <span class="t-lc"><a href="../thread/thrd_create.html" title="c/thread/thrd create">thrd_create</a></span> or other means.
</p><p>Any thread can potentially access any object in the program (objects with automatic and thread-local <a href="storage_duration.html" title="c/language/storage duration">storage duration</a> may still be accessed by another thread through a pointer).
</p><p>Different threads of execution are always allowed to access (read and modify) different <i>memory locations</i> concurrently, with no interference and no synchronization requirements. (note that it is not safe to concurrently update two non-atomic bit-fields in the same structure if all members declared between them are also (non-zero-length) bit-fields, no matter what the sizes of those intervening bit-fields happen to be)
</p><p>When an <a href="eval_order.html" title="c/language/eval order">evaluation</a> of an expression writes to a memory location and another evaluation reads or modifies the same memory location, the expressions are said to <i>conflict</i>. A program that has two conflicting evaluations has a <i>data race</i> unless either
</p>
<ul><li> both conflicting evaluations are <a href="atomic.html" title="c/language/atomic">atomic operations</a>
</li><li> one of the conflicting evaluations <i>happens-before</i> another (see <span class="t-lc"><a href="../atomic/memory_order.html" title="c/atomic/memory order">memory_order</a></span>)
</li></ul>
<p>If a data race occurs, the behavior of the program is undefined.
</p>
(in particular, <span class="t-lc"><a href="../thread/mtx_unlock.html" title="c/thread/mtx unlock">mtx_unlock</a></span> is <i>synchronized-with</i>, and therefore, <i>happens-before</i> <span class="t-lc"><a href="../thread/mtx_lock.html" title="c/thread/mtx lock">mtx_lock</a></span> of the same mutex by another thread, which makes it possible to use mutex locks to guard against data races) <table class="metadata plainlinks ambox mbox-small-left ambox-notice" style=""><tr><td class="mbox-empty-cell"></td><td class="mbox-text" style="">This section is incomplete<br />Reason: small example or two </td></tr></table>
<h3> <span class="mw-headline" id="Memory_order">Memory order</span></h3>
<p>When a thread reads a value from a memory location, it may see the initial value, the value written in the same thread, or the value written in another thread. See <span class="t-lc"><a href="../atomic/memory_order.html" title="c/atomic/memory order">memory_order</a></span> for details on the order in which writes made from threads become visible to other threads.
</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/memory_model&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> 3.6 byte (p: 4)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 3.14 memory location (p: 5)
</li></ul>
</dd></dl>
<dl><dd><ul><li> 5.1.2.4 Multi-threaded executions and data races (p: 17-21)
</li></ul>
</dd></dl>
<div class="t-ref-std-c99">
<ul><li> C99 standard (ISO/IEC 9899:1999):
</li></ul>
<dl><dd><ul><li> 3.6 byte (p: 4)
</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/memory_model&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/memory_model.html" title="cpp/language/memory model">C++ documentation</a></span> for <span class="t-dsc-see-tt"><span>Memory model</span></span></div>
</td></tr>
</table></div>
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