1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
|
<!DOCTYPE html>
<html lang="en" dir="ltr" class="client-nojs">
<head>
<title>Memory model - cppreference.com</title>
<meta charset="UTF-8" />
<meta name="generator" content="MediaWiki 1.21.2" />
<link rel="alternate" type="application/x-wiki" title="Edit" href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&action=edit" />
<link rel="edit" title="Edit" href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&action=edit" />
<link rel="shortcut icon" href="../../../favicon.ico" />
<link rel="search" type="application/opensearchdescription+xml" href="../../../mwiki/opensearch_desc.php" title="cppreference.com (en)" />
<link rel="EditURI" type="application/rsd+xml" href="http://en.cppreference.com/mwiki/api.php?action=rsd" />
<link rel="alternate" type="application/atom+xml" title="cppreference.com Atom feed" href="http://en.cppreference.com/mwiki/index.php?title=Special:RecentChanges&feed=atom" />
<link rel="stylesheet" href="../../../mwiki/load.php%3Fdebug=false&lang=en&modules=ext.gadget.ColiruCompiler%257Cext.rtlcite%257Cmediawiki.legacy.commonPrint%252Cshared%257Cskins.cppreference2&only=styles&skin=cppreference2&*.css" />
<meta name="ResourceLoaderDynamicStyles" content="" />
<link rel="stylesheet" href="../../../mwiki/load.php%3Fdebug=false&lang=en&modules=site&only=styles&skin=cppreference2&*.css" />
<style>a:lang(ar),a:lang(ckb),a:lang(fa),a:lang(kk-arab),a:lang(mzn),a:lang(ps),a:lang(ur){text-decoration:none}#toc{display:none}.editsection{display:none}
/* cache key: mwiki1-mwiki_en_:resourceloader:filter:minify-css:7:472787eddcf4605d11de8c7ef047234f */</style>
<script src="../../../mwiki/load.php%3Fdebug=false&lang=en&modules=startup&only=scripts&skin=cppreference2&*"></script>
<script>if(window.mw){
mw.config.set({"wgCanonicalNamespace":"","wgCanonicalSpecialPageName":false,"wgNamespaceNumber":0,"wgPageName":"cpp/language/memory_model","wgTitle":"cpp/language/memory model","wgCurRevisionId":78340,"wgArticleId":13490,"wgIsArticle":true,"wgAction":"view","wgUserName":null,"wgUserGroups":["*"],"wgCategories":["Todo with reason"],"wgBreakFrames":false,"wgPageContentLanguage":"en","wgSeparatorTransformTable":["",""],"wgDigitTransformTable":["",""],"wgDefaultDateFormat":"dmy","wgMonthNames":["","January","February","March","April","May","June","July","August","September","October","November","December"],"wgMonthNamesShort":["","Jan","Feb","Mar","Apr","May","Jun","Jul","Aug","Sep","Oct","Nov","Dec"],"wgRelevantPageName":"cpp/language/memory_model","wgRestrictionEdit":[],"wgRestrictionMove":[]});
}</script><script>if(window.mw){
mw.loader.implement("user.options",function(){mw.user.options.set({"ccmeonemails":0,"cols":80,"date":"default","diffonly":0,"disablemail":0,"disablesuggest":0,"editfont":"default","editondblclick":0,"editsection":0,"editsectiononrightclick":0,"enotifminoredits":0,"enotifrevealaddr":0,"enotifusertalkpages":1,"enotifwatchlistpages":0,"extendwatchlist":0,"externaldiff":0,"externaleditor":0,"fancysig":0,"forceeditsummary":0,"gender":"unknown","hideminor":0,"hidepatrolled":0,"imagesize":2,"justify":0,"math":1,"minordefault":0,"newpageshidepatrolled":0,"nocache":0,"noconvertlink":0,"norollbackdiff":0,"numberheadings":0,"previewonfirst":0,"previewontop":1,"quickbar":5,"rcdays":7,"rclimit":50,"rememberpassword":0,"rows":25,"searchlimit":20,"showhiddencats":0,"showjumplinks":1,"shownumberswatching":1,"showtoc":0,"showtoolbar":1,"skin":"cppreference2","stubthreshold":0,"thumbsize":2,"underline":2,"uselivepreview":0,"usenewrc":0,"watchcreations":0,"watchdefault":0,"watchdeletion":0,
"watchlistdays":3,"watchlisthideanons":0,"watchlisthidebots":0,"watchlisthideliu":0,"watchlisthideminor":0,"watchlisthideown":0,"watchlisthidepatrolled":0,"watchmoves":0,"wllimit":250,"variant":"en","language":"en","searchNs0":true,"searchNs1":false,"searchNs2":false,"searchNs3":false,"searchNs4":false,"searchNs5":false,"searchNs6":false,"searchNs7":false,"searchNs8":false,"searchNs9":false,"searchNs10":false,"searchNs11":false,"searchNs12":false,"searchNs13":false,"searchNs14":false,"searchNs15":false,"gadget-ColiruCompiler":1});;},{},{});mw.loader.implement("user.tokens",function(){mw.user.tokens.set({"editToken":"+\\","patrolToken":false,"watchToken":false});;},{},{});
/* cache key: mwiki1-mwiki_en_:resourceloader:filter:minify-js:7:ca03345b1e2c4d90a25d968753a73b92 */
}</script>
<script>if(window.mw){
mw.loader.load(["mediawiki.page.startup","mediawiki.legacy.wikibits","mediawiki.legacy.ajax"]);
}</script>
<style type="text/css">/*<![CDATA[*/
.source-cpp {line-height: normal;}
.source-cpp li, .source-cpp pre {
line-height: normal; border: 0px none white;
}
/**
* GeSHi Dynamically Generated Stylesheet
* --------------------------------------
* Dynamically generated stylesheet for cpp
* CSS class: source-cpp, CSS id:
* GeSHi (C) 2004 - 2007 Nigel McNie, 2007 - 2008 Benny Baumann
* (http://qbnz.com/highlighter/ and http://geshi.org/)
* --------------------------------------
*/
.cpp.source-cpp .de1, .cpp.source-cpp .de2 {font: normal normal 1em/1.2em monospace; margin:0; padding:0; background:none; vertical-align:top;}
.cpp.source-cpp {font-family:monospace;}
.cpp.source-cpp .imp {font-weight: bold; color: red;}
.cpp.source-cpp li, .cpp.source-cpp .li1 {font-weight: normal; vertical-align:top;}
.cpp.source-cpp .ln {width:1px;text-align:right;margin:0;padding:0 2px;vertical-align:top;}
.cpp.source-cpp .li2 {font-weight: bold; vertical-align:top;}
.cpp.source-cpp .kw1 {color: #0000dd;}
.cpp.source-cpp .kw2 {color: #0000ff;}
.cpp.source-cpp .kw3 {color: #0000dd;}
.cpp.source-cpp .kw4 {color: #0000ff;}
.cpp.source-cpp .co1 {color: #909090;}
.cpp.source-cpp .co2 {color: #339900;}
.cpp.source-cpp .coMULTI {color: #ff0000; font-style: italic;}
.cpp.source-cpp .es0 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es1 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es2 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es3 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es4 {color: #008000; font-weight: bold;}
.cpp.source-cpp .es5 {color: #008000; font-weight: bold;}
.cpp.source-cpp .br0 {color: #008000;}
.cpp.source-cpp .sy0 {color: #008000;}
.cpp.source-cpp .sy1 {color: #000080;}
.cpp.source-cpp .sy2 {color: #000040;}
.cpp.source-cpp .sy3 {color: #000040;}
.cpp.source-cpp .sy4 {color: #008080;}
.cpp.source-cpp .st0 {color: #008000;}
.cpp.source-cpp .nu0 {color: #000080;}
.cpp.source-cpp .nu6 {color: #000080;}
.cpp.source-cpp .nu8 {color: #000080;}
.cpp.source-cpp .nu12 {color: #000080;}
.cpp.source-cpp .nu16 {color:#000080;}
.cpp.source-cpp .nu17 {color:#000080;}
.cpp.source-cpp .nu18 {color:#000080;}
.cpp.source-cpp .nu19 {color:#000080;}
.cpp.source-cpp .ln-xtra, .cpp.source-cpp li.ln-xtra, .cpp.source-cpp div.ln-xtra {background-color: #ffc;}
.cpp.source-cpp span.xtra { display:block; }
/*]]>*/
</style><!--[if lt IE 7]><style type="text/css">body{behavior:url("/mwiki/skins/cppreference2/csshover.min.htc")}</style><![endif]--></head>
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_language_memory_model skin-cppreference2 action-view cpp-navbar">
<!-- header -->
<div id="mw-head" class="noprint">
<div id="cpp-head-first-base">
<div id="cpp-head-first">
<h5><a href="../../../index.html">
cppreference.com </a></h5>
<div id="cpp-head-search">
<!-- 0 -->
<div id="p-search">
<h5><label for="searchInput">Search</label></h5>
<form action="http://en.cppreference.com/mwiki/index.php" id="searchform">
<input type='hidden' name="title" value="Special:Search"/>
<div id="simpleSearch">
<input name="search" title="Search cppreference.com [f]" accesskey="f" id="searchInput" /> <button type="submit" name="button" title="Search the pages for this text" id="searchButton"><img src="../../../mwiki/skins/cppreference2/images/search-ltr.png%3F303" alt="Search" /></button> </div>
</form>
</div>
<!-- /0 -->
</div>
<div id="cpp-head-personal">
<!-- 0 -->
<div id="p-personal" class="">
<span id="pt-createaccount"><a href="http://en.cppreference.com/mwiki/index.php?title=Special:UserLogin&returnto=cpp%2Flanguage%2Fmemory+model&type=signup">Create account</a></span> <div class="menu">
<ul>
<li id="pt-login"><a href="http://en.cppreference.com/mwiki/index.php?title=Special:UserLogin&returnto=cpp%2Flanguage%2Fmemory+model" title="You are encouraged to log in; however, it is not mandatory [o]" accesskey="o">Log in</a></li> </ul>
</div>
</div>
<!-- /0 -->
</div>
</div>
</div>
<div id="cpp-head-second-base">
<div id="cpp-head-second">
<div id="cpp-head-tools-left">
<!-- 0 -->
<div id="p-namespaces" class="vectorTabs">
<h5>Namespaces</h5>
<ul>
<li id="ca-nstab-main" class="selected"><span><a href="memory_model.html" title="View the content page [c]" accesskey="c">Page</a></span></li>
<li id="ca-talk"><span><a href="http://en.cppreference.com/w/Talk:cpp/language/memory_model" title="Discussion about the content page [t]" accesskey="t">Discussion</a></span></li>
</ul>
</div>
<!-- /0 -->
<!-- 1 -->
<div id="p-variants" class="vectorMenu emptyPortlet">
<h5><span>Variants</span><a href="memory_model.html#"></a></h5>
<div class="menu">
<ul>
</ul>
</div>
</div>
<!-- /1 -->
</div>
<div id="cpp-head-tools-right">
<!-- 0 -->
<div id="p-views" class="vectorTabs">
<h5>Views</h5>
<ul>
<li id="ca-view" class="selected"><span><a href="memory_model.html" >View</a></span></li>
<li id="ca-edit"><span><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&action=edit" title="You can edit this page. Please use the preview button before saving [e]" accesskey="e">Edit</a></span></li>
<li id="ca-history" class="collapsible"><span><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&action=history" title="Past revisions of this page [h]" accesskey="h">History</a></span></li>
</ul>
</div>
<!-- /0 -->
<!-- 1 -->
<div id="p-cactions" class="vectorMenu emptyPortlet">
<h5><span>Actions</span><a href="memory_model.html#"></a></h5>
<div class="menu">
<ul>
</ul>
</div>
</div>
<!-- /1 -->
</div>
</div>
</div>
</div>
<!-- /header -->
<!-- content -->
<div id="cpp-content-base">
<div id="content">
<a id="top"></a>
<div id="mw-js-message" style="display:none;"></div>
<!-- firstHeading -->
<h1 id="firstHeading" class="firstHeading">Memory model</h1>
<!-- /firstHeading -->
<!-- bodyContent -->
<div id="bodyContent">
<!-- tagline -->
<div id="siteSub">From cppreference.com</div>
<!-- /tagline -->
<!-- subtitle -->
<div id="contentSub"><span class="subpages">< <a href="../../cpp.html" title="cpp">cpp</a>‎ | <a href="../language.1.html" title="cpp/language">language</a></span></div>
<!-- /subtitle -->
<!-- bodycontent -->
<div id="mw-content-text" lang="en" dir="ltr" class="mw-content-ltr"><div class="t-navbar" style=""><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../../cpp.html" title="cpp"> C++</a><div class="t-navbar-menu"><div><div><table class="t-nv-begin" cellpadding="0" style="line-height:1.1em;">
<tr class="t-nv"><td colspan="5"> <a href="../language.1.html" title="cpp/language"> Language</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../header.html" title="cpp/header"> Standard library headers</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../concept.html" title="cpp/concept"> Concepts</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../utility.html" title="cpp/utility"> Utilities library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../string.html" title="cpp/string"> Strings library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../container.html" title="cpp/container"> Containers library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../algorithm.html" title="cpp/algorithm"> Algorithms library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../iterator.html" title="cpp/iterator"> Iterators library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../numeric.html" title="cpp/numeric"> Numerics library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../io.html" title="cpp/io"> Input/output library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../locale.html" title="cpp/locale"> Localizations library</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../regex.html" title="cpp/regex"> Regular expressions library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../atomic.html" title="cpp/atomic"> Atomic operations library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../thread.html" title="cpp/thread"> Thread support library</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../experimental.html" title="cpp/experimental"> Technical Specifications</a> </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:cpp/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="../language.1.html" title="cpp/language"> C++ language</a></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="basic_concepts.html" title="cpp/language/basic concepts"> Basic Concepts</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="../keywords.html" title="cpp/keyword"> language keywords</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="translation_phases.html" title="cpp/language/translation phases"> phases of translation</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="../comments.html" title="cpp/comment"> comments </a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="main_function.html" title="cpp/language/main function"> the main() function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="name.html" title="cpp/language/identifiers"> names and identifiers</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="type-id.html" title="cpp/language/type"> types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="types.html" title="cpp/language/types"> fundamental types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="objects.html" title="cpp/language/object"> objects</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="scope.html" title="cpp/language/scope"> scope</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lifetime.html" title="cpp/language/lifetime"> object lifetime</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="storage_duration.html" title="cpp/language/storage duration"> storage duration and linkage</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="definition.html" title="cpp/language/definition"> definitions and ODR</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="lookup.html" title="cpp/language/lookup"> name lookup</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="qualified_lookup.html" title="cpp/language/qualified lookup">qualified name lookup</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="unqualified_lookup.html" title="cpp/language/unqualified lookup">unqualified name lookup</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink"> memory model</strong> </td></tr>
</table></div>
</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:cpp/language/basics/navbar_content&action=edit">[edit]</a></span></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Defines the semantics of computer memory storage for the purpose of the C++ abstract machine.
</p><p>The 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 tocsection-3"><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 tocsection-4"><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#See_also"><span class="tocnumber">5</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=cpp/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 <span class="t-rev-inl t-since-cxx14"><span>the value of any <code>UTF-8</code> code unit (256 distinct values) and of </span> <span><span class="t-mark-rev t-since-cxx14">(since C++14)</span></span></span>any member of the <i>basic execution character set</i> (<a href="translation_phases.html" title="cpp/language/translation phases">the 96 characters</a> that are required to be single-byte). Similar to C, C++ supports bytes of sizes 8 bits and greater.
</p><p>The <a href="types.html" title="cpp/language/types">types</a> <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">char</span></span></span>, <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">unsigned</span> <span class="kw4">char</span></span></span>, and <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">signed</span> <span class="kw4">char</span></span></span> use one byte for both storage and <a href="objects.html" title="cpp/language/object">value representation</a>. The number of bits in a byte is accessible as <span class="t-lc"><a href="../types/climits.1.html" title="cpp/types/climits">CHAR_BIT</a></span> or <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="../types/numeric_limits.1.html"><span class="kw344">std::<span class="me2">numeric_limits</span></span></a><span class="sy1"><</span><span class="kw4">unsigned</span> <span class="kw4">char</span><span class="sy1">></span><span class="sy4">::</span><span class="me2">digits</span></span></span>.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/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="type-id.html" title="cpp/language/type">scalar type</a> (arithmetic type, pointer type, enumeration type, or std::nullptr_t)
</li><li> or the largest contiguous sequence of <a href="bit_field.html" title="cpp/language/bit field">bit fields</a> of non-zero length
</li></ul>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><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>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&action=edit&section=3" title="Edit section: Threads and data races">edit</a>]</span> <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/thread/thread.html" title="cpp/thread/thread/thread">std::thread::thread</a></span>, <span class="t-lc"><a href="../thread/async.html" title="cpp/thread/async">std::async</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="cpp/language/storage duration">storage duration</a> may still be accessed by another thread through a pointer or by reference).
</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.
</p><p>When an <a href="eval_order.html" title="cpp/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 atomic operations (see <span class="t-lc"><a href="../atomic/atomic.html" title="cpp/atomic/atomic">std::atomic</a></span>)
</li><li> one of the conflicting evaluations <i>happens-before</i> another (see <span class="t-lc"><a href="../atomic/memory_order.html" title="cpp/atomic/memory order">std::memory_order</a></span>)
</li></ul>
<p>If a data race occurs, the behavior of the program is undefined.
</p>
(in particular, release of a <span class="t-lc"><a href="../thread/mutex.html" title="cpp/thread/mutex">std::mutex</a></span> is <i>synchronized-with</i>, and therefore, <i>happens-before</i> acquisition 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="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&action=edit&section=4" title="Edit section: Memory order">edit</a>]</span> <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="cpp/atomic/memory order">std::memory_order</a></span> for details on the order in which writes made from threads become visible to other threads.
</p>
<h3><span class="editsection">[<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&action=edit&section=5" 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="../../c/language/memory_model.html" title="c/language/memory model">C documentation</a></span> for <span class="t-dsc-see-tt"><span>Memory model</span></span></div>
</td></tr>
</table>
<!--
NewPP limit report
Preprocessor visited node count: 1006/1000000
Preprocessor generated node count: 5858/1000000
Post‐expand include size: 27103/2097152 bytes
Template argument size: 3397/2097152 bytes
Highest expansion depth: 17/40
Expensive parser function count: 0/100
-->
<!-- Saved in parser cache with key mwiki1-mwiki_en_:pcache:idhash:13490-0!*!0!!en!*!* and timestamp 20150517191553 -->
</div> <!-- /bodycontent -->
<!-- printfooter -->
<div class="printfooter">
Retrieved from "<a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&oldid=78340">http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&oldid=78340</a>" </div>
<!-- /printfooter -->
<!-- catlinks -->
<div id='catlinks' class='catlinks'><div id="mw-normal-catlinks" class="mw-normal-catlinks"><a href="http://en.cppreference.com/w/Special:Categories" title="Special:Categories">Category</a>: <ul><li><a href="http://en.cppreference.com/w/Category:Todo_with_reason" title="Category:Todo with reason">Todo with reason</a></li></ul></div></div> <!-- /catlinks -->
<div class="visualClear"></div>
<!-- debughtml -->
<!-- /debughtml -->
</div>
<!-- /bodyContent -->
</div>
</div>
<!-- /content -->
<!-- footer -->
<div id="cpp-footer-base" class="noprint">
<div id="footer">
<div id="cpp-navigation">
<h5>Navigation</h5>
<ul>
<li id="n-Support-us"><a href="http://www.cppreference.com/support" rel="nofollow">Support us</a></li><li id="n-recentchanges"><a href="http://en.cppreference.com/w/Special:RecentChanges" title="A list of recent changes in the wiki [r]" accesskey="r">Recent changes</a></li><li id="n-FAQ"><a href="http://en.cppreference.com/w/Cppreference:FAQ">FAQ</a></li><li id="n-Offline-version"><a href="http://en.cppreference.com/w/Cppreference:Archives">Offline version</a></li> </ul>
</div>
<div id="cpp-toolbox">
<h5><span>Toolbox</span><a href="memory_model.html#"></a></h5>
<ul>
<li id="t-whatlinkshere"><a href="http://en.cppreference.com/w/Special:WhatLinksHere/cpp/language/memory_model" title="A list of all wiki pages that link here [j]" accesskey="j">What links here</a></li><li id="t-recentchangeslinked"><a href="http://en.cppreference.com/w/Special:RecentChangesLinked/cpp/language/memory_model" title="Recent changes in pages linked from this page [k]" accesskey="k">Related changes</a></li><li id="t-upload"><a href="http://upload.cppreference.com/w/Special:Upload" title="Upload files [u]" accesskey="u">Upload file</a></li><li id="t-specialpages"><a href="http://en.cppreference.com/w/Special:SpecialPages" title="A list of all special pages [q]" accesskey="q">Special pages</a></li><li id="t-print"><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&printable=yes" rel="alternate" title="Printable version of this page [p]" accesskey="p">Printable version</a></li><li id="t-permalink"><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&oldid=78340" title="Permanent link to this revision of the page">Permanent link</a></li><li id="t-info"><a href="http://en.cppreference.com/mwiki/index.php?title=cpp/language/memory_model&action=info">Page information</a></li> </ul>
</div>
<ul id="footer-info">
<li id="footer-info-lastmod"> This page was last modified on 17 May 2015, at 12:15.</li>
<li id="footer-info-viewcount">This page has been accessed 5,690 times.</li>
</ul>
<ul id="footer-places">
<li id="footer-places-privacy"><a href="http://en.cppreference.com/w/Cppreference:Privacy_policy" title="Cppreference:Privacy policy">Privacy policy</a></li>
<li id="footer-places-about"><a href="http://en.cppreference.com/w/Cppreference:About" title="Cppreference:About">About cppreference.com</a></li>
<li id="footer-places-disclaimer"><a href="http://en.cppreference.com/w/Cppreference:General_disclaimer" title="Cppreference:General disclaimer">Disclaimers</a></li>
</ul>
<ul id="footer-icons" class="noprint">
<li id="footer-poweredbyico">
<a href="http://www.mediawiki.org/"><img src="../../../mwiki/skins/common/images/poweredby_mediawiki_88x31.png" alt="Powered by MediaWiki" width="88" height="31" /></a> <a href="http://qbnz.com/highlighter/"><img src="../../../../upload.cppreference.com/mwiki/images/2/2b/powered_by_geshi_88x31.png" alt="Powered by GeSHi" height="31" width="88" /></a> <a href="http://www.tigertech.net/referral/cppreference.com"><img src="../../../../upload.cppreference.com/mwiki/images/9/94/powered_by_tigertech_88x31.png" alt="Hosted by Tiger Technologies" height="31" width="88" /></a> </li>
</ul>
<div style="clear:both">
</div>
</div>
</div>
<!-- /footer -->
<script>if(window.mw){
mw.loader.state({"site":"loading","user":"missing","user.groups":"ready"});
}</script>
<script src="../../../mwiki/load.php%3Fdebug=false&lang=en&modules=skins.cppreference2&only=scripts&skin=cppreference2&*"></script>
<script>if(window.mw){
mw.loader.load(["mediawiki.action.view.postEdit","mediawiki.user","mediawiki.page.ready","mediawiki.searchSuggest","mediawiki.hidpi","ext.gadget.ColiruCompiler"], null, true);
}</script>
<script src="../../../mwiki/load.php%3Fdebug=false&lang=en&modules=site&only=scripts&skin=cppreference2&*"></script>
<script type="text/javascript">
var gaJsHost = (("https:" == document.location.protocol) ? "https://ssl." : "http://www.");
document.write(unescape("%3Cscript src='" + gaJsHost + "google-analytics.com/ga.js' type='text/javascript'%3E%3C/script%3E"));
</script>
<script type="text/javascript">
var _gaq = _gaq || [];
_gaq.push(['_setAccount', 'UA-2828341-1']);
_gaq.push(['_setDomainName', 'cppreference.com']);
_gaq.push(['_trackPageview']);
</script><!-- Served in 0.077 secs. -->
</body>
<!-- Cached 20150518025135 -->
</html>
|
