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<html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8" /><title>Hyperlinked ECMA C# Language Specification</title><meta name="author" content="Jon Jagger" /><link rel="stylesheet" href="ecma334.css"></link></head><body><div align="right"><em><a href="http://www.jaggersoft.com">Jon Jagger</a></em></div><div align="right"><a href="mailto:jon@jaggersoft.com">jon@jaggersoft.com</a></div><form method="get" action="http://search.atomz.com/search/"><input size="30" name="sp-q"></input><input type="submit" value="Search C# Spec"></input><input type="hidden" name="sp-a" value="sp10024177"></input><input type="hidden" name="sp-f" value="ISO-8859-1"></input></form><a href="toc.htm">Table of Contents</a> <a href="1.htm">1</a> <a href="2.htm">2</a> <a href="3.htm">3</a> <a href="4.htm">4</a> <a href="5.htm">5</a> <a href="6.htm">6</a> <a href="7.htm">7</a> <a href="8.htm">8</a> <a href="9.htm">9</a> <a href="10.htm">10</a> <a href="11.htm">11</a> <a href="12.htm">12</a> <a href="13.htm">13</a> <a href="14.htm">14</a> <a href="15.htm">15</a> <a href="16.htm">16</a> <a href="17.htm">17</a> <a href="18.htm">18</a> <a href="19.htm">19</a> <a href="20.htm">20</a> <a href="21.htm">21</a> <a href="22.htm">22</a> <a href="23.htm">23</a> <a href="24.htm">24</a> <a href="25.htm">25</a> <a href="notes.htm">Notes</a> <a href="HyperlinkedCSharpECMA.zip">Download</a><span class="ruler"></span><span class="heading">ECMA-334 C# Language Specification</span><span class="navigate"><a href="17.4.2.2.htm"><img src="previous.gif" alt="previous" border="0" /></a><a href="17.4.4.htm"><img src="next.gif" alt="next" border="0" /></a></span><span class="clause-depth"><a href="16.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="18.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="17.htm">17</a></span><span class="clause-title-previous"> Classes</span></span><span class="clause-depth"><a href="17.3.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.5.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number-link"><a href="17.4.htm">17.4</a></span><span class="clause-title-previous"> Fields</span></span><span class="clause-depth"><a href="17.4.2.htm"><img src="previous.gif" alt="previous at this level" border="0" /></a><a href="17.4.4.htm"><img src="next.gif" alt="next at this level" border="0" /></a> <span class="clause-number">17.4.3</span><span class="clause-title"> Volatile fields</span></span><span class="locator">
Paragraph 1</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P1S1"></a>When a <span class="non-terminal"><a href="17.4.htm#field-declaration">field-declaration</a></span> includes a volatile modifier, the fields introduced by that declaration are volatile fields.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P1S2"></a>For non-volatile fields, optimization techniques that reorder instructions can lead to unexpected and unpredictable results in multi-threaded programs that access fields without synchronization such as that provided by the <span class="non-terminal"><a href="15.12.htm#lock-statement">lock-statement</a></span> (<a href="15.12.htm">§15.12</a>).</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P1S3"></a>These optimizations can be performed by the compiler, by the runtime system, or by hardware.</span> <span class="sentence"><span class="sentence-number">4</span> <a name="P1S4"></a>For volatile fields, such reordering optimizations are restricted: </span><ul><li><span class="sentence"><span class="sentence-number">5</span> <a name="P1S5"></a> A read of a volatile field is called a volatile read.</span> <span class="sentence"><span class="sentence-number">6</span> <a name="P1S6"></a>A volatile read has "acquire semantics"; that is, it is guaranteed to occur prior to any references to memory that occur after it in the instruction sequence.</span> </li><li><span class="sentence"><span class="sentence-number">7</span> <a name="P1S7"></a> A write of a volatile field is called a volatile write.</span> <span class="sentence"><span class="sentence-number">8</span> <a name="P1S8"></a>A volatile write has "release semantics"; that is, it is guaranteed to happen after any memory references prior to the write instruction in the instruction sequence.</span> </li></ul></span><span class="locator">
Paragraph 2</span><span class="paragraph"><span class="sentence"><span class="sentence-number">1</span> <a name="P2S1"></a>These restrictions ensure that all threads will observe volatile writes performed by any other thread in the order in which they were performed.</span> <span class="sentence"><span class="sentence-number">2</span> <a name="P2S2"></a>A conforming implementation is not required to provide a single total ordering of volatile writes as seen from all threads of execution.</span> <span class="sentence"><span class="sentence-number">3</span> <a name="P2S3"></a>The type of a volatile field must be one of the following: </span><ul><li><span class="sentence"><span class="sentence-number">4</span> <a name="P2S4"></a> A <span class="non-terminal"><a href="11.2.htm#reference-type">reference-type</a></span>.</span> </li><li><span class="sentence"><span class="sentence-number">5</span> <a name="P2S5"></a> The type <span class="keyword">byte</span>, <span class="keyword">sbyte</span>, <span class="keyword">short</span>, <span class="keyword">ushort</span>, <span class="keyword">int</span>, <span class="keyword">uint</span>, <span class="keyword">char</span>, <span class="keyword">float</span>, or <span class="keyword">bool</span>.</span> </li><li><span class="sentence"><span class="sentence-number">6</span> <a name="P2S6"></a> An <span class="non-terminal"><a href="11.1.htm#enum-type">enum-type</a></span> having an enum base type of <span class="keyword">byte</span>, <span class="keyword">sbyte</span>, <span class="keyword">short</span>, <span class="keyword">ushort</span>, <span class="keyword">int</span>, or <span class="keyword">uint</span>.</span> </li></ul></span><span class="paragraph"><span class="example">[Example: The example <pre class="code-example">
using System;
using System.Threading;
class Test
{
public static int result;
public static volatile bool finished;
static void Thread2() {
result = 143;
finished = true;
}
static void Main() {
finished = false;
// Run Thread2() in a new thread
new Thread(new ThreadStart(Thread2)).Start();
// Wait for Thread2 to signal that it has a result by setting
// finished to true.
for (;;) {
if (finished) {
Console.WriteLine("result = {0}", result);
return;
}
}
}
}
</pre>produces the output: <pre class="code-example">
result = 143
</pre></span></span><span class="paragraph"><span class="example">In this example, the method Main starts a new thread that runs the method Thread2. This method stores a value into a non-volatile field called result, then stores true in the volatile field finished. The main thread waits for the field finished to be set to true, then reads the field result. Since result has been declared volatile, the main thread must read the value 143 from the field result. If the field finished had not been declared volatile, then it would be permissible for the store to result to be visible to the main thread after the store to finished, and hence for the main thread to read the value 0 from the field result. Declaring finished as a volatile field prevents any such inconsistency. end example]</span> </span><span class="ruler"></span><table><tr><td><table align="left" bgcolor="navy"><tr bgcolor="navy"><td><font face="Arial,sans-serif" size="6" color="yellow"><strong>{ JSL }</strong></font></td></tr></table></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>Jagger Software Ltd</strong></font></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>Company # 4070126</strong></font></td></tr><tr><td><font face="Arial,sans-serif" size="2" color="navy"><strong>VAT # 762 5213 42</strong></font></td></tr></table><img src="valid-html401.png" align="left" height="31" width="88" alt="Valid HTML 4.01" /><img src="vcss.gif" align="left" height="31" width="88" alt="Valid CSS" /></body></html>⌨️ 快捷键说明
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