Volatile (computer programming): Difference between revisions

Revision as of 04:09, 21 November 2020

Revision as of 04:13, 21 November 2020

Line 235:

In [[C Sharp (programming language)|C#]], <code>volatile</code> ensures that code accessing the field is not subject to some thread-unsafe optimizations that may be performed by the compiler, the CLR, or by hardware. When a field is marked <code>volatile</code>, the compiler is instructed to generate a "memory barrier" or "fence" around it, which prevents instruction reordering or caching tied to the field. When reading a <code>volatile</code> field, the compiler generates an ''acquire-fence'', which prevents other reads and writes to the field, including those in other threads, from being moved ''before'' the fence. When writing to a <code>volatile</code> field, the compiler generates a ''release-fence''; this fence prevents other reads and writes to the field from being moved ''after'' the fence.<ref name="Albahari">{{cite web |last1=Albahari |first1=Joseph |title=Part 4: Advanced Threading |url=http://www.albahari.com/threading/part4.aspx#_Nonblocking_Synchronization |website=Threading in C# |publisher=O'Reilly Media |accessdate=9 December 2019 |archiveurl=http://www.albahari.info/threading/threading.pdf|archivedate=27 April 2011|url-status=live}}</ref>

Only the following types can be marked <code>volatile</code>: all valuereference types, <code>Single</code>, <code>Boolean</code>, <code>Byte</code>, <code>SByte</code>, <code>Int16</code>, <code>UInt16</code>, <code>Int32</code>, <code>UInt32</code>, <code>Char</code>, and all enumerated types with an underlying type of <code>Byte</code>, <code>SByte</code>, <code>Int16</code>, <code>UInt16</code>, <code>Int32</code>, or <code>UInt32</code>.<ref>{{cite book |last1=Richter |first1=Jeffrey |title=CLR Via C# |url=https://archive.org/details/clrviac00rich_000 |url-access=limited |publisher=Microsoft Press |date=February 11, 2010 |pages=[https://archive.org/details/clrviac00rich_000/page/n200 183] |chapter=Chapter 7: Constants and Fields |isbn=978-0-7356-2704-8}}</ref> (This excludes value [[struct]]s, as well as the primitive types <code>Double</code>, <code>Int64</code>, <code>UInt64</code> and <code>Decimal</code>.)

Using the <code>volatile</code> keyword does not support fields that are [[Evaluation strategy#Call by reference|passed by reference]] or [[Closure (computer programming)|captured local variables]]; in these cases, <code>Thread.VolatileRead</code> and <code>Thread.VolatileWrite</code> must be used instead.<ref name="Albahari"/>