Volatile (computer programming): Difference between revisions

Revision as of 13:10, 28 August 2024

Revision as of 13:18, 28 August 2024

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=== Multi-Threading ===

It is a common misconception that the <code>volatile</code> keyword is useful in portable [[thread (computing)|multi-threading]] code in C and C++. Unlike the [[Java (programming language)|Java]] and [[C Sharp (programming language)|C#]] programming languages, operations on <code>volatile</code> variables in C and C++ are not [[atomic operation|atomic]]. The <code>volatile</code> keyword has ''never'' functioned as a useful, portable tool for ''any'' multi-threading scenario.<ref>{{cite web |title=Volatile Keyword In Visual C++ |url=http://msdn2.microsoft.com/en-us/library/12a04hfd.aspx |work=Microsoft MSDN|date=21 September 2021 }}</ref><ref>{{cite web |title=Linux Kernel Documentation – Why the "volatile" type class should not be used |url=https://www.kernel.org/doc/html/latest/process/volatile-considered-harmful.html |work=kernel.org}}</ref><ref>{{cite web |author1=Scott Meyers |author2=Andrei Alexandrescu |year=2004 |title=C++ and the Perils of Double-Checked Locking |url=http://www.aristeia.com/Papers/DDJ_Jul_Aug_2004_revised.pdf |work=DDJ}}</ref><ref>{{cite web |author1=Jeremy Andrews |year=2007 |title=Linux: Volatile Superstition |url=http://kerneltrap.org/Linux/Volatile_Superstition |archive-url=https://web.archive.org/web/20100620121940/http://kerneltrap.org/Linux/Volatile_Superstition |archive-date=2010-06-20 |access-date=Jan 9, 2011 |publisher=kerneltrap.org}}</ref> Most C and C++ compilers, linkers, and runtimes simply do not provide the necessary guarantees (such as [[memory barrier|memory barriers]]) to make the <code>volatile</code> keyword useful for ''any'' multi-threading scenario. Before the C11 and C++11 standards, programmers were forced to rely on guarantees from the individual implementations and platforms (e.g. POSIX and WIN32) to write [[thread (computing)|multi-threading]] code. With the modern C11 and C++11 standards, programmers can write portable [[thread (computing)|multi-threading]] code using new portable constructs such as the <code>std::atomic<T></code> templates.<ref>{{cite web |title=volatile (C++) |url=https://msdn.microsoft.com/en-us/library/12a04hfd.aspx |work=Microsoft MSDN|date=21 September 2021 }}</ref>

===Example of memory-mapped I/O in C===

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=== Compiler Defects ===

Unlike other language features of C and C++, the <code>volatile</code> keyword is not well supported by most C/C++ implementations - even for portable uses according to the C and C++ standards. Most C/C++ implementations are buggy regarding the behavior of the <code>volatile</code> keyword.<ref>{{Cite journal |lastlast1=Eide |firstfirst1=Eric |last2=Regehr |first2=John |date=Oct. 2008 |title=Volatiles Are Miscompiled, and What to Do about It |url=https://users.cs.utah.edu/~regehr/papers/emsoft08-preprint.pdf |journal=Proceedings of the Eighth ACM and IEEE International Conference on Embedded Software (EMSOFT), Atlanta, Georgia, USA |via=cs.utah.edu}}</ref><ref>{{Cite web |title=Volatile Bugs, Three Years Later – Embedded in Academia |url=https://blog.regehr.org/archives/503 |access-date=2024-08-28 |website=blog.regehr.org}}</ref> Programmers should take great care whenever using the <code>volatile</code> keyword in C and C++.

and IEEE International Conference on Embedded Software (EMSOFT),

Atlanta, Georgia, USA, |via=cs.utah.edu}}</ref><ref>{{Cite web |title=Volatile Bugs, Three Years Later – Embedded in Academia |url=https://blog.regehr.org/archives/503 |access-date=2024-08-28 |website=blog.regehr.org}}</ref> Programmers should take great care whenever using the <code>volatile</code> keyword in C and C++.

==In Java==