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Slight rewrite in C/C++ multi-threading to emphasize that to be a useful threading construct in most cases, it must offer atomic semantics and also memory ordering guarantees. Atomic does not imply memory ordering e.g. fences, and fences do not imply atomic. Not-atomic simply means that you might observe word tearing. You need fences on top of avoiding word-tearing to be a useful threading construct in most cases. |
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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++. The <code>volatile</code> keyword in C and C++ has ''never'' functioned as a useful, portable tool for ''any'' multi-threading scenario.<ref>{{cite web |date=21 September 2021 |title=Volatile Keyword In Visual C++ |url=http://msdn2.microsoft.com/en-us/library/12a04hfd.aspx |work=Microsoft MSDN}}</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> 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]], and operations on <code>volatile</code> variables do not have sufficient memory ordering guarantees (i.e. [[memory barrier|memory barriers]]). Most C and C++ compilers, linkers, and runtimes simply do not provide
===Example of memory-mapped I/O in C===
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