22b4470f2a
Update all links to C++ Core Guidelines to `e49158a`.
Refresh done using the following script and some manual edits:
db76e34e74/personal/fred-tingaud/rspec/refresh-cppcoreguidelines.py
When re-using this script, be mindful that:
- it does not cover `shared_content`
- it does not properly escape inline code in links (e.g., "[=]" or "`mutex`es")
- it does not change `C++` to `{cpp}` in link titles.
Co-authored-by: Marco Borgeaud <marco.borgeaud@sonarsource.com>
71 lines
2.2 KiB
Plaintext
71 lines
2.2 KiB
Plaintext
== Why is this an issue?
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The main intended use-case for ``++volatile++`` in C and {cpp} is to access data that can be modified by something external to the program, typically some hardware register. In contrast with other languages that provide a ``++volatile++`` keyword, it does not provide any useful guarantees related to atomicity, memory ordering, or inter-thread synchronization. It is only really needed for the kind of low-level code found in kernels or embedded software, i.e. using memory-mapped I/O registers to manipulate hardware directly.
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According to the C standard:
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____
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``++volatile++`` is a hint to the implementation to avoid aggressive optimization involving the object because the value of the object might be changed by means undetectable by an implementation.
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____
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Only C11/{cpp}11 "atomic types" are free from data races, and you should use them or synchronization primitives if you want to avoid race conditions.
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This rule raises an issue when a local variable or class data member is declared as ``++volatile++`` (at the top level of the type, pointers to volatile are not reported).
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=== Noncompliant code example
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[source,cpp]
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----
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volatile int counter; // Noncompliant
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User * volatile vpUser; // Noncompliant; pointer is volatile
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User volatile * pvUser; // Compliant; User instance is volatile, not the pointer
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----
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=== Compliant solution
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[source,cpp]
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----
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atomic_int counter;
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std::atomic<User*> vpUser;
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User volatile * pvUser;
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----
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== Resources
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* https://wiki.sei.cmu.edu/confluence/display/c/CON02-C.+Do+not+use+volatile+as+a+synchronization+primitive[CERT CON02-C] - Do not use volatile as a synchronization primitive
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* {cpp} Core Guidelines - https://github.com/isocpp/CppCoreGuidelines/blob/e49158a/CppCoreGuidelines.md#cp200-use-volatile-only-to-talk-to-non-c-memory[CP.200: Use `volatile` only to talk to non-{cpp} memory]
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ifdef::env-github,rspecator-view[]
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'''
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== Implementation Specification
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(visible only on this page)
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=== Message
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Convert this "volatile" type into an atomic type.
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=== Highlighting
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volatile keyword
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'''
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== Comments And Links
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(visible only on this page)
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=== is related to: S3475
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=== on 27 Jul 2016, 15:49:26 Ann Campbell wrote:
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Okay [~alban.auzeill], double-check me.
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endif::env-github,rspecator-view[]
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