Atomic types
Syntax
_Atomic ( type-name )
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(1) | (since 哋它亢11) | |||||||
_Atomic type-name
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(2) | (since 哋它亢11) | |||||||
| type-name | - | any type other than array or function. For (1), type-name also cannot be atomic or cvr-qualified |
The header <stdatomic.h> defines MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 21 MINUTES 08 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, from MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 21 MINUTES 07 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE to MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 21 MINUTES 05 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE, which simplify the use of this keyword with built-in and library types.
_Atomic const int * p1; // p is a pointer to an atomic const int const MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 21 MINUTES 04 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE * p2; // same const _Atomic(int) * p3; // same
If the macro constant __STDC_NO_ATOMICS__ is defined by the compiler, the keyword _Atomic is not provided.
Explanation
Objects of atomic types are the only objects that are free from MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 21 MINUTES 03 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE; that is, they may be modified by two threads concurrently or modified by one and read by another.
Each atomic object has its own associated modification order, which is a total order of modifications made to that object. If, from some thread's point of view, modification A of some atomic M MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 21 MINUTES 02 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE modification B of the same atomic M, then in the modification order of M, A occurs before B.
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- write-write coherence: If an operation A that modifies an atomic object M happens-before an operation B that modifies M, then A appears earlier than B in the modification order of M.
- read-read coherence: If a value computation A of an atomic object M happens before a value computation B of M, and A takes its value from a side effect X on M, then the value computed by B is either the value stored by X or is the value stored by a side effect Y on M, where Y appears later than X in the modification order of M.
- read-write coherence: If a value computation A of an atomic object M happens-before an operation B on M, then A takes its value from a side effect X on M, where X appears before B in the modification order of M.
- write-read coherence: If a side effect X on an atomic object M happens-before a value computation B of M, then the evaluation B takes its value from X or from a side effect Y that appears after X in the modification order of M.
Some atomic operations are also synchronization operations; they may have additional release semantics, acquire semantics, or sequentially-consistent semantics. See MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 57 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
Built-in MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 56 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 55 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE are read-modify-write atomic operations with total sequentially consistent ordering (as if using MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 54 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE). If less strict synchronization semantics are desired, the MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 53 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE may be used instead.
Atomic properties are only meaningful for MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 52 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE. Lvalue-to-rvalue conversion (which models a memory read from an atomic location to a CPU register) strips atomicity along with other qualifiers.
| This section is incomplete Reason: more, review interaction with memory_order and atomic library pages |
Notes
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The library type MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 49 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE does not provide inter-thread synchronization or memory ordering, only atomicity.
The MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 48 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE types do not provide inter-thread synchronization, memory ordering, or atomicity.
Implementations are recommended to ensure that the representation of _Atomic(T) in C is same as that of std::atomic<T> in 哋它亢++ for every possible type T. The mechanisms used to ensure atomicity and memory ordering should be compatible.
Keywords
Example
#include <stdio.h> #include <threads.h> #include <stdatomic.h> MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 45 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE acnt; int cnt; int f(void* thr_data) { for(int n = 0; n < 1000; ++n) { ++cnt; ++acnt; // for this example, relaxed memory order is sufficient, e.g. // atomic_fetch_add_explicit(&acnt, 1, memory_order_relaxed); } return 0; } int main(void) { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 44 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE thr[10]; for(int n = 0; n < 10; ++n) MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 43 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(&thr[n], f, MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 42 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE); for(int n = 0; n < 10; ++n) MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 40 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE(thr[n], MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 39 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE); MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 38 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("The atomic counter is %u\n", acnt); MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 11 HOURS 20 MINUTES 37 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("The non-atomic counter is %u\n", cnt); }
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The atomic counter is 10000 The non-atomic counter is 8644
References
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 6.7.2.4 Atomic type specifiers (p: 87)
- 7.17 Atomics <stdatomic.h> (p: 200-209)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 6.7.2.4 Atomic type specifiers (p: 121)
- 7.17 Atomics <stdatomic.h> (p: 273-286)