Relating to atoms or the energy that can be produced by them (Definition of atomic from the Cambridge Academic Content Dictionary © Cambridge University Press). Find your perfect ski equipment Alpine Skis Touring Skis Nordic Skis Skiboots, Goggles, Helmets ★ Engineered in Austria Find a dealer nearby! “All of our investors have been instrumental in helping our success, but Atomic is unique in its holistic approach to founder support.” – Clay Kellogg, CEO of Terminal “All of our investors have been instrumental in helping our success, but Atomic is unique in its holistic approach to founder support.” – Clay Kellogg, CEO of Terminal.

[edit]Syntax

The header <stdatomic.h> defines 37 convenience macros, from atomic_bool to atomic_uintmax_t, which simplify the use of this keyword with built-in and library types.

[edit]Explanation

Objects of atomic types are the only objects that are free from data races, 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 happens-before modification B of the same atomic M, then in the modification order of M, A occurs before B.

Note that although each atomic object has its own modification order, there is no single total order; different threads may observe modifications to different atomic objects in different orders.

There are four coherences that are guaranteed for all atomic operations:

• 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 memory_order.

Built-in increment and decrement operators and compound assignment are read-modify-write atomic operations with total sequentially consistent ordering (as if using memory_order_seq_cst). If less strict synchronization semantics are desired, the standard library functions may be used instead.

Atomic properties are only meaningful for lvalue expressions. Lvalue-to-rvalue conversion (which models a memory read from an atomic location to a CPU register) strips atomicity along with other qualifiers.

[edit]Notes

If the macro constant __STDC_NO_ATOMICS__ is defined by the compiler, the keyword _Atomic as well as the header <stdatomic.h>, is not provided.

Accessing a member of an atomic struct/union is undefined behavior.

The library type sig_atomic_t does not provide inter-thread synchronization or memory ordering, only atomicity.

The volatile 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 C++ for every possible type T. The mechanisms used to ensure atomicity and memory ordering should be compatible.

[edit]Keywords

[edit]Example

Possible output:

[edit]References

• C17 standard (ISO/IEC 9899:2018):

• 6.7.2.4 Atomic type specifiers (p: 87)

• 7.17 Atomics <stdatomic.h> (p: 200-209)

Atomic Number

• C11 standard (ISO/IEC 9899:2011):

Atomic Blonde

• 6.7.2.4 Atomic type specifiers (p: 121)

Atomic Clock

• 7.17 Atomics <stdatomic.h> (p: 273-286)

Atomic Mass

[edit]See also

Atomic Filament