Suppose one thread sets a shared `finished` flag while another checks it. An ordinary `bool` is not enough: even its update may be divisible between threads, and conflicting unprotected accesses can leave the program in an undefined state. Declaring the member `_Atomic(bool) finished;` tells C that it needs atomic access. section 18.1
40 // Parameters that will dynamically be changed by
41 // different threads
42 _Atomic(size_t) constellations; //< Constellations visited
43 _Atomic(size_t) x0; //< Cursor position, row
44 _Atomic(size_t) x1; //< Cursor position, column
45 _Atomic(size_t) frames; //< FPS for display
46 _Atomic(bool) finished; //< This game is finished.
!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":0},{\"line\":0,\"ch\":2}],[{\"line\":1,\"ch\":0},{\"line\":1,\"ch\":2}],[{\"line\":2,\"ch\":0},{\"line\":2,\"ch\":2}],[{\"line\":2,\"ch\":7},{\"line\":2,\"ch\":14}],[{\"line\":3,\"ch\":7},{\"line\":3,\"ch\":14}],[{\"line\":4,\"ch\":7},{\"line\":4,\"ch\":14}],[{\"line\":5,\"ch\":7},{\"line\":5,\"ch\":14}],[{\"line\":6,\"ch\":7},{\"line\":6,\"ch\":14}],[{\"line\":2,\"ch\":15},{\"line\":2,\"ch\":21}],[{\"line\":3,\"ch\":15},{\"line\":3,\"ch\":21}],[{\"line\":4,\"ch\":15},{\"line\":4,\"ch\":21}],[{\"line\":5,\"ch\":15},{\"line\":5,\"ch\":21}],[{\"line\":3,\"ch\":0},{\"line\":3,\"ch\":2}],[{\"line\":2,\"ch\":7},{\"line\":2,\"ch\":14}],[{\"line\":3,\"ch\":7},{\"line\":3,\"ch\":14}],[{\"line\":4,\"ch\":7},{\"line\":4,\"ch\":14}],[{\"line\":5,\"ch\":7},{\"line\":5,\"ch\":14}],[{\"line\":6,\"ch\":7},{\"line\":6,\"ch\":14}],[{\"line\":2,\"ch\":15},{\"line\":2,\"ch\":21}],[{\"line\":3,\"ch\":15},{\"line\":3,\"ch\":21}],[{\"line\":4,\"ch\":15},{\"line\":4,\"ch\":21}],[{\"line\":5,\"ch\":15},{\"line\":5,\"ch\":21}],[{\"line\":4,\"ch\":0},{\"line\":4,\"ch\":2}],[{\"line\":2,\"ch\":7},{\"line\":2,\"ch\":14}],[{\"line\":3,\"ch\":7},{\"line\":3,\"ch\":14}],[{\"line\":4,\"ch\":7},{\"line\":4,\"ch\":14}],[{\"line\":5,\"ch\":7},{\"line\":5,\"ch\":14}],[{\"line\":6,\"ch\":7},{\"line\":6,\"ch\":14}],[{\"line\":2,\"ch\":15},{\"line\":2,\"ch\":21}],[{\"line\":3,\"ch\":15},{\"line\":3,\"ch\":21}],[{\"line\":4,\"ch\":15},{\"line\":4,\"ch\":21}],[{\"line\":5,\"ch\":15},{\"line\":5,\"ch\":21}],[{\"line\":5,\"ch\":0},{\"line\":5,\"ch\":2}],[{\"line\":2,\"ch\":7},{\"line\":2,\"ch\":14}],[{\"line\":3,\"ch\":7},{\"line\":3,\"ch\":14}],[{\"line\":4,\"ch\":7},{\"line\":4,\"ch\":14}],[{\"line\":5,\"ch\":7},{\"line\":5,\"ch\":14}],[{\"line\":6,\"ch\":7},{\"line\":6,\"ch\":14}],[{\"line\":2,\"ch\":15},{\"line\":2,\"ch\":21}],[{\"line\":3,\"ch\":15},{\"line\":3,\"ch\":21}],[{\"line\":4,\"ch\":15},{\"line\":4,\"ch\":21}],[{\"line\":5,\"ch\":15},{\"line\":5,\"ch\":21}],[{\"line\":6,\"ch\":0},{\"line\":6,\"ch\":2}],[{\"line\":2,\"ch\":7},{\"line\":2,\"ch\":14}],[{\"line\":3,\"ch\":7},{\"line\":3,\"ch\":14}],[{\"line\":4,\"ch\":7},{\"line\":4,\"ch\":14}],[{\"line\":5,\"ch\":7},{\"line\":5,\"ch\":14}],[{\"line\":6,\"ch\":7},{\"line\":6,\"ch\":14}]]"}
!@%STYLE%@!
The declarations apply `_Atomic` to the flag, cursor coordinates, and other members changed by different threads. Each member has its own atomic type. section 18.1
TAKEAWAY: In different threads, standard operations on atomic objects are indivisible and linearizable. Indivisible means an operation cannot be observed partway through. Linearizability also lets threads reason about the ordering of computations: when one thread observes `finished` set to true, it can rely on the setting thread having performed the actions that precede that signal. Atomic operations thus help establish synchronization, rather than merely protecting a single value. section 18.1 section 19.5
The primary declaration syntax is the atomic specifier `_Atomic(T)`, where `T` is the type from which the atomic type is derived. `_Atomic` also has a qualifier form. Placement matters in compound declarations: an atomic pointer and a pointer to an atomic object are different types. Nor can an array itself be atomic, although its elements can have an atomic type. section 18.1
_Atomic(double[45]) C; // Invalid: atomic cannot be applied to arrays.
_Atomic(double) D[45]; // Valid: atomic can be applied to array base.
!@%STYLE%@!
{"css":"{\"css\": \"font-weight: bold;\"}","target":"[[{\"line\":0,\"ch\":3},{\"line\":0,\"ch\":10}],[{\"line\":1,\"ch\":3},{\"line\":1,\"ch\":10}],[{\"line\":0,\"ch\":11},{\"line\":0,\"ch\":17}],[{\"line\":1,\"ch\":11},{\"line\":1,\"ch\":17}],[{\"line\":0,\"ch\":3},{\"line\":0,\"ch\":10}],[{\"line\":1,\"ch\":3},{\"line\":1,\"ch\":10}],[{\"line\":0,\"ch\":11},{\"line\":0,\"ch\":17}],[{\"line\":1,\"ch\":11},{\"line\":1,\"ch\":17}]]"}
!@%STYLE%@!
`_Atomic(double[45])` attempts to make an array atomic and is invalid; `_Atomic(double) D[45]` declares an array whose elements are atomic. section 18.1
Signal handlers need a related but narrower guarantee: an operation must appear either not to have started or to have completed when the handler runs. `sig_atomic_t` provides uninterruptible loads and stores, not a general-purpose atomic counter; `++` may involve separate load, increment, and store steps. C11 also provides `atomic_flag` and other atomic types with the lock-free property for this setting, where supported. section 17.6
Use atomic types for small, critical data shared between threads. They do not replace every synchronization tool: code operating on unprotected shared data generally needs a mutex. Atomics also have a memory-consistency model. Sequential consistency is the default; explicit acquire-release consistency can be more efficient, but requires careful design. section 18.7 section 19.5
