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    Atomic types

    From cppreference.com
     
    C
     
    C language
     
    Declarations
     

    Syntax

    _Atomic ( type-name ) (1) (since C11)
    _Atomic type-name (2) (since C11)
    1) Use as a type specifier; this designates a new atomic type
    2) Use as a type qualifier; this designates the atomic version of type-name. In this role, it may be mixed with const, volatile, and restrict, although unlike other qualifiers, the atomic version of type-name may have a different size, alignment, and object representation.
    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 many convenience type aliases, from atomic_bool to atomic_uintmax_t, 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 atomic_int* 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