33 Concurrency support library [thread]

33.5 Atomic operations [atomics]

33.5.5 Lock-free property [atomics.lockfree]

#define ATOMIC_BOOL_LOCK_FREE unspecified #define ATOMIC_CHAR_LOCK_FREE unspecified #define ATOMIC_CHAR8_T_LOCK_FREE unspecified #define ATOMIC_CHAR16_T_LOCK_FREE unspecified #define ATOMIC_CHAR32_T_LOCK_FREE unspecified #define ATOMIC_WCHAR_T_LOCK_FREE unspecified #define ATOMIC_SHORT_LOCK_FREE unspecified #define ATOMIC_INT_LOCK_FREE unspecified #define ATOMIC_LONG_LOCK_FREE unspecified #define ATOMIC_LLONG_LOCK_FREE unspecified #define ATOMIC_POINTER_LOCK_FREE unspecified
The ATOMIC_..._LOCK_FREE macros indicate the lock-free property of the corresponding atomic types, with the signed and unsigned variants grouped together.
The properties also apply to the corresponding (partial) specializations of the atomic template.
A value of 0 indicates that the types are never lock-free.
A value of 1 indicates that the types are sometimes lock-free.
A value of 2 indicates that the types are always lock-free.
On a hosted implementation ([compliance]), at least one signed integral specialization of the atomic template, along with the specialization for the corresponding unsigned type ([basic.fundamental]), is always lock-free.
The functions atomic<T>​::​is_lock_free and atomic_is_lock_free ([atomics.types.operations]) indicate whether the object is lock-free.
In any given program execution, the result of the lock-free query is the same for all atomic objects of the same type.
Atomic operations that are not lock-free are considered to potentially block ([intro.progress]).
Recommended practice: Operations that are lock-free should also be address-free.301
The implementation of these operations should not depend on any per-process state.
[Note 1: 
This restriction enables communication by memory that is mapped into a process more than once and by memory that is shared between two processes.
— end note]
That is, atomic operations on the same memory location via two different addresses will communicate atomically.