32 Concurrency support library [thread]

32.5 Atomic operations [atomics]

32.5.5 Lock-free property [atomics.lockfree]

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#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
1
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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.
2
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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.
3
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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.
4
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Atomic operations that are not lock-free are considered to potentially block ([intro.progress]).
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Recommended practice: Operations that are lock-free should also be address-free.300
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]
300)300)
That is, atomic operations on the same memory location via two different addresses will communicate atomically.