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atomics.tex
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atomics.tex
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%!TEX root = std.tex
\rSec0[atomics]{Atomic operations library}
\rSec1[atomics.general]{General}
\pnum
This Clause describes components for fine-grained atomic access. This access is
provided via operations on atomic objects.
\pnum
The following subclauses describe atomics requirements and components for types
and operations, as summarized in \tref{atomics.summary}.
\begin{libsumtab}{Atomics library summary}{atomics.summary}
\ref{atomics.alias} & Type aliases & \tcode{<atomic>} \\
\ref{atomics.order} & Order and consistency & \\
\ref{atomics.lockfree} & Lock-free property & \\
\ref{atomics.wait} & Waiting and notifying & \\
\ref{atomics.ref.generic} & Class template \tcode{atomic_ref} & \\
\ref{atomics.types.generic} & Class template \tcode{atomic} & \\
\ref{atomics.nonmembers} & Non-member functions & \\
\ref{atomics.flag} & Flag type and operations & \\
\ref{atomics.fences} & Fences & \\
\end{libsumtab}
\rSec1[atomics.syn]{Header \tcode{<atomic>} synopsis}
\indexheader{atomic}%
\begin{codeblock}
namespace std {
// \ref{atomics.order}, order and consistency
enum class memory_order : @\unspec@;
template<class T>
T kill_dependency(T y) noexcept;
// \ref{atomics.lockfree}, lock-free property
#define ATOMIC_BOOL_LOCK_FREE @\unspec@
#define ATOMIC_CHAR_LOCK_FREE @\unspec@
#define ATOMIC_CHAR8_T_LOCK_FREE @\unspec@
#define ATOMIC_CHAR16_T_LOCK_FREE @\unspec@
#define ATOMIC_CHAR32_T_LOCK_FREE @\unspec@
#define ATOMIC_WCHAR_T_LOCK_FREE @\unspec@
#define ATOMIC_SHORT_LOCK_FREE @\unspec@
#define ATOMIC_INT_LOCK_FREE @\unspec@
#define ATOMIC_LONG_LOCK_FREE @\unspec@
#define ATOMIC_LLONG_LOCK_FREE @\unspec@
#define ATOMIC_POINTER_LOCK_FREE @\unspec@
// \ref{atomics.ref.generic}, class template \tcode{atomic_ref}
template<class T> struct atomic_ref;
// \ref{atomics.ref.pointer}, partial specialization for pointers
template<class T> struct atomic_ref<T*>;
// \ref{atomics.types.generic}, class template \tcode{atomic}
template<class T> struct atomic;
// \ref{atomics.types.pointer}, partial specialization for pointers
template<class T> struct atomic<T*>;
// \ref{atomics.nonmembers}, non-member functions
template<class T>
bool atomic_is_lock_free(const volatile atomic<T>*) noexcept;
template<class T>
bool atomic_is_lock_free(const atomic<T>*) noexcept;
template<class T>
void atomic_store(volatile atomic<T>*, typename atomic<T>::value_type) noexcept;
template<class T>
void atomic_store(atomic<T>*, typename atomic<T>::value_type) noexcept;
template<class T>
void atomic_store_explicit(volatile atomic<T>*, typename atomic<T>::value_type,
memory_order) noexcept;
template<class T>
void atomic_store_explicit(atomic<T>*, typename atomic<T>::value_type,
memory_order) noexcept;
template<class T>
T atomic_load(const volatile atomic<T>*) noexcept;
template<class T>
T atomic_load(const atomic<T>*) noexcept;
template<class T>
T atomic_load_explicit(const volatile atomic<T>*, memory_order) noexcept;
template<class T>
T atomic_load_explicit(const atomic<T>*, memory_order) noexcept;
template<class T>
T atomic_exchange(volatile atomic<T>*, typename atomic<T>::value_type) noexcept;
template<class T>
T atomic_exchange(atomic<T>*, typename atomic<T>::value_type) noexcept;
template<class T>
T atomic_exchange_explicit(volatile atomic<T>*, typename atomic<T>::value_type,
memory_order) noexcept;
template<class T>
T atomic_exchange_explicit(atomic<T>*, typename atomic<T>::value_type,
memory_order) noexcept;
template<class T>
bool atomic_compare_exchange_weak(volatile atomic<T>*,
typename atomic<T>::value_type*,
typename atomic<T>::value_type) noexcept;
template<class T>
bool atomic_compare_exchange_weak(atomic<T>*,
typename atomic<T>::value_type*,
typename atomic<T>::value_type) noexcept;
template<class T>
bool atomic_compare_exchange_strong(volatile atomic<T>*,
typename atomic<T>::value_type*,
typename atomic<T>::value_type) noexcept;
template<class T>
bool atomic_compare_exchange_strong(atomic<T>*,
typename atomic<T>::value_type*,
typename atomic<T>::value_type) noexcept;
template<class T>
bool atomic_compare_exchange_weak_explicit(volatile atomic<T>*,
typename atomic<T>::value_type*,
typename atomic<T>::value_type,
memory_order, memory_order) noexcept;
template<class T>
bool atomic_compare_exchange_weak_explicit(atomic<T>*,
typename atomic<T>::value_type*,
typename atomic<T>::value_type,
memory_order, memory_order) noexcept;
template<class T>
bool atomic_compare_exchange_strong_explicit(volatile atomic<T>*,
typename atomic<T>::value_type*,
typename atomic<T>::value_type,
memory_order, memory_order) noexcept;
template<class T>
bool atomic_compare_exchange_strong_explicit(atomic<T>*,
typename atomic<T>::value_type*,
typename atomic<T>::value_type,
memory_order, memory_order) noexcept;
template<class T>
T atomic_fetch_add(volatile atomic<T>*, typename atomic<T>::difference_type) noexcept;
template<class T>
T atomic_fetch_add(atomic<T>*, typename atomic<T>::difference_type) noexcept;
template<class T>
T atomic_fetch_add_explicit(volatile atomic<T>*, typename atomic<T>::difference_type,
memory_order) noexcept;
template<class T>
T atomic_fetch_add_explicit(atomic<T>*, typename atomic<T>::difference_type,
memory_order) noexcept;
template<class T>
T atomic_fetch_sub(volatile atomic<T>*, typename atomic<T>::difference_type) noexcept;
template<class T>
T atomic_fetch_sub(atomic<T>*, typename atomic<T>::difference_type) noexcept;
template<class T>
T atomic_fetch_sub_explicit(volatile atomic<T>*, typename atomic<T>::difference_type,
memory_order) noexcept;
template<class T>
T atomic_fetch_sub_explicit(atomic<T>*, typename atomic<T>::difference_type,
memory_order) noexcept;
template<class T>
T atomic_fetch_and(volatile atomic<T>*, typename atomic<T>::value_type) noexcept;
template<class T>
T atomic_fetch_and(atomic<T>*, typename atomic<T>::value_type) noexcept;
template<class T>
T atomic_fetch_and_explicit(volatile atomic<T>*, typename atomic<T>::value_type,
memory_order) noexcept;
template<class T>
T atomic_fetch_and_explicit(atomic<T>*, typename atomic<T>::value_type,
memory_order) noexcept;
template<class T>
T atomic_fetch_or(volatile atomic<T>*, typename atomic<T>::value_type) noexcept;
template<class T>
T atomic_fetch_or(atomic<T>*, typename atomic<T>::value_type) noexcept;
template<class T>
T atomic_fetch_or_explicit(volatile atomic<T>*, typename atomic<T>::value_type,
memory_order) noexcept;
template<class T>
T atomic_fetch_or_explicit(atomic<T>*, typename atomic<T>::value_type,
memory_order) noexcept;
template<class T>
T atomic_fetch_xor(volatile atomic<T>*, typename atomic<T>::value_type) noexcept;
template<class T>
T atomic_fetch_xor(atomic<T>*, typename atomic<T>::value_type) noexcept;
template<class T>
T atomic_fetch_xor_explicit(volatile atomic<T>*, typename atomic<T>::value_type,
memory_order) noexcept;
template<class T>
T atomic_fetch_xor_explicit(atomic<T>*, typename atomic<T>::value_type,
memory_order) noexcept;
template<class T>
void atomic_wait(const volatile atomic<T>*, typename atomic<T>::value_type);
template<class T>
void atomic_wait(const atomic<T>*, typename atomic<T>::value_type);
template<class T>
void atomic_wait_explicit(const volatile atomic<T>*, typename atomic<T>::value_type,
memory_order);
template<class T>
void atomic_wait_explicit(const atomic<T>*, typename atomic<T>::value_type,
memory_order);
template<class T>
void atomic_notify_one(volatile atomic<T>*);
template<class T>
void atomic_notify_one(atomic<T>*);
template<class T>
void atomic_notify_all(volatile atomic<T>*);
template<class T>
void atomic_notify_all(atomic<T>*);
// \ref{atomics.alias}, type aliases
using atomic_bool = atomic<bool>;
using atomic_char = atomic<char>;
using atomic_schar = atomic<signed char>;
using atomic_uchar = atomic<unsigned char>;
using atomic_short = atomic<short>;
using atomic_ushort = atomic<unsigned short>;
using atomic_int = atomic<int>;
using atomic_uint = atomic<unsigned int>;
using atomic_long = atomic<long>;
using atomic_ulong = atomic<unsigned long>;
using atomic_llong = atomic<long long>;
using atomic_ullong = atomic<unsigned long long>;
using atomic_char8_t = atomic<char8_t>;
using atomic_char16_t = atomic<char16_t>;
using atomic_char32_t = atomic<char32_t>;
using atomic_wchar_t = atomic<wchar_t>;
using atomic_int8_t = atomic<int8_t>;
using atomic_uint8_t = atomic<uint8_t>;
using atomic_int16_t = atomic<int16_t>;
using atomic_uint16_t = atomic<uint16_t>;
using atomic_int32_t = atomic<int32_t>;
using atomic_uint32_t = atomic<uint32_t>;
using atomic_int64_t = atomic<int64_t>;
using atomic_uint64_t = atomic<uint64_t>;
using atomic_int_least8_t = atomic<int_least8_t>;
using atomic_uint_least8_t = atomic<uint_least8_t>;
using atomic_int_least16_t = atomic<int_least16_t>;
using atomic_uint_least16_t = atomic<uint_least16_t>;
using atomic_int_least32_t = atomic<int_least32_t>;
using atomic_uint_least32_t = atomic<uint_least32_t>;
using atomic_int_least64_t = atomic<int_least64_t>;
using atomic_uint_least64_t = atomic<uint_least64_t>;
using atomic_int_fast8_t = atomic<int_fast8_t>;
using atomic_uint_fast8_t = atomic<uint_fast8_t>;
using atomic_int_fast16_t = atomic<int_fast16_t>;
using atomic_uint_fast16_t = atomic<uint_fast16_t>;
using atomic_int_fast32_t = atomic<int_fast32_t>;
using atomic_uint_fast32_t = atomic<uint_fast32_t>;
using atomic_int_fast64_t = atomic<int_fast64_t>;
using atomic_uint_fast64_t = atomic<uint_fast64_t>;
using atomic_intptr_t = atomic<intptr_t>;
using atomic_uintptr_t = atomic<uintptr_t>;
using atomic_size_t = atomic<size_t>;
using atomic_ptrdiff_t = atomic<ptrdiff_t>;
using atomic_intmax_t = atomic<intmax_t>;
using atomic_uintmax_t = atomic<uintmax_t>;
using atomic_signed_lock_free = @\seebelow@;
using atomic_unsigned_lock_free = @\seebelow@;
// \ref{atomics.flag}, flag type and operations
struct atomic_flag;
bool atomic_flag_test(const volatile atomic_flag*) noexcept;
bool atomic_flag_test(const atomic_flag*) noexcept;
bool atomic_flag_test_explicit(const volatile atomic_flag*, memory_order) noexcept;
bool atomic_flag_test_explicit(const atomic_flag*, memory_order) noexcept;
bool atomic_flag_test_and_set(volatile atomic_flag*) noexcept;
bool atomic_flag_test_and_set(atomic_flag*) noexcept;
bool atomic_flag_test_and_set_explicit(volatile atomic_flag*, memory_order) noexcept;
bool atomic_flag_test_and_set_explicit(atomic_flag*, memory_order) noexcept;
void atomic_flag_clear(volatile atomic_flag*) noexcept;
void atomic_flag_clear(atomic_flag*) noexcept;
void atomic_flag_clear_explicit(volatile atomic_flag*, memory_order) noexcept;
void atomic_flag_clear_explicit(atomic_flag*, memory_order) noexcept;
void atomic_flag_wait(const volatile atomic_flag*, bool) noexcept;
void atomic_flag_wait(const atomic_flag*, bool) noexcept;
void atomic_flag_wait_explicit(const volatile atomic_flag*,
bool, memory_order) noexcept;
void atomic_flag_wait_explicit(const atomic_flag*,
bool, memory_order) noexcept;
void atomic_flag_notify_one(volatile atomic_flag*) noexcept;
void atomic_flag_notify_one(atomic_flag*) noexcept;
void atomic_flag_notify_all(volatile atomic_flag*) noexcept;
void atomic_flag_notify_all(atomic_flag*) noexcept;
// \ref{atomics.fences}, fences
extern "C" void atomic_thread_fence(memory_order) noexcept;
extern "C" void atomic_signal_fence(memory_order) noexcept;
}
\end{codeblock}
\rSec1[atomics.alias]{Type aliases}
\indexlibraryglobal{atomic_bool}%
\indexlibraryglobal{atomic_char}%
\indexlibraryglobal{atomic_schar}%
\indexlibraryglobal{atomic_uchar}%
\indexlibraryglobal{atomic_short}%
\indexlibraryglobal{atomic_ushort}%
\indexlibraryglobal{atomic_int}%
\indexlibraryglobal{atomic_uint}%
\indexlibraryglobal{atomic_long}%
\indexlibraryglobal{atomic_ulong}%
\indexlibraryglobal{atomic_llong}%
\indexlibraryglobal{atomic_ullong}%
\indexlibraryglobal{atomic_char8_t}%
\indexlibraryglobal{atomic_char16_t}%
\indexlibraryglobal{atomic_char32_t}%
\indexlibraryglobal{atomic_wchar_t}%
\indexlibraryglobal{atomic_int8_t}%
\indexlibraryglobal{atomic_uint8_t}%
\indexlibraryglobal{atomic_int16_t}%
\indexlibraryglobal{atomic_uint16_t}%
\indexlibraryglobal{atomic_int32_t}%
\indexlibraryglobal{atomic_uint32_t}%
\indexlibraryglobal{atomic_int64_t}%
\indexlibraryglobal{atomic_uint64_t}%
\indexlibraryglobal{atomic_int_least8_t}%
\indexlibraryglobal{atomic_uint_least8_t}%
\indexlibraryglobal{atomic_int_least16_t}%
\indexlibraryglobal{atomic_uint_least16_t}%
\indexlibraryglobal{atomic_int_least32_t}%
\indexlibraryglobal{atomic_uint_least32_t}%
\indexlibraryglobal{atomic_int_least64_t}%
\indexlibraryglobal{atomic_uint_least64_t}%
\indexlibraryglobal{atomic_int_fast8_t}%
\indexlibraryglobal{atomic_uint_fast8_t}%
\indexlibraryglobal{atomic_int_fast16_t}%
\indexlibraryglobal{atomic_uint_fast16_t}%
\indexlibraryglobal{atomic_int_fast32_t}%
\indexlibraryglobal{atomic_uint_fast32_t}%
\indexlibraryglobal{atomic_int_fast64_t}%
\indexlibraryglobal{atomic_uint_fast64_t}%
\indexlibraryglobal{atomic_intptr_t}%
\indexlibraryglobal{atomic_uintptr_t}%
\indexlibraryglobal{atomic_size_t}%
\indexlibraryglobal{atomic_ptrdiff_t}%
\indexlibraryglobal{atomic_intmax_t}%
\indexlibraryglobal{atomic_uintmax_t}%
\pnum
The type aliases \tcode{atomic_int$N$_t}, \tcode{atomic_uint$N$_t},
\tcode{atomic_intptr_t}, and \tcode{atomic_uintptr_t}
are defined if and only if
\tcode{int$N$_t}, \tcode{uint$N$_t},
\tcode{intptr_t}, and \tcode{uintptr_t}
are defined, respectively.
\pnum
\indexlibraryglobal{atomic_signed_lock_free}%
\indexlibraryglobal{atomic_unsigned_lock_free}%
The type aliases
\tcode{atomic_signed_lock_free} and \tcode{atomic_unsigned_lock_free}
name specializations of \tcode{atomic}
whose template arguments are integral types, respectively signed and unsigned,
and whose \tcode{is_always_lock_free} property is \tcode{true}.
\begin{note}
These aliases are optional in freestanding implementations\iref{compliance}.
\end{note}
Implementations should choose for these aliases
the integral specializations of \tcode{atomic}
for which the atomic waiting and notifying operations\iref{atomics.wait}
are most efficient.
\rSec1[atomics.order]{Order and consistency}
\indexlibraryglobal{memory_order}%
\indexlibrarymember{relaxed}{memory_order}%
\indexlibrarymember{consume}{memory_order}%
\indexlibrarymember{acquire}{memory_order}%
\indexlibrarymember{release}{memory_order}%
\indexlibrarymember{acq_rel}{memory_order}%
\indexlibrarymember{seq_cst}{memory_order}%
\indexlibraryglobal{memory_order_relaxed}%
\indexlibraryglobal{memory_order_consume}%
\indexlibraryglobal{memory_order_acquire}%
\indexlibraryglobal{memory_order_release}%
\indexlibraryglobal{memory_order_acq_rel}%
\indexlibraryglobal{memory_order_seq_cst}%
\begin{codeblock}
namespace std {
enum class memory_order : @\unspec@ {
relaxed, consume, acquire, release, acq_rel, seq_cst
};
inline constexpr memory_order memory_order_relaxed = memory_order::relaxed;
inline constexpr memory_order memory_order_consume = memory_order::consume;
inline constexpr memory_order memory_order_acquire = memory_order::acquire;
inline constexpr memory_order memory_order_release = memory_order::release;
inline constexpr memory_order memory_order_acq_rel = memory_order::acq_rel;
inline constexpr memory_order memory_order_seq_cst = memory_order::seq_cst;
}
\end{codeblock}
\pnum
The enumeration \tcode{memory_order} specifies the detailed regular
(non-atomic) memory synchronization order as defined in
\ref{intro.multithread} and may provide for operation ordering. Its
enumerated values and their meanings are as follows:
\begin{itemize}
\item \tcode{memory_order::relaxed}: no operation orders memory.
\item \tcode{memory_order::release}, \tcode{memory_order::acq_rel}, and
\tcode{memory_order::seq_cst}: a store operation performs a release operation on the
affected memory location.
\item \tcode{memory_order::consume}: a load operation performs a consume operation on the
affected memory location.
\begin{note}
Prefer \tcode{memory_order::acquire}, which provides stronger guarantees
than \tcode{memory_order::consume}. Implementations have found it infeasible
to provide performance better than that of \tcode{memory_order::acquire}.
Specification revisions are under consideration.
\end{note}
\item \tcode{memory_order::acquire}, \tcode{memory_order::acq_rel}, and
\tcode{memory_order::seq_cst}: a load operation performs an acquire operation on the
affected memory location.
\end{itemize}
\begin{note}
Atomic operations specifying \tcode{memory_order::relaxed} are relaxed
with respect to memory ordering. Implementations must still guarantee that any
given atomic access to a particular atomic object be indivisible with respect
to all other atomic accesses to that object.
\end{note}
\pnum
An atomic operation \placeholder{A} that performs a release operation on an atomic
object \placeholder{M} synchronizes with an atomic operation \placeholder{B} that performs
an acquire operation on \placeholder{M} and takes its value from any side effect in the
release sequence headed by \placeholder{A}.
\pnum
An atomic operation \placeholder{A} on some atomic object \placeholder{M} is
\defn{coherence-ordered before}
another atomic operation \placeholder{B} on \placeholder{M} if
\begin{itemize}
\item \placeholder{A} is a modification, and
\placeholder{B} reads the value stored by \placeholder{A}, or
\item \placeholder{A} precedes \placeholder{B}
in the modification order of \placeholder{M}, or
\item \placeholder{A} and \placeholder{B} are not
the same atomic read-modify-write operation, and
there exists an atomic modification \placeholder{X} of \placeholder{M}
such that \placeholder{A} reads the value stored by \placeholder{X} and
\placeholder{X} precedes \placeholder{B}
in the modification order of \placeholder{M}, or
\item there exists an atomic modification \placeholder{X} of \placeholder{M}
such that \placeholder{A} is coherence-ordered before \placeholder{X} and
\placeholder{X} is coherence-ordered before \placeholder{B}.
\end{itemize}
\pnum
There is a single total order \placeholder{S}
on all \tcode{memory_order::seq_cst} operations, including fences,
that satisfies the following constraints.
First, if \placeholder{A} and \placeholder{B} are
\tcode{memory_order::seq_cst} operations and
\placeholder{A} strongly happens before \placeholder{B},
then \placeholder{A} precedes \placeholder{B} in \placeholder{S}.
Second, for every pair of atomic operations \placeholder{A} and
\placeholder{B} on an object \placeholder{M},
where \placeholder{A} is coherence-ordered before \placeholder{B},
the following four conditions are required to be satisfied by \placeholder{S}:
\begin{itemize}
\item if \placeholder{A} and \placeholder{B} are both
\tcode{memory_order::seq_cst} operations,
then \placeholder{A} precedes \placeholder{B} in \placeholder{S}; and
\item if \placeholder{A} is a \tcode{memory_order::seq_cst} operation and
\placeholder{B} happens before
a \tcode{memory_order::seq_cst} fence \placeholder{Y},
then \placeholder{A} precedes \placeholder{Y} in \placeholder{S}; and
\item if a \tcode{memory_order::seq_cst} fence \placeholder{X}
happens before \placeholder{A} and
\placeholder{B} is a \tcode{memory_order::seq_cst} operation,
then \placeholder{X} precedes \placeholder{B} in \placeholder{S}; and
\item if a \tcode{memory_order::seq_cst} fence \placeholder{X}
happens before \placeholder{A} and
\placeholder{B} happens before
a \tcode{memory_order::seq_cst} fence \placeholder{Y},
then \placeholder{X} precedes \placeholder{Y} in \placeholder{S}.
\end{itemize}
\pnum
\begin{note}
This definition ensures that \placeholder{S} is consistent with
the modification order of any atomic object \placeholder{M}.
It also ensures that
a \tcode{memory_order::seq_cst} load \placeholder{A} of \placeholder{M}
gets its value either from the last modification of \placeholder{M}
that precedes \placeholder{A} in \placeholder{S} or
from some non-\tcode{memory_order::seq_cst} modification of \placeholder{M}
that does not happen before any modification of \placeholder{M}
that precedes \placeholder{A} in \placeholder{S}.
\end{note}
\pnum
\begin{note}
We do not require that \placeholder{S} be consistent with
``happens before''\iref{intro.races}.
This allows more efficient implementation
of \tcode{memory_order::acquire} and \tcode{memory_order::release}
on some machine architectures.
It can produce surprising results
when these are mixed with \tcode{memory_order::seq_cst} accesses.
\end{note}
\pnum
\begin{note}
\tcode{memory_order::seq_cst} ensures sequential consistency only
for a program that is free of data races and
uses exclusively \tcode{memory_order::seq_cst} atomic operations.
Any use of weaker ordering will invalidate this guarantee
unless extreme care is used.
In many cases, \tcode{memory_order::seq_cst} atomic operations are reorderable
with respect to other atomic operations performed by the same thread.
\end{note}
\pnum
Implementations should ensure that no ``out-of-thin-air'' values are computed that
circularly depend on their own computation.
\begin{note}
For example, with \tcode{x} and \tcode{y} initially zero,
\begin{codeblock}
// Thread 1:
r1 = y.load(memory_order::relaxed);
x.store(r1, memory_order::relaxed);
\end{codeblock}
\begin{codeblock}
// Thread 2:
r2 = x.load(memory_order::relaxed);
y.store(r2, memory_order::relaxed);
\end{codeblock}
should not produce \tcode{r1 == r2 == 42}, since the store of 42 to \tcode{y} is only
possible if the store to \tcode{x} stores \tcode{42}, which circularly depends on the
store to \tcode{y} storing \tcode{42}. Note that without this restriction, such an
execution is possible.
\end{note}
\pnum
\begin{note}
The recommendation similarly disallows \tcode{r1 == r2 == 42} in the
following example, with \tcode{x} and \tcode{y} again initially zero:
\begin{codeblock}
// Thread 1:
r1 = x.load(memory_order::relaxed);
if (r1 == 42) y.store(42, memory_order::relaxed);
\end{codeblock}
\begin{codeblock}
// Thread 2:
r2 = y.load(memory_order::relaxed);
if (r2 == 42) x.store(42, memory_order::relaxed);
\end{codeblock}
\end{note}
\pnum
Atomic read-modify-write operations shall always read the last value
(in the modification order) written before the write associated with
the read-modify-write operation.
\pnum
Implementations should make atomic stores visible to atomic loads within a reasonable
amount of time.
\indexlibraryglobal{kill_dependency}%
\begin{itemdecl}
template<class T>
T kill_dependency(T y) noexcept;
\end{itemdecl}
\begin{itemdescr}
\pnum
\effects
The argument does not carry a dependency to the return
value\iref{intro.multithread}.
\pnum
\returns
\tcode{y}.
\end{itemdescr}
\rSec1[atomics.lockfree]{Lock-free property}
\indexlibraryglobal{ATOMIC_BOOL_LOCK_FREE}%
\indexlibraryglobal{ATOMIC_CHAR_LOCK_FREE}%
\indexlibraryglobal{ATOMIC_CHAR8_T_LOCK_FREE}%
\indexlibraryglobal{ATOMIC_CHAR16_T_LOCK_FREE}%
\indexlibraryglobal{ATOMIC_CHAR32_T_LOCK_FREE}%
\indexlibraryglobal{ATOMIC_WCHAR_T_LOCK_FREE}%
\indexlibraryglobal{ATOMIC_SHORT_LOCK_FREE}%
\indexlibraryglobal{ATOMIC_INT_LOCK_FREE}%
\indexlibraryglobal{ATOMIC_LONG_LOCK_FREE}%
\indexlibraryglobal{ATOMIC_LLONG_LOCK_FREE}%
\indexlibraryglobal{ATOMIC_POINTER_LOCK_FREE}%
\indeximpldef{values of various \tcode{ATOMIC_..._LOCK_FREE} macros}
\begin{codeblock}
#define ATOMIC_BOOL_LOCK_FREE @\unspec@
#define ATOMIC_CHAR_LOCK_FREE @\unspec@
#define ATOMIC_CHAR8_T_LOCK_FREE @\unspec@
#define ATOMIC_CHAR16_T_LOCK_FREE @\unspec@
#define ATOMIC_CHAR32_T_LOCK_FREE @\unspec@
#define ATOMIC_WCHAR_T_LOCK_FREE @\unspec@
#define ATOMIC_SHORT_LOCK_FREE @\unspec@
#define ATOMIC_INT_LOCK_FREE @\unspec@
#define ATOMIC_LONG_LOCK_FREE @\unspec@
#define ATOMIC_LLONG_LOCK_FREE @\unspec@
#define ATOMIC_POINTER_LOCK_FREE @\unspec@
\end{codeblock}
\pnum
The \tcode{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
\tcode{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.
\pnum
At least one signed integral specialization of the \tcode{atomic} template,
along with the specialization
for the corresponding unsigned type\iref{basic.fundamental},
is always lock-free.
\begin{note}
This requirement is optional in freestanding implementations\iref{compliance}.
\end{note}
\pnum
The function \tcode{atomic_is_lock_free}\iref{atomics.types.operations}
indicates whether the object is lock-free. In any given program execution, the
result of the lock-free query shall be consistent for all pointers of the same
type.
\pnum
Atomic operations that are not lock-free are considered to potentially
block\iref{intro.progress}.
\pnum
\begin{note}
Operations that are lock-free should also be address-free. That is,
atomic operations on the same memory location via two different addresses will
communicate atomically. The implementation should not depend on any
per-process state. 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}
\rSec1[atomics.wait]{Waiting and notifying}
\pnum
\defnx{Atomic waiting operations}{atomic!waiting operation}
and \defnx{atomic notifying operations}{atomic!notifying operation}
provide a mechanism to wait for the value of an atomic object to change
more efficiently than can be achieved with polling.
An atomic waiting operation may block until it is unblocked
by an atomic notifying operation, according to each function's effects.
\begin{note}
Programs are not guaranteed to observe transient atomic values,
an issue known as the A-B-A problem,
resulting in continued blocking if a condition is only temporarily met.
\end{note}
\pnum
\begin{note}
The following functions are atomic waiting operations:
\begin{itemize}
\item \tcode{atomic<T>::wait},
\item \tcode{atomic_flag::wait},
\item \tcode{atomic_wait} and \tcode{atomic_wait_explicit},
\item \tcode{atomic_flag_wait} and \tcode{atomic_flag_wait_explicit}, and
\item \tcode{atomic_ref<T>::wait}.
\end{itemize}
\end{note}
\pnum
\begin{note}
The following functions are atomic notifying operations:
\begin{itemize}
\item \tcode{atomic<T>::notify_one} and \tcode{atomic<T>::notify_all},
\item \tcode{atomic_flag::notify_one} and \tcode{atomic_flag::notify_all},
\item \tcode{atomic_notify_one} and \tcode{atomic_notify_all},
\item \tcode{atomic_flag_notify_one} and \tcode{atomic_flag_notify_all}, and
\item \tcode{atomic_ref<T>::notify_one} and \tcode{atomic_ref<T>::notify_all}.
\end{itemize}
\end{note}
\indextext{atomic!waiting operation!eligible to be unblocked}%
\pnum
A call to an atomic waiting operation on an atomic object \tcode{M}
is \defn{eligible to be unblocked}
by a call to an atomic notifying operation on \tcode{M}
if there exist side effects \tcode{X} and \tcode{Y} on \tcode{M} such that:
\begin{itemize}
\item the atomic waiting operation has blocked after observing the result of \tcode{X},
\item \tcode{X} precedes \tcode{Y} in the modification order of \tcode{M}, and
\item \tcode{Y} happens before the call to the atomic notifying operation.
\end{itemize}
\rSec1[atomics.ref.generic]{Class template \tcode{atomic_ref}}
\indexlibraryglobal{atomic_ref}%
\indexlibrarymember{value_type}{atomic_ref}%
\begin{codeblock}
namespace std {
template<class T> struct atomic_ref {
private:
T* ptr; // \expos
public:
using value_type = T;
static constexpr size_t required_alignment = @\impdefx{required alignment for \tcode{atomic_ref} type's operations}@;
static constexpr bool is_always_lock_free = @\impdefx{whether a given \tcode{atomic_ref} type's operations are always lock free}@;
bool is_lock_free() const noexcept;
explicit atomic_ref(T&);
atomic_ref(const atomic_ref&) noexcept;
atomic_ref& operator=(const atomic_ref&) = delete;
void store(T, memory_order = memory_order_seq_cst) const noexcept;
T operator=(T) const noexcept;
T load(memory_order = memory_order_seq_cst) const noexcept;
operator T() const noexcept;
T exchange(T, memory_order = memory_order_seq_cst) const noexcept;
bool compare_exchange_weak(T&, T,
memory_order, memory_order) const noexcept;
bool compare_exchange_strong(T&, T,
memory_order, memory_order) const noexcept;
bool compare_exchange_weak(T&, T,
memory_order = memory_order_seq_cst) const noexcept;
bool compare_exchange_strong(T&, T,
memory_order = memory_order_seq_cst) const noexcept;
void wait(T, memory_order = memory_order::seq_cst) const noexcept;
void notify_one() const noexcept;
void notify_all() const noexcept;
};
}
\end{codeblock}
\pnum
An \tcode{atomic_ref} object applies atomic operations\iref{atomics.general} to
the object referenced by \tcode{*ptr} such that,
for the lifetime\iref{basic.life} of the \tcode{atomic_ref} object,
the object referenced by \tcode{*ptr} is an atomic object\iref{intro.races}.
\pnum
The template argument for \tcode{T}
shall be trivially copyable\iref{basic.types}.
\pnum
The lifetime\iref{basic.life} of an object referenced by \tcode{*ptr}
shall exceed the lifetime of all \tcode{atomic_ref}s that reference the object.
While any \tcode{atomic_ref} instances exist
that reference the \tcode{*ptr} object,
all accesses to that object shall exclusively occur
through those \tcode{atomic_ref} instances.
No subobject of the object referenced by \tcode{atomic_ref}
shall be concurrently referenced by any other \tcode{atomic_ref} object.
\pnum
Atomic operations applied to an object
through a referencing \tcode{atomic_ref} are atomic with respect to
atomic operations applied through any other \tcode{atomic_ref}
referencing the same object.
\begin{note}
Atomic operations or the \tcode{atomic_ref} constructor could acquire
a shared resource, such as a lock associated with the referenced object,
to enable atomic operations to be applied to the referenced object.
\end{note}
\rSec2[atomics.ref.ops]{Operations}
\indexlibrarymember{required_alignment}{atomic_ref}%
\indexlibrarymember{required_alignment}{atomic_ref<T*>}%
\indexlibrarymember{required_alignment}{atomic_ref<\placeholder{integral}>}%
\indexlibrarymember{required_alignment}{atomic_ref<\placeholder{floating-point}>}%
\begin{itemdecl}
static constexpr size_t required_alignment;
\end{itemdecl}
\begin{itemdescr}
\pnum
The alignment required for an object to be referenced by an atomic reference,
which is at least \tcode{alignof(T)}.
\pnum
\begin{note}
Hardware could require an object
referenced by an \tcode{atomic_ref}
to have stricter alignment\iref{basic.align}
than other objects of type \tcode{T}.
Further, whether operations on an \tcode{atomic_ref}
are lock-free could depend on the alignment of the referenced object.
For example, lock-free operations on \tcode{std::complex<double>}
could be supported only if aligned to \tcode{2*alignof(double)}.
\end{note}
\end{itemdescr}
\indexlibrarymember{is_always_lock_free}{atomic_ref}%
\indexlibrarymember{is_always_lock_free}{atomic_ref<T*>}%
\indexlibrarymember{is_always_lock_free}{atomic_ref<\placeholder{integral}>}%
\indexlibrarymember{is_always_lock_free}{atomic_ref<\placeholder{floating-point}>}%
\begin{itemdecl}
static constexpr bool is_always_lock_free;
\end{itemdecl}
\begin{itemdescr}
\pnum
The static data member \tcode{is_always_lock_free} is \tcode{true}
if the \tcode{atomic_ref} type's operations are always lock-free,
and \tcode{false} otherwise.
\end{itemdescr}
\indexlibrarymember{is_lock_free}{atomic_ref}%
\indexlibrarymember{is_lock_free}{atomic_ref<T*>}%
\indexlibrarymember{is_lock_free}{atomic_ref<\placeholder{integral}>}%
\indexlibrarymember{is_lock_free}{atomic_ref<\placeholder{floating-point}>}%
\begin{itemdecl}
bool is_lock_free() const noexcept;
\end{itemdecl}
\begin{itemdescr}
\pnum
\returns
\tcode{true} if operations on all objects of the type \tcode{atomic_ref<T>}
are lock-free,
\tcode{false} otherwise.
\end{itemdescr}
\indexlibraryctor{atomic_ref}%
\indexlibraryctor{atomic_ref<T*>}%
\indexlibrary{\idxcode{atomic_ref<\placeholder{integral}>}!constructor}%
\indexlibrary{\idxcode{atomic_ref<\placeholder{floating-point}>}!constructor}%
\begin{itemdecl}
atomic_ref(T& obj);
\end{itemdecl}
\begin{itemdescr}
\pnum
\requires The referenced object shall be aligned to \tcode{required_alignment}.
\pnum
\effects
Constructs an atomic reference that references the object.
\pnum
\throws
Nothing.
\end{itemdescr}
\indexlibraryctor{atomic_ref}%
\indexlibraryctor{atomic_ref<T*>}%
\indexlibrary{\idxcode{atomic_ref<\placeholder{integral}>}!constructor}%
\indexlibrary{\idxcode{atomic_ref<\placeholder{floating-point}>}!constructor}%
\begin{itemdecl}
atomic_ref(const atomic_ref& ref) noexcept;
\end{itemdecl}
\begin{itemdescr}
\pnum
\effects
Constructs an atomic reference
that references the object referenced by \tcode{ref}.
\end{itemdescr}
\indexlibrarymember{store}{atomic_ref}%
\indexlibrarymember{store}{atomic_ref<T*>}%
\indexlibrarymember{store}{atomic_ref<\placeholder{integral}>}%
\indexlibrarymember{store}{atomic_ref<\placeholder{floating-point}>}%
\begin{itemdecl}
void store(T desired, memory_order order = memory_order_seq_cst) const noexcept;
\end{itemdecl}
\begin{itemdescr}
\pnum
\requires The \tcode{order} argument shall not be
\tcode{memory_order_consume},
\tcode{memory_order_acquire}, nor
\tcode{memory_order_acq_rel}.
\pnum
\effects
Atomically replaces the value referenced by \tcode{*ptr}
with the value of \tcode{desired}.
Memory is affected according to the value of \tcode{order}.
\end{itemdescr}
\indexlibrarymember{operator=}{atomic_ref}%
\indexlibrarymember{operator=}{atomic_ref<T*>}%
\indexlibrarymember{operator=}{atomic_ref<\placeholder{integral}>}%
\indexlibrarymember{operator=}{atomic_ref<\placeholder{floating-point}>}%
\begin{itemdecl}
T operator=(T desired) const noexcept;
\end{itemdecl}
\begin{itemdescr}
\pnum
\effects
Equivalent to:
\begin{codeblock}
store(desired);
return desired;
\end{codeblock}
\end{itemdescr}
\indexlibrarymember{load}{atomic_ref}%
\indexlibrarymember{load}{atomic_ref<T*>}%
\indexlibrarymember{load}{atomic_ref<\placeholder{integral}>}%
\indexlibrarymember{load}{atomic_ref<\placeholder{floating-point}>}%
\begin{itemdecl}
T load(memory_order order = memory_order_seq_cst) const noexcept;
\end{itemdecl}
\begin{itemdescr}
\pnum
\requires The \tcode{order} argument shall not be
\tcode{memory_order_release} nor \tcode{memory_order_acq_rel}.
\pnum
\effects
Memory is affected according to the value of \tcode{order}.
\pnum
\returns
Atomically returns the value referenced by \tcode{*ptr}.
\end{itemdescr}
\indexlibrarymember{operator \placeholder{type}}{atomic_ref}%
\indexlibrarymember{operator T*}{atomic_ref<T*>}%
\indexlibrarymember{operator \placeholder{integral}}{atomic_ref<\placeholder{integral}>}%
\indexlibrarymember{operator \placeholder{floating-point}}{atomic_ref<\placeholder{floating-point}>}%
\begin{itemdecl}
operator T() const noexcept;
\end{itemdecl}
\begin{itemdescr}
\pnum
\effects
Equivalent to: \tcode{return load();}
\end{itemdescr}
\indexlibrarymember{exchange}{atomic_ref}%
\indexlibrarymember{exchange}{atomic_ref<T*>}%
\indexlibrarymember{exchange}{atomic_ref<\placeholder{integral}>}%
\indexlibrarymember{exchange}{atomic_ref<\placeholder{floating-point}>}%
\begin{itemdecl}
T exchange(T desired, memory_order order = memory_order_seq_cst) const noexcept;
\end{itemdecl}
\begin{itemdescr}
\pnum
\effects
Atomically replaces the value referenced by \tcode{*ptr}
with \tcode{desired}.
Memory is affected according to the value of \tcode{order}.
This operation is an atomic read-modify-write operation\iref{intro.multithread}.
\pnum
\returns
Atomically returns the value referenced by \tcode{*ptr}
immediately before the effects.
\end{itemdescr}
\indexlibrarymember{compare_exchange_weak}{atomic_ref}%
\indexlibrarymember{compare_exchange_weak}{atomic_ref<T*>}%
\indexlibrarymember{compare_exchange_weak}{atomic_ref<\placeholder{integral}>}%
\indexlibrarymember{compare_exchange_weak}{atomic_ref<\placeholder{floating-point}>}%
\indexlibrarymember{compare_exchange_strong}{atomic_ref}%
\indexlibrarymember{compare_exchange_strong}{atomic_ref<T*>}%
\indexlibrarymember{compare_exchange_strong}{atomic_ref<\placeholder{integral}>}%
\indexlibrarymember{compare_exchange_strong}{atomic_ref<\placeholder{floating-point}>}%
\begin{itemdecl}
bool compare_exchange_weak(T& expected, T desired,
memory_order success, memory_order failure) const noexcept;
bool compare_exchange_strong(T& expected, T desired,
memory_order success, memory_order failure) const noexcept;
bool compare_exchange_weak(T& expected, T desired,
memory_order order = memory_order_seq_cst) const noexcept;
bool compare_exchange_strong(T& expected, T desired,
memory_order order = memory_order_seq_cst) const noexcept;
\end{itemdecl}
\begin{itemdescr}
\pnum
\requires The \tcode{failure} argument shall not be
\tcode{memory_order_release} nor \tcode{memory_order_acq_rel}.
\pnum
\effects
Retrieves the value in \tcode{expected}.
It then atomically compares the value representation of
the value referenced by \tcode{*ptr} for equality
with that previously retrieved from \tcode{expected},
and if \tcode{true}, replaces the value referenced by \tcode{*ptr}
with that in \tcode{desired}.
If and only if the comparison is \tcode{true},
memory is affected according to the value of \tcode{success}, and
if the comparison is \tcode{false},