31 Atomic operations library [atomics]

31.10 Flag type and operations [atomics.flag]

namespace std {
  struct atomic_flag {
    constexpr atomic_flag() noexcept;
    atomic_flag(const atomic_flag&) = delete;
    atomic_flag& operator=(const atomic_flag&) = delete;
    atomic_flag& operator=(const atomic_flag&) volatile = delete;

    bool test(memory_order = memory_order::seq_cst) const volatile noexcept;
    bool test(memory_order = memory_order::seq_cst) const noexcept;
    bool test_and_set(memory_order = memory_order::seq_cst) volatile noexcept;
    bool test_and_set(memory_order = memory_order::seq_cst) noexcept;
    void clear(memory_order = memory_order::seq_cst) volatile noexcept;
    void clear(memory_order = memory_order::seq_cst) noexcept;

    void wait(bool, memory_order = memory_order::seq_cst) const volatile noexcept;
    void wait(bool, memory_order = memory_order::seq_cst) const noexcept;
    void notify_one() volatile noexcept;
    void notify_one() noexcept;
    void notify_all() volatile noexcept;
    void notify_all() noexcept;
  };
}
1
#
The atomic_­flag type provides the classic test-and-set functionality.
It has two states, set and clear.
2
#
Operations on an object of type atomic_­flag shall be lock-free.
[Note
:
Hence the operations should also be address-free.
— end note
]
3
#
The atomic_­flag type is a standard-layout struct.
It has a trivial destructor.
🔗
constexpr atomic_flag::atomic_flag() noexcept;
4
#
Effects: Initializes *this to the clear state.
🔗
bool atomic_flag_test(const volatile atomic_flag* object) noexcept; bool atomic_flag_test(const atomic_flag* object) noexcept; bool atomic_flag_test_explicit(const volatile atomic_flag* object, memory_order order) noexcept; bool atomic_flag_test_explicit(const atomic_flag* object, memory_order order) noexcept; bool atomic_flag::test(memory_order order = memory_order::seq_cst) const volatile noexcept; bool atomic_flag::test(memory_order order = memory_order::seq_cst) const noexcept;
5
#
For atomic_­flag_­test, let order be memory_­order​::​seq_­cst.
6
#
Preconditions: order is neither memory_­order​::​release nor memory_­order​::​acq_­rel.
7
#
Effects: Memory is affected according to the value of order.
8
#
Returns: Atomically returns the value pointed to by object or this.
🔗
bool atomic_flag_test_and_set(volatile atomic_flag* object) noexcept; bool atomic_flag_test_and_set(atomic_flag* object) noexcept; bool atomic_flag_test_and_set_explicit(volatile atomic_flag* object, memory_order order) noexcept; bool atomic_flag_test_and_set_explicit(atomic_flag* object, memory_order order) noexcept; bool atomic_flag::test_and_set(memory_order order = memory_order::seq_cst) volatile noexcept; bool atomic_flag::test_and_set(memory_order order = memory_order::seq_cst) noexcept;
9
#
Effects: Atomically sets the value pointed to by object or by this to true.
Memory is affected according to the value of order.
These operations are atomic read-modify-write operations ([intro.multithread]).
10
#
Returns: Atomically, the value of the object immediately before the effects.
🔗
void atomic_flag_clear(volatile atomic_flag* object) noexcept; void atomic_flag_clear(atomic_flag* object) noexcept; void atomic_flag_clear_explicit(volatile atomic_flag* object, memory_order order) noexcept; void atomic_flag_clear_explicit(atomic_flag* object, memory_order order) noexcept; void atomic_flag::clear(memory_order order = memory_order::seq_cst) volatile noexcept; void atomic_flag::clear(memory_order order = memory_order::seq_cst) noexcept;
11
#
Preconditions: The order argument is neither memory_­order​::​consume, memory_­order​::​acquire, nor memory_­order​::​acq_­rel.
12
#
Effects: Atomically sets the value pointed to by object or by this to false.
Memory is affected according to the value of order.
🔗
void atomic_flag_wait(const volatile atomic_flag* object, bool old) noexcept; void atomic_flag_wait(const atomic_flag* object, bool old) noexcept; void atomic_flag_wait_explicit(const volatile atomic_flag* object, bool old, memory_order order) noexcept; void atomic_flag_wait_explicit(const atomic_flag* object, bool old, memory_order order) noexcept; void atomic_flag::wait(bool old, memory_order order = memory_order::seq_cst) const volatile noexcept; void atomic_flag::wait(bool old, memory_order order = memory_order::seq_cst) const noexcept;
13
#
For atomic_­flag_­wait, let order be memory_­order​::​seq_­cst.
Let flag be object for the non-member functions and this for the member functions.
14
#
Preconditions: order is neither memory_­order​::​release nor memory_­order​::​acq_­rel.
15
#
Effects: Repeatedly performs the following steps, in order:
  • (15.1)
    Evaluates flag->test(order) != old.
  • (15.2)
    If the result of that evaluation is true, returns.
  • (15.3)
    Blocks until it is unblocked by an atomic notifying operation or is unblocked spuriously.
16
#
Remarks: This function is an atomic waiting operation ([atomics.wait]).
🔗
void atomic_flag_notify_one(volatile atomic_flag* object) noexcept; void atomic_flag_notify_one(atomic_flag* object) noexcept; void atomic_flag::notify_one() volatile noexcept; void atomic_flag::notify_one() noexcept;
17
#
Effects: Unblocks the execution of at least one atomic waiting operation that is eligible to be unblocked ([atomics.wait]) by this call, if any such atomic waiting operations exist.
18
#
Remarks: This function is an atomic notifying operation ([atomics.wait]).
🔗
void atomic_flag_notify_all(volatile atomic_flag* object) noexcept; void atomic_flag_notify_all(atomic_flag* object) noexcept; void atomic_flag::notify_all() volatile noexcept; void atomic_flag::notify_all() noexcept;
19
#
Effects: Unblocks the execution of all atomic waiting operations that are eligible to be unblocked ([atomics.wait]) by this call.
20
#
Remarks: This function is an atomic notifying operation ([atomics.wait]).