32 Atomic operations library [atomics]

Effects: Leaves the atomic object in an uninitialized state. [ Note: These semantics ensure compatibility with C.  — end note ]

Effects: Initializes the object with the value desired. Initialization is not an atomic operation ([intro.multithread]). [ Note: It is possible to have an access to an atomic object A race with its construction, for example by communicating the address of the just-constructed object A to another thread via memory_­order_­relaxed operations on a suitable atomic pointer variable, and then immediately accessing A in the receiving thread. This results in undefined behavior.  — end note ]

The macro expands to a token sequence suitable for constant initialization of an atomic variable of static storage duration of a type that is initialization-compatible with value. [ Note: This operation may need to initialize locks.  — end note ] Concurrent access to the variable being initialized, even via an atomic operation, constitutes a data race. [ Example:

atomic<int> v = ATOMIC_VAR_INIT(5);

 — end example ]

The static data member is_­always_­lock_­free is true if the atomic type's operations are always lock-free, and false otherwise. [ Note: The value of is_­always_­lock_­free is consistent with the value of the corresponding ATOMIC_­..._­LOCK_­FREE macro, if defined.  — end note ]

Returns: true if the object's operations are lock-free, false otherwise. [ Note: The return value of the is_­lock_­free member function is consistent with the value of is_­always_­lock_­free for the same type.  — end note ]

Requires: The order argument shall not be memory_­order_­consume, memory_­order_­acquire, nor memory_­order_­acq_­rel.

Effects: Atomically replaces the value pointed to by this with the value of desired. Memory is affected according to the value of order.

Effects: Equivalent to: store(desired).

Returns: desired.

Requires: The order argument shall not be memory_­order_­release nor memory_­order_­acq_­rel.

Effects: Memory is affected according to the value of order.

Returns: Atomically returns the value pointed to by this.

Effects: Equivalent to: return load();

Effects: Atomically replaces the value pointed to by this with desired. Memory is affected according to the value of order. These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically returns the value pointed to by this immediately before the effects.

Requires: The failure argument shall not be memory_­order_­release nor memory_­order_­acq_­rel.

Effects: Retrieves the value in expected. It then atomically compares the contents of the memory pointed to by this for equality with that previously retrieved from expected, and if true, replaces the contents of the memory pointed to by this with that in desired. If and only if the comparison is true, memory is affected according to the value of success, and if the comparison is false, memory is affected according to the value of failure. When only one memory_­order argument is supplied, the value of success is order, and the value of failure is order except that a value of memory_­order_­acq_­rel shall be replaced by the value memory_­order_­acquire and a value of memory_­order_­release shall be replaced by the value memory_­order_­relaxed. If and only if the comparison is false then, after the atomic operation, the contents of the memory in expected are replaced by the value read from the memory pointed to by this during the atomic comparison. If the operation returns true, these operations are atomic read-modify-write operations ([intro.multithread]) on the memory pointed to by this. Otherwise, these operations are atomic load operations on that memory.

Returns: The result of the comparison.

[ Note: For example, the effect of compare_­exchange_­strong is

if (memcmp(this, &expected, sizeof(*this)) == 0)
  memcpy(this, &desired, sizeof(*this));
else
  memcpy(expected, this, sizeof(*this));

 — end note ] [ Example: The expected use of the compare-and-exchange operations is as follows. The compare-and-exchange operations will update expected when another iteration of the loop is needed.

expected = current.load();
do {
  desired = function(expected);
} while (!current.compare_exchange_weak(expected, desired));

 — end example ] [ Example: Because the expected value is updated only on failure, code releasing the memory containing the expected value on success will work. E.g. list head insertion will act atomically and would not introduce a data race in the following code:

do {
  p->next = head; // make new list node point to the current head
} while (!head.compare_exchange_weak(p->next, p)); // try to insert

 — end example ]

Implementations should ensure that weak compare-and-exchange operations do not consistently return false unless either the atomic object has value different from expected or there are concurrent modifications to the atomic object.

Remarks: A weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory referred to by expected and this are equal, it may return false and store back to expected the same memory contents that were originally there. [ Note: This spurious failure enables implementation of compare-and-exchange on a broader class of machines, e.g., load-locked store-conditional machines. A consequence of spurious failure is that nearly all uses of weak compare-and-exchange will be in a loop.

When a compare-and-exchange is in a loop, the weak version will yield better performance on some platforms. When a weak compare-and-exchange would require a loop and a strong one would not, the strong one is preferable.  — end note ]

[ Note: The memcpy and memcmp semantics of the compare-and-exchange operations may result in failed comparisons for values that compare equal with operator== if the underlying type has padding bits, trap bits, or alternate representations of the same value. Thus, compare_­exchange_­strong should be used with extreme care. On the other hand, compare_­exchange_­weak should converge rapidly.  — end note ]

Effects: Atomically replaces the value pointed to by this with the result of the computation applied to the value pointed to by this and the given operand. Memory is affected according to the value of order. These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically, the value pointed to by this immediately before the effects.

Remarks: For signed integer types, arithmetic is defined to use two's complement representation. There are no undefined results.

Effects: Equivalent to: return fetch_­key(operand) op operand;

Requires: T shall be an object type, otherwise the program is ill-formed. [ Note: Pointer arithmetic on void* or function pointers is ill-formed.  — end note ]

Effects: Atomically replaces the value pointed to by this with the result of the computation applied to the value pointed to by this and the given operand. Memory is affected according to the value of order. These operations are atomic read-modify-write operations ([intro.multithread]).

Returns: Atomically, the value pointed to by this immediately before the effects.

Remarks: The result may be an undefined address, but the operations otherwise have no undefined behavior.

Effects: Equivalent to: return fetch_­key(operand) op operand;

Effects: Equivalent to: return fetch_­add(1) + 1;

Effects: Equivalent to: return fetch_­sub(1) - 1;