29 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 ]

Returns: True if the object's operations are lock-free, false otherwise.

Effects: Non-atomically initializes *object with value desired. This function shall only be applied to objects that have been default constructed, and then only once. [ Note: These semantics ensure compatibility with C.  — end note ] [ Note: Concurrent access from another thread, even via an atomic operation, constitutes a data race.  — 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 object or by this with the value of desired. Memory is affected according to the value of order.

Effects: 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 object or by this.

Effects: load()

Returns: The result of load().

Effects: Atomically replaces the value pointed to by object or 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 object or by this immediately before the effects.

Requires: The failure argument shall not be memory_order_release nor memory_order_acq_rel. The failure argument shall be no stronger than the success argument.

Effects: Atomically, compares the contents of the memory pointed to by object or by this for equality with that in expected, and if true, replaces the contents of the memory pointed to by object or by this with that in desired, and if false, updates the contents of the memory in expected with the contents of the memory pointed to by object or by this. Further, 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 the operation returns true, these operations are atomic read-modify-write operations ([intro.multithread]). Otherwise, these operations are atomic load operations.

Returns: The result of the comparison.

[ Note: For example, the effect of atomic_compare_exchange_strong is

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

 — 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 ]

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.

Remark: A weak compare-and-exchange operation may fail spuriously. That is, even when the contents of memory referred to by expected and object 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 object or by this with the result of the computation applied to the value pointed to by object or 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 object or by this immediately before the effects.

Remark: For signed integer types, arithmetic is defined to use two's complement representation. There are no undefined results. For address types, the result may be an undefined address, but the operations otherwise have no undefined behavior.

Effects: fetch_key(operand)

Returns: fetch_key(operand) op operand

Returns: fetch_add(1)

Returns: fetch_sub(1)

Effects: fetch_add(1)

Returns: fetch_add(1) + 1

Effects: fetch_sub(1)

Returns: fetch_sub(1) - 1