std::atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE
<atomic>
bool atomic_is_lock_free( const volatile std::atomic <T>* obj ) noexcept;
bool atomic_is_lock_free( const std::atomic <T>* obj ) noexcept;
#define ATOMIC_CHAR_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 */
- 0 for the built-in atomic types that are never lock-free,
- 1 for the built-in atomic types that are sometimes lock-free,
- 2 for the built-in atomic types that are always lock-free.
[edit] Parameters
[edit] Return value
true if *obj is a lock-free atomic, false otherwise.
[edit] Notes
All atomic types except for std::atomic_flag may be implemented using mutexes or other locking operations, rather than using the lock-free atomic CPU instructions. Atomic types are also allowed to be sometimes lock-free: for example, if only some sub-architectures support lock-free atomic access for a given type (such as the CMPXCHG16B instruction on x86-64), whether atomics are lock-free may not be known until runtime.
The C++ standard recommends (but does not require) that lock-free atomic operations are also address-free, that is, suitable for communication between processes using shared memory.
[edit] Example
#include <atomic> #include <iostream> #include <utility> struct A { int a[4]; }; struct B { int x, y; }; int main() { std::atomic <A> a; std::atomic <B> b; std::cout << std::boolalpha << "std::atomic<A> is lock free? " << std::atomic_is_lock_free(&a) << '\n' << "std::atomic<B> is lock free? " << std::atomic_is_lock_free(&b) << '\n'; }
Possible output:
std::atomic<A> is lock free? false std::atomic<B> is lock free? true
[edit] Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3249 | C++11 | atomic_is_lock_free was specified via pointers, whichwas ambiguous and might accept invalid pointer values |
specified via atomic objects |
[edit] See also
(public static member constant of
std::atomic<T>) [edit]
(function template)