NAME

std::atomic_is_lock_free,ATOMIC_xxx_LOCK_FREE - std::atomic_is_lock_free,ATOMIC_xxx_LOCK_FREE

Synopsis

Defined in header <atomic>
template< class T >
bool atomic_is_lock_free( const volatile
std::atomic<T>* obj ) noexcept;
template< class T >
bool atomic_is_lock_free( const std::atomic<T>*
obj ) noexcept;
#define ATOMIC_BOOL_LOCK_FREE /* unspecified */

#define ATOMIC_CHAR_LOCK_FREE /* unspecified */
#define ATOMIC_CHAR16_T_LOCK_FREE /* unspecified
*/
#define ATOMIC_CHAR32_T_LOCK_FREE /* unspecified (1) (since C++11)
*/
#define ATOMIC_WCHAR_T_LOCK_FREE /* unspecified (2) (since C++11)
*/
#define ATOMIC_SHORT_LOCK_FREE /* unspecified */
#define ATOMIC_INT_LOCK_FREE /* unspecified */
#define ATOMIC_LONG_LOCK_FREE /* unspecified */
#define ATOMIC_LLONG_LOCK_FREE /* unspecified */

#define ATOMIC_POINTER_LOCK_FREE /* unspecified
*/
#define ATOMIC_CHAR8_T_LOCK_FREE /* unspecified (3) (since C++20)
*/

1) Determines if the atomic object pointed to by obj is implemented lock-free, as if
by calling obj->is_lock_free(). In any given program execution, the result of the
lock-free query is the same for all atomic objects of the same type.
2,3) Expands to an integer constant expression with value

* 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.

Parameters

obj - pointer to the atomic object to examine

Return value

true if *obj is a lock-free atomic, false otherwise.

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
subarchitectures 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.

Example

// Run this code

#include <iostream>
#include <utility>
#include <atomic>

struct A { int a[100]; };
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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
atomic_is_lock_free was specified via
LWG 3249 C++11 pointers, which was specified via atomic
ambiguous and might accept invalid pointer objects
values

See also

is_lock_free checks if the atomic object is lock-free
(public member function of std::atomic<T>)
specializes atomic operations for
std::atomic_is_lock_free(std::shared_ptr) std::shared_ptr
(function template)
atomic_flag the lock-free boolean atomic type
(C++11) (class)
indicates that the type is always
is_always_lock_free lock-free
[static] (C++17) (public static member constant of
std::atomic<T>)