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 ) (1) (since C++11)
noexcept;
template< class T > (2) (since C++11)
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 */
#define ATOMIC_WCHAR_T_LOCK_FREE /* unspecified */ (3) (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 */ (4) (since C++20)

1,2) 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.
3,4) 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 <atomic>
#include <iostream>
#include <utility>

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 pointers,
LWG 3249 C++11 which specified via
was ambiguous and might accept invalid pointer atomic objects
values

See also¶

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

std::atomic_is_lock_free(std::shared_ptr) specializes atomic operations for
(deprecated in C++20) std::shared_ptr
(removed in C++26) (function template)
C documentation for
atomic_is_lock_free
C documentation for
ATOMIC_*_LOCK_FREE