std::atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE

Defined in header <atomic>
(1) (since C++11)
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 */
#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 */
#define ATOMIC_POINTER_LOCK_FREE  /* unspecified */
(2) (since C++11)
#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.


obj - pointer to the atomic object to examine

Return value

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


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.


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

See also

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

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