std::atomic_is_lock_free,ATOMIC_xxx_LOCK_FREE (3) - Linux Manuals

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


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


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 pointers 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.


// 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

See also

                                          checks if the atomic object is lock-free
is_lock_free (public member function of std::atomic<T>)
                                          specializes atomic operations for std::shared_ptr
std::atomic_is_lock_free(std::shared_ptr) (function template)

atomic_flag the lock-free boolean atomic type

is_always_lock_free indicates that the type is always lock-free
                                          (public static member constant of std::atomic<T>)
[static] (C++17)