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.

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
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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Technical specifications
Symbols index
External libraries

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