std::atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE

< cpp‎ | atomic
Concurrency support library
this_thread namespace
Mutual exclusion
Generic lock management
Condition variables
Latches and Barriers
Safe Reclamation
Hazard Pointers

Atomic types
Initialization of atomic types
(C++11)(deprecated in C++20)
(C++11)(deprecated in C++20)
Memory ordering
Free functions for atomic operations
Free functions for atomic flags
Defined in header <atomic>
template< class T >
bool atomic_is_lock_free( const volatile std::atomic<T>* obj ) noexcept;
(1) (since C++11)
template< class T >
bool atomic_is_lock_free( const std::atomic<T>* obj ) noexcept;
(2) (since C++11)
#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 */
(3) (since C++11)
#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.


[edit] Parameters

obj - pointer to the atomic object to examine

[edit] Return value

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

[edit] 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.

[edit] Example

#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

[edit] 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

[edit] See also

checks if the atomic object is lock-free
(public member function of std::atomic<T>) [edit]
the lock-free boolean atomic type
(class) [edit]
[static] (C++17)
indicates that the type is always lock-free
(public static member constant of std::atomic<T>) [edit]
(deprecated in C++20)(removed in C++26)
specializes atomic operations for std::shared_ptr
(function template)
C documentation for atomic_is_lock_free
C documentation for ATOMIC_*_LOCK_FREE