std::atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE

 

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Threads thread (C++11) jthread (C++20) stop_token (C++20) stop_source (C++20) stop_callback (C++20) hardware_destructive_interference_sizehardware_constructive_interference_size (C++17)(C++17)     this_thread namespace get_id (C++11) yield (C++11) sleep_for (C++11) sleep_until (C++11) Atomic types atomic (C++11) atomic_ref (C++20) atomic_flag (C++11) Initialization of atomic types atomic_init (C++11)(deprecated in C++20) ATOMIC_VAR_INIT (C++11)(deprecated in C++20) ATOMIC_FLAG_INIT (C++11) Free functions for atomic operations atomic_storeatomic_store_explicit (C++11)(C++11) atomic_loadatomic_load_explicit (C++11)(C++11) atomic_exchangeatomic_exchange_explicit (C++11)(C++11) atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11) atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11) atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11) atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11) atomic_fetch_oratomic_fetch_or_explicit (C++11)(C++11) atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11) atomic_is_lock_free (C++11) atomic_waitatomic_wait_explicit (C++20)(C++20) atomic_notify_one (C++20) atomic_notify_all (C++20)

Free functions for atomic flags atomic_flag_test_and_setatomic_flag_test_and_set_explicit (C++11)(C++11) atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11) atomic_flag_testatomic_flag_test_explicit (C++20)(C++20) atomic_flag_waitatomic_flag_wait_explicit (C++20)(C++20) atomic_flag_notify_one (C++20) atomic_flag_notify_all (C++20) Memory ordering memory_order (C++11) kill_dependency (C++11) atomic_thread_fence (C++11) atomic_signal_fence (C++11) Mutual exclusion mutex (C++11) recursive_mutex (C++11) shared_mutex (C++17) timed_mutex (C++11) recursive_timed_mutex (C++11) shared_timed_mutex (C++14) Generic lock management lock_guard (C++11) scoped_lock (C++17) unique_lock (C++11) shared_lock (C++14) defer_lock_ttry_to_lock_tadopt_lock_t (C++11)(C++11)(C++11) lock (C++11) try_lock (C++11) defer_locktry_to_lockadopt_lock (C++11)(C++11)(C++11) once_flag (C++11) call_once (C++11) Condition variables condition_variable (C++11) condition_variable_any (C++11) notify_all_at_thread_exit (C++11) cv_status (C++11) Semaphores counting_semaphorebinary_semaphore (C++20)(C++20) Latches and barriers latch (C++20) barrier (C++20) Futures promise (C++11) future (C++11) shared_future (C++11) packaged_task (C++11) async (C++11) launch (C++11) future_status (C++11) future_error (C++11) future_category (C++11) future_errc (C++11)

 

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

See also

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>)
C documentation for atomic_is_lock_free
C documentation for ATOMIC_*_LOCK_FREE