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std::atomic_is_lock_free,ATOMIC_xxx_LOCK_FREE(3) | C++ Standard Libary | std::atomic_is_lock_free,ATOMIC_xxx_LOCK_FREE(3) |
NAME¶
std::atomic_is_lock_free,ATOMIC_xxx_LOCK_FREE - std::atomic_is_lock_free,ATOMIC_xxx_LOCK_FREE
Synopsis¶
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 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
atomic_is_lock_free was specified via
LWG 3249 C++11 pointers, which was specified via atomic
ambiguous and might accept invalid pointer objects
values
See also¶
is_lock_free checks if the atomic object is lock-free
(public member function of std::atomic<T>)
specializes atomic operations for
std::atomic_is_lock_free(std::shared_ptr) std::shared_ptr
(function template)
atomic_flag the lock-free boolean atomic type
(C++11) (class)
indicates that the type is always
is_always_lock_free lock-free
[static] (C++17) (public static member
constant of
std::atomic<T>)
2022.07.31 | http://cppreference.com |