std::atomic_is_lock_free, ATOMIC_xxx_LOCK_FREE
Header: <atomic>
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.
# Declarations
template< class T >
bool atomic_is_lock_free( const volatile std::atomic<T>* obj ) noexcept;
(since C++11)
template< class T >
bool atomic_is_lock_free( const std::atomic<T>* obj ) noexcept;
(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 */
(since C++11)
#define ATOMIC_CHAR8_T_LOCK_FREE /* unspecified */
(since C++20)
# 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
#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';
}
# Defect reports
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3249 | C++11 | atomic_is_lock_free was specified via pointers, whichwas ambiguous and might accept invalid pointer values | specified viaatomic objects |