Atomics

Overview

The _Atomic(...) specifier is used to declare an atomic object. Operations on atomic objects are synchronized across threads. Additionally, if the atomic type is lock-free, then the corresponding operations are uninterruptible.

_Atomic cannot be applied to array types.

Test-and-set

The atomic_flag type, found in <stdatomic.h>, provides the test-and-set functionality. It should be initialized to the clear state by assigning to macro ATOMIC_FLAG_INIT.

Lock-Free Property

The lock-free property of a C type indicates that no locking occurs internally. As a result, operations on objects of the type are uninterruptible. This property can be checked for _Atomic(T) via macro ATOMIC_<T>_LOCK_FREE. Values are interpreted as:

• 0 indicates the type is never lock-free.
• 1 indicates the type is sometimes lock-free.
• 2 indicates the type is always lock-free.