Annex D (normative) Compatibility features [depr]

D.25 Deprecated atomic operations [depr.atomics]

D.25.5 Flag type and operations [depr.atomics.flag]

#define ATOMIC_FLAG_INIT see below
Remarks: The macro ATOMIC_­FLAG_­INIT is defined in such a way that it can be used to initialize an object of type atomic_­flag to the clear state.
The macro can be used in the form: atomic_flag guard = ATOMIC_FLAG_INIT;
It is unspecified whether the macro can be used in other initialization contexts.
For a complete static-duration object, that initialization shall be static.