This macro provides a way to cancel once processing within a BSLMT_ONCE_DO construct. It will not compile outside of a BSLMT_ONCE_DO construct. Executing this function‐like macro will set the state of the BSLMT_ONCE_DO construct to "not entered", possibly unblocking a thread waiting to enter the one‐time code region. Note that this macro does not exit the BSLMT_ONCE_DO construct (i.e., it does not have break or return semantics).