This macro provides a simple control construct to bracket a piece of code that should only be executed once during the course of a multithreaded program. Usage: ` BSLMT_ONCE_DO { /* one‐time code goes here */ } ` Leaving a BSLMT_ONCE_DO construct via break, continue, or return will put the construct in a "done" state (unless BSLMT_ONCE_CANCEL has been called) and will unblock all threads waiting to enter the one‐time region. Note that a break or continue within the one‐time code will terminate only the BSLMT_ONCE_DO construct, not any surrounding loop or switch statement. Due to a bug in the Microsoft Visual C++ 2003 compiler, the behavior is undefined if an exception is thrown from within this construct and is not caught within the same construct. Only one call to BSLMT_ONCE_DO may appear on a single source‐code line in any code block.