Cancel all operations associated with the signal set.
void cancel( asio::error_code & ec);
This function forces the completion of any pending asynchronous wait
operations against the signal set. The handler for each cancelled operation
will be invoked with the
Cancellation does not alter the set of registered signals.
Set to indicate what error occurred, if any.
If a registered signal occurred before
cancel() is called,
then the handlers for asynchronous wait operations will:
These handlers can no longer be cancelled, and therefore are passed an error code that indicates the successful completion of the wait operation.