Construct a signal set and add three signals.
basic_signal_set( asio::io_service & io_service, int signal_number_1, int signal_number_2, int signal_number_3);
This constructor creates a signal set and registers for three signals.
object that the signal set will use to dispatch handlers for any
asynchronous operations performed on the set.
The first signal number to be added.
The second signal number to be added.
The third signal number to be added.
This constructor is equivalent to performing:
asio::signal_set signals(io_service); signals.add(signal_number_1); signals.add(signal_number_2); signals.add(signal_number_3);