From docs:
EV_DEFAULT_UC, EV_DEFAULT_UC_
Usage identical to EV_DEFAULT and EV_DEFAULT_, but requires that the
default loop has been initialised (UC == unchecked). Their behaviour
is undefined when the default loop has not been initialised by a
previous execution of EV_DEFAULT, EV_DEFAULT_ or ev_default_init
(...). It is often prudent to use EV_DEFAULT when initialising the
first watcher in a function but use EV_DEFAULT_UC afterwards.