is_engine_available
if (engine->funcs->is_engine_available) {
if (!engine->funcs->is_engine_available(engine))
.is_engine_available = is_engine_available,
if (aux_engine->funcs->is_engine_available) {
if (!aux_engine->funcs->is_engine_available(aux_engine)) {
bool (*is_engine_available) (
.is_engine_available = is_engine_available,
bool (*is_engine_available)(struct aux_engine *engine);