void init_sync_flag(struct sync_flag *s) {
assert(pthread_mutex_init(&s->mutex, NULL) == 0);
assert(pthread_cond_init(&s->cond, NULL) == 0);
void init_sync_flag(struct sync_flag *s) {
assert(pthread_mutex_init(&s->mutex, NULL) == 0);
assert(pthread_cond_init(&s->cond, NULL) == 0);