Mosquitto-1.5.4源码分析，主题订阅的数据结构及SUBSCRIBE的函数跳转关系

int db__open(struct mosquitto__config *config, struct mosquitto_db *db)
{
  struct mosquitto__subhier *subhier;
  if(!config || !db) return MOSQ_ERR_INVAL;
  db->last_db_id = 0;
  db->contexts_by_id = NULL;
  db->contexts_by_sock = NULL;
  db->contexts_for_free = NULL;
#ifdef WITH_BRIDGE
  db->bridges = NULL;
  db->bridge_count = 0;
#endif
  // Initialize the hashtable
  db->clientid_index_hash = NULL;
  db->subs = NULL;
    subhier = sub__add_hier_entry(NULL, &db->subs, "", strlen(""));//业务子树根节点
  if(!subhier) return MOSQ_ERR_NOMEM;
    subhier = sub__add_hier_entry(NULL, &db->subs, "$SYS", strlen("$SYS"));//系统子树根节点
  if(!subhier) return MOSQ_ERR_NOMEM;
subs.c，会创建内存
child = mosquitto__malloc(sizeof(struct mosquitto__subhier));
struct mosquitto__subhier *sub__add_hier_entry(struct mosquitto__subhier *parent, struct mosquitto__subhier **sibling, const char *topic, size_t len)
{
  struct mosquitto__subhier *child;
  assert(sibling);
  child = mosquitto__malloc(sizeof(struct mosquitto__subhier));
  if(!child){
  log__printf(NULL, MOSQ_LOG_ERR, "Error: Out of memory.");
  return NULL;
  }
  child->parent = parent;
  child->topic_len = len;
  if(UHPA_ALLOC_TOPIC(child) == 0){
  child->topic_len = 0;
  mosquitto__free(child);
  log__printf(NULL, MOSQ_LOG_ERR, "Error: Out of memory.");
  return NULL;
  }else{
  strncpy(UHPA_ACCESS_TOPIC(child), topic, child->topic_len+1);
  }
  child->subs = NULL;
  child->children = NULL;
  child->retained = NULL;
  if(child->topic_len+1 > sizeof(child->topic.array)){
  if(child->topic.ptr){
    HASH_ADD_KEYPTR(hh, *sibling, child->topic.ptr, child->topic_len, child);
  }else{
    mosquitto__free(child);
    return NULL;
  }
  }else{
  HASH_ADD(hh, *sibling, topic.array, child->topic_len, child);
  }
  return child;
}