typedef struct SignalPool {
    GsNode* free_head; /* the head of free signal list */
    GsNode* free_tail; /* the tail of free signal list  */
    GsNode* used_head; /* the head of used signal list */
    GsNode* used_tail; /* the tail of used signal list */
    int pool_size;     /* the size of the array list */
    pthread_mutex_t sigpool_lock;
} SignalPool;