Tail Queues

在看别人代码的时候看到了  Tail Queues，于是查了 manual,但是还不是很懂，

于是查了源码，明白了些。

 TAILQ_HEAD(HEADNAME, TYPE) head;
       where  HEADNAME is the name of the structure to be defined, and TYPE is      

the type of the elements to be linked into the tail queue.   A  pointer to the

head of the tail queue.

在源码中这样实现：

#define TAILQ_HEAD(name, type)                                          
 struct name {                                                           
         struct type *tqh_first;      /* first element */                     
         struct type **tqh_last;    /* addr of last next element */         
 }
 

The  macro  CIRCLEQ_INIT  initializes  the circular queue referenced by head.

这个宏是这样实现：

#define TAILQ_FIRST(head)       ((head)->tqh_first

#define TAILQ_INIT(head) do {                                           
         TAILQ_FIRST((head)) = NULL;                                     
         (head)->tqh_last = &TAILQ_FIRST((head));                        
 } while (0)
 

 The macro CIRCLEQ_INSERT_HEAD inserts the new element elm at  the  head

 of the circular queue.

这个宏的实现：

#define CIRCLEQ_NEXT(elm,field) ((elm)->field.cqe_next)
 
#define CIRCLEQ_PREV(elm,field) ((elm)->field.cqe_prev)


 #define CIRCLEQ_LAST(head)      ((head)->cqh_last)

#define CIRCLEQ_INSERT_HEAD(head, elm, field) do {                      
        CIRCLEQ_NEXT((elm), field) = CIRCLEQ_FIRST((head));             
         CIRCLEQ_PREV((elm), field) = (void *)(head);                    
         if (CIRCLEQ_LAST((head)) == (void *)(head))                     
                 CIRCLEQ_LAST((head)) = (elm);                           
         else                                                            
                 CIRCLEQ_PREV(CIRCLEQ_FIRST((head)), field) = (elm);     
         CIRCLEQ_FIRST((head)) = (elm);                                  
 } while (0)