c - 我在 C 中看到了什么奇怪的东西？

Question

   typedef struct stage_tag {
   pthread_mutex_t     mutex;          /* Protect data */
    pthread_cond_t      avail;          /* Data available */
   pthread_cond_t      ready;          /* Ready for data */
   int                 data_ready;     /* Data present */
   long                data;           /* Data to process */
    pthread_t           thread;         /* Thread for stage */
   struct stage_tag    *next;          /* Next stage */
  } stage_t;

 typedef struct pipe_tag {
  pthread_mutex_t     mutex;          /* Mutex to protect pipe */
  stage_t             *head;          /* First stage */
  stage_t             *tail;          /* Final stage */
  int                 stages;         /* Number of stages */
  int                 active;         /* Active data elements */

} 管道_t；

  int pipe_create (pipe_t *pipe, int stages)
{
int pipe_index;
stage_t **link = &pipe->head, *new_stage, *stage;
int status;

......//问题出在pipe_create方法上。有一个 stage_t 类型的双指针，它具有三个值...这是什么样的声明？我真的很困惑。

Answer 1

单个声明语句可以声明类型相关的多个变量(从某种意义上说，它们仅在限定方面有所不同):

T **x, *y, z;

等同于:

T **x;
T *y;
T z;

一个有点荒谬的例子:

int main(void)
{
    int a = 1, * const b = &a, * const * c = &b;
    return a + *b + **c;
}