二级指针的使用

用法：用一个变量管理多个指针型的变量，即动态给某个（些）指针型变量赋值

1.给一个指针变量赋值，如(见后面的例子程序)：
osalTaskRec_t **ptr;
ptr = &tasksHead;
*ptr = newTask;
其中：
osalTaskRec_t *tasksHead;
osalTaskRec_t *newTask;
ptr可以通过&获得一个指针变量的地址(tasksHead是一个指针变量)，以后可以通过*ptr给这个指针变量赋值，赋的值应该也是指针类型类型的变量或值

又如：
ptr = &srchTask->next;
srchTask = srchTask->next;
ptr首先获取srchTask结构体中的next变量的地址，而srchTask则赋值为srchTask结构体中next所指结构体地址。因此可以通过*ptr指针对上一个结构体中的next指针进行赋值（达到通过一个变量ptr管理多个指针变量的目的），而srchTask则可以操纵下一个结构体内部的变量。

eg:该代码取自：ZStack1.4.2中的OSAL_Tasks.c中的osalTaskAdd函数
/***************************************************************************
 * @fn      osalTaskAdd
 *
 * @brief   Add a task to the task list. Keep task queue in priority order.
 *
 * @param   none
 *
 * @return
 */
void osalTaskAdd( pTaskInitFn pfnInit,
                  pTaskEventHandlerFn pfnEventProcessor,
                  byte taskPriority)
{
  osalTaskRec_t *newTask;
  osalTaskRec_t *srchTask;
  osalTaskRec_t **ptr;

  newTask = osal_mem_alloc( sizeof( osalTaskRec_t ) );
  if ( newTask )
  {
      // Fill in new task
      newTask->pfnInit           = pfnInit;
      newTask->pfnEventProcessor = pfnEventProcessor;
      newTask->taskID            = taskIDs++;
      newTask->taskPriority      = taskPriority;
      newTask->events            = 0;
      newTask->next              = (osalTaskRec_t *)NULL;

      // 'ptr' is the address of the pointer to the new task when the new task is
      // inserted. Initially it is set to address of 'tasksHead' in case the new
      // task is higher priority than the existing head or the queue is empty.
      ptr      = &tasksHead;
      srchTask = tasksHead;
      while (srchTask)  {
          if (newTask->taskPriority > srchTask->taskPriority)  {
              // insert here. New task has a higher priority than the task
              // with which is being compared and a lower or equal priority
              // to any task that precedes it.
              newTask->next = srchTask;
              *ptr          = newTask;
              return;
          }
          // set 'ptr' to address of the pointer to 'next' in the current
          // (soon to be previous) task control block
          ptr      = &srchTask->next;
          srchTask = srchTask->next;
      }

      // We're at the end of the current queue. New task is not higher
      // priority than any other already in the list. Make it the tail.
      // (It is also the head if the queue was initially empty.)
      *ptr = newTask;
  }
  return;
}