Thread: pointer to a pointer

I need help in understanding some example code. If you want to delete the last node of the list L, why does the parameter have to be a pointer to a variable containing a node pointer? In other words why is it **L? Is there any list example that would not have its last node deleted if the parameter had been only a node pointer as in the function prototype void DeleteLastNode(NodeType *L)

Code:

void DeleteLastNode(NodeType **L)
{
   NodeType *PreviousNode, *CurrentNode;

   if (*L!=NULL)
   {
      if ((*L)->Link == NULL)
      {    
         free(*L);
         *L=NULL;
      }

      else
      {
         PreviousNode=*L;
         CurrentNode=(*L)->Link;

         while (CurrentNode->Link!=NULL)
         {
            PreviousNode=CurrentNode;
            CurrentNode=CurrentNode->Link;
         }

         PreviousNode->Link=NULL;
         free(CurrentNode);

      }
   }
}

If you had only passed a single pointer to the routine then the routine would have the address of the memory pointer only. By passing a double pointer you are getting the address of what the pointer points to and that is the memory that you are trying to alter.

The only analogy that I could think of to help explain this is to think of it in terms of a post office and your home:

A post office knows where your house is based upon your address. However, the post office is itself a location which has an address but obviously not your house. The post office is the pointer. Your address (where your actual house resides) is the data. I don't know if this elaboration helps at all or has confused you more.