Question

I’m trying to understand this C code for modifying queues:

  /*
    * create or delete a queue
    * PARAMETERS: QUEUE **qptr - space for, or pointer to, queue
    * int flag -  1 for create, 0 for delete
    * int size - max elements in queue
    */
    void qManage(QUEUE **qptr, int flag, int size){
         if(flag){
                  /* allocate a new queue */
                  *qptr = malloc(sizeof(QUEUE));
                  (*qptr)->head = (*qptr)->count = 0;
                  (*qptr)->que = malloc(size * sizeof(int));
                  (*qptr)->size = size;
                  }
                  else{
                       // delete the current queue
                       (void) free((*qptr)->que);
                       (void) free(*qptr);
                 }
           }

What is the **qptr parameter? What does (*qptr)->head mean? I know that -> is a pointer to a structure member reference, but I’m lost on what’s going on here. I appreciate any tips or advice.

Answer 1

QUEUE** qptr means that qptr is a pointer to a pointer to a QUEUE (whatever that is).

*qptr is “the memory pointed to by qptr“, which is thus a pointer to a QUEUE.

x->y is the same as (*x).y. In other words, “take the thing pointed to by x, then get its y“. See https://stackoverflow.com/a/3479169/383402 for reference.

So, (*qptr)->head is the head of the QUEUE which is pointed to by the thing which is pointed to by qptr.

The extra layer of indirection is so that the function can effectively return a QUEUE*. In order to return the QUEUE*, it takes in a QUEUE**, and makes it point to the newly-allocated memory.