I’m learning queues from a book. I’ve got into a problem while learning circular queue. The author from which I’m learning uses the following piece of code to explain how an element is inserted in a circular queue.

#define MAX 100
char *p[MAX];
int spos = 0; // spos: holds the index of the **next free** storage location

int rpos = 0;// rpos: holds the index of the next item to retrieve

void qstore(char *q)
{
  /* The queue is full if either spos is one less than rpos
      or if spos is at the end of the queue array and rpos
      is at the beginning.
  */
  if(spos+1= =rpos || (spos+1==MAX && !rpos)) <-- /***Problem is here**. Is it even
                                                    correct?*/
  {
     printf(''List Fulln");
     return;
  }
  p[spos] = q;
  spos++;

  if(spos==MAX)
  spos = 0; /* loop back */
}

He further states that: The queue is full when the store index is one less than the retrieve index; otherwise, there is room in the queue for another event.

SO, acc. to the author, if spos (which holds the index of the next free storage location) is equal to 4 and rpos=5, then the queue is full. Isn’t this incorrect? Because spos=3 means that the memory location at p[3] is empty.

So I decided to change the program.

#define MAX 100
char *p[MAX];
int spos = 0; // spos: holds the index of the **last allocated** storage location

int rpos = 0;// rpos: holds the index of the next item to retrieve

void qstore(char *q)
{
  /* The condition for queue full is same as the previous program*/

 /* The queue is full if either spos is one less than rpos
      or if spos is at the end of the queue array and rpos
      is at the beginning.
  */

if((spos+1==rpos) || (spos+1==MAX && rpos==0)) // Also changed syntax of test condition.
 {
   printf("Queue Fulln");
 } 

spos++

if((spos+1==MAX) && (rpos!=0))
 {
   spos=0;
 }
else
{
  spos++;
}

 p[spos]=q;

}

Is my code correct?