Queue trouble!

This is a discussion on Queue trouble! within the C Programming forums, part of the General Programming Boards category; Hey guys, I am in the midsts of writing code that involves managing two queues of names, and having the ...

  1. #1
    Registered User
    Join Date
    Sep 2007
    Posts
    25

    Queue trouble!

    Hey guys,

    I am in the midsts of writing code that involves managing two queues of names, and having the ability to call one name from the queue, and then deleting it. For some reason, I can't get it to properly work. I was wondering if someone could look over these couple functions and see if you notice anything wrong with them.

    Here's the two struct definitions:
    Code:
    #define MAX 50
    #define EMPTY 0
    
    struct elem {
       char name[MAX];
       struct elem *next;
    };
    
    struct queue {
       int num;
       struct elem *front;
       struct elem *rear;
    };
    Function enqueue:
    Code:
    void enqueue(queue *qu, char pass[]) {
       elem *p;
    
       strcpy(p->name, pass);
       p->next=NULL;
       if (!empty(qu)) {
          qu->rear->next = p;
          qu->rear = p;
       }
       else qu->front = qu->rear = p;
       printf("name = %s\n", qu->rear->name);
       qu->num++;
    }
    Function dequeue:
    Code:
    char *dequeue(queue *qu) {
       char *pass = malloc(sizeof(char)*50);
       elem *p;
    
       p = qu->front;
       strcpy(pass, qu->front->name);
       qu->front = qu->front->next;
       qu->num--;
       free(p);
       return (pass);
    }
    I also have:
    Code:
    typedef struct queue queue;
    typedef struct elem elem;
    Now, what seems to happen is that when I call dequeue, the name it gives me is always the last name I add to either queue. I can't seem to get it to work.

  2. #2
    Registered User
    Join Date
    Sep 2007
    Posts
    25
    No ideas?

  3. #3
    Kernel hacker
    Join Date
    Jul 2007
    Location
    Farncombe, Surrey, England
    Posts
    15,677
    Code:
    void enqueue(queue *qu, char pass[]) {
       elem *p;
    
       strcpy(p->name, pass);
       p->next=NULL;
       if (!empty(qu)) {
          qu->rear->next = p;
          qu->rear = p;
       }
       else qu->front = qu->rear = p;
       printf("name = %s\n", qu->rear->name);
       qu->num++;
    }
    What does p point at - shouldn't you malloc() a new elem each time?

    [I'm very surprised that this doesn't crash immediately, but I guess you may have some pointer value floating about on the stack that just happens to be valid].

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  4. #4
    Registered User
    Join Date
    Sep 2007
    Posts
    25
    Ahha, that did it. I could have sworn I had the code to alloc the memory in there before.. must have lost it in my many attempts to fix the code after the first run through. Thanks again!

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