What is wrong with my queue ADT?

This is a discussion on What is wrong with my queue ADT? within the C Programming forums, part of the General Programming Boards category; Code: #include "queue.h" QUEUE* CreateQueue(void) { QUEUE* queue; queue = (QUEUE*)malloc(sizeof(QUEUE)); if(queue) { int count = 0; queue->front = NULL; ...

  1. #1
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    What is wrong with my queue ADT?

    Code:
    #include "queue.h"
    
    QUEUE* CreateQueue(void)
    {
        QUEUE* queue;
    
    
        queue = (QUEUE*)malloc(sizeof(QUEUE));
    
    
        if(queue)
        {
            int count = 0;
            queue->front = NULL;
            queue->rear = NULL;
        }
        else
            return NULL;
        return queue;
    }
    
    
    bool Enqueue(QUEUE* queue, void* dataptr)
    {
        NODE* newnode;
        
        if(!(newnode = (NODE*) malloc(sizeof(NODE))))
            return FALSE;
    
    
        newnode->dataptr = dataptr;
        newnode->next = NULL;
    
    
        if(queue->count == 0)
            queue->front = newnode;
        else
            queue->rear->next = newnode;
        (queue->count)++;
        queue->rear = newnode;
        return TRUE;
    }
    
    
    void* Dequeue (QUEUE* queue)
    {
        void* dataOut;
        NODE* temp;
        
        if(queue->count == 0)
            return NULL;
        dataOut = queue->front->dataptr;
        temp = queue->front;
        queue->front = queue->front->next;
        if(queue->count == 1)
            queue->rear = NULL;
        (queue->count)--;
        free(temp);
        return dataOut;
    }
    
    
    void* QueueFront (QUEUE* queue)
    {
        void* dataOut;
        
        if(queue->count == 0)
            return NULL;
        else
            return queue->front->dataptr;
    }
    
    
    void* QueueRear (QUEUE* queue)
    {
        void* dataOut;
        if(queue->count == 0)
            return NULL;
        else
            return queue->rear->dataptr;
    }
    
    
    bool EmptyQueue (QUEUE* queue)
    {
        return (queue->count == 0);
    }    
    
    
    bool FullQueue (QUEUE* queue)
    {
        NODE* temp;
    
    
        temp = (NODE*)malloc(sizeof(*(queue->rear)));
        if(temp)
        {
            free(temp);
            return FALSE;
        }
        return TRUE;
    }
    
    
    int QueueCount(QUEUE* queue)
    {
        return queue->count;
    }
    
    
    void DestroyQueue(QUEUE* queue)
    {
        NODE* temp;
        if(queue->count != 0)
        {
            while(queue->front != NULL)
            {
                temp = queue->front;
                queue->front = queue->front->next;
                free(temp->dataptr);
                free(temp);
            }
        }
        free(queue);
    }
    So I tried to enqueue something very simple in main as so:
    Code:
    #include "queue.h"
    
    
    int main(void)
    {
        int* a;
        QUEUE* queue;
    
    
        queue = CreateQueue();
        a = (int*)malloc(sizeof(int));
        *a = 1;
        Enqueue(queue, a);
        return 0;
    }
    When I compile, the program crashes and my debugger states that there was an exception at this line:
    Code:
    bool Enqueue(QUEUE* queue, void* dataptr){
        NODE* newnode;
        
        if(!(newnode = (NODE*) malloc(sizeof(NODE))))
            return FALSE;
    
    
        newnode->dataptr = dataptr;
        newnode->next = NULL;
    
    
        if(queue->count == 0)
            queue->front = newnode;
        else
            queue->rear->next = newnode;
        (queue->count)++;
        queue->rear = newnode;
        return TRUE;
    }
    I am using Microsoft visual studio 2010.

    Any help is appreciated!
    Last edited by slee8251; 10-08-2012 at 01:30 PM.

  2. #2
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    Your code for queue.h is what?

    Edit: A random guess of the problem; you forgot to include the header "stdlib.h".
    http://faq.cprogramming.com/cgi-bin/...&id=1043284351

    There is nothing wrong with this except in the event that stdlib.h, the header which declares malloc, is not included.
    Tim S.
    Last edited by stahta01; 10-08-2012 at 01:19 PM.
    "Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the universe trying to produce bigger and better idiots. So far, the Universe is winning." Rick Cook

  3. #3
    Algorithm Dissector iMalc's Avatar
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    On line 13 you are not initialising queue->count, you are instead creating and not using a separate variable then goes out of scope shortly after.
    Also, in main, everything is going okay and then you igmore your malloc'ed int and just pass in 5 directly. What's with that?
    My homepage
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  4. #4
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    Code:
    #include <stdio.h>#include <stdlib.h>
    
    
    typedef struct node
    {
        void* dataptr;
        struct node* next;
    } NODE;
    
    
    typedef struct queue
    {
        int count;
        NODE* front;
        NODE* rear;
    } QUEUE;
    
    
    typedef enum {FALSE, TRUE} bool;
    
    
    QUEUE* CreateQueue(void);
    bool Enqueue(QUEUE* queue, void* dataptr);
    void* Dequeue(QUEUE* queue);
    void* QueueFront(QUEUE* queue);
    void* QueueRear (QUEUE* queue);
    bool EmptyQueue (QUEUE* queue);
    bool FullQueue (QUEUE* queue);
    int QueueCount (QUEUE* queue);
    void DestroyQueue(QUEUE* queue);
    that is my header file.

    Also, in main, everything is going okay and then you igmore your malloc'ed int and just pass in 5 directly. What's with that?
    typo, I fixed it
    Last edited by slee8251; 10-08-2012 at 01:35 PM.

  5. #5
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    Quote Originally Posted by iMalc View Post
    On line 13 you are not initialising queue->count, you are instead creating and not using a separate variable then goes out of scope shortly after.
    Also, in main, everything is going okay and then you igmore your malloc'ed int and just pass in 5 directly. What's with that?
    Did the problem go away after fixing "queue->count" initialization?
    Note, if you did not fix it, why not?

    Tim S.
    "Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the universe trying to produce bigger and better idiots. So far, the Universe is winning." Rick Cook

  6. #6
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    Yes, the problem did go away after I fixed the initialization.
    Appreciate the second pair of eyes to look over my dumb mistakes.

    Thanks!

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