Thread: 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!

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.

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?

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

Originally Posted by iMalc

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.