Question

I am working with a double linked list and I have run into a problem with my pop() function.

//QueueElement describe the block in the cache
typedef struct _queue_ele_
{
    char *content;   //the data of the block
    struct _queue_ele_ *prev;
    struct _queue_ele_ *next;
}QueueElement;

typedef struct _queue_
{
    int queue_len;
    int max_queue_size;
    QueueElement *head;
    QueueElement *tail;
}MyQueue;

The pop function works until there is an input of 2 elements ( I clear the queue by poping one by one and freeing the memory)

pop:

// head is removed and returned
QueueElement* pop(MyQueue* myqueue)
{
    // if empty
    if(myqueue->queue_len == 0) return NULL;

    QueueElement *p = myqueue->head;

    // if one element
    if(myqueue->queue_len == 1) 
    {
        myqueue->queue_len--;
        myqueue->head = NULL;
        myqueue->tail = NULL;

        return p;
    }
    else
    {
        myqueue->queue_len--;

        //remove the head from the queue
        myqueue->head = myqueue->head->prev;
        myqueue->head->next = NULL; //******************Seg Fault here

        p->prev = NULL;

        return p;
        }   
}

The error I get when there are two elements is a segmentation fault in line shown, but it works for queues with more. Why wont it let me assign NULL to myqueue->head->next???

Answer 1

Change this:

myqueue->head = myqueue->head->prev;
myqueue->head->next = NULL; //******************Seg Fault here

To:

myqueue->head = myqueue->head->prev;
if (myqueue->head != NULL) {
    myqueue->head->next = NULL;
}

It is likely that you are trying to dereference a NULL pointer. It also would appear that you may have a memory leak from not calling free on the nodes you are deleting, but it is possible you do that elsewhere in the code.