Exception thrown when using dequeue method from queue

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I am trying to make my queue library and when I implement the dequeue method, an exception is throw "0xC0000374: A heap has been corrupted (parameters: 0x77475910)." This only gets thrown AFTER I already remove an item from the queue, the item is returned successfully however the first element is not removed, here is how I wrote the code :

typedef struct {
   int value;
   struct queue_item * next;
} queue_item;
typedef queue_item * queue;

and this is the method:

int dequeue(queue q)
{
    queue fi = q;
    int value = q->value;
    fi = q;
    q = q->next;
    free(fi);
    return  value;
}
c
pointers
linked-list
queue
singly-linked-list
asked on Stack Overflow Dec 13, 2017 by Matthew • edited Dec 13, 2017 by Vlad from Moscow

2 Answers

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 int dequeue(queue q)
 {
   queue fi = q;

You pass q by value so even though you make q point to next element inside dequeue(), outside dequeue(), it is still pointing to freed memory and using q will crash.

Pass a pointer to q to dequeue

answered on Stack Overflow Dec 13, 2017 by Gaurav Sehgal • edited Dec 13, 2017 by Gaurav Sehgal
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For starters this declaration

typedef struct {
   int value;
   struct queue_item * next;
} queue_item;

is invalid.

It should look like

typedef struct queue_item {
   int value;
   struct queue_item * next;
} queue_item;

The function dequeue deals with a copy of the original queue pointer because it is passed to the function by value.

So this statement in the function

q = q->next;

does not change the original pointer.

You should pass the pointer by reference. For example

int dequeue(queue *q)
{
    queue fi = *q;
    int value = ( *q )->value;

    *q = ( *q )->next;
    free( fi );

    return  value;
}
answered on Stack Overflow Dec 13, 2017 by Vlad from Moscow

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