C -- (void*) to int

Question

I'm implementing a simple priority queue in C for a kernel and so I can't use any standard libraries. The queue holds a head node and each node points to the next in the queue.

typedef struct node node;
struct node {
    node *next;
    void *data;
};

typedef struct  {
    node *head;
    int n;
} queue;

As you can see, each node holds it data in a void*. I'm having trouble converting this data to lets say an int when I pop the data off the stack.

//push data
int int_data = 100;
push(q, &int_data);
//...
//pop data
node* popped = pop(q);
int *pop_data = popped->data;
printf("pop data (100): %d\n", *pop_data);

Why can't I get the original value here? I seem to be printing a pointer value. Alternatively, is there a better way to handle this?

== edit (sorry should have included these):

void push(queue *q, void *data)
{
    node new;
    new.data = data;
    node *new_ptr = &new;

    if(is_empty(q))
    {
        q->head = new_ptr;
        q->n++;
        return;
    }

    int i;
    node *curr = q->head;
    for(i=0; i<q->n; i++)
    {
        curr = curr->next;
    }
    curr->next = new_ptr;
    q->n++;
}

node* pop(queue *q)
{
    node *curr = q->head;
    q->head = curr->next;
    return curr;
}

From what you've provided I don't think we have enough information to answer your question; provided that you set everything up correctly, this should work just fine. What is the program actually printing? Can you provide some more context for how these functions work? — templatetypedef
You are storing the address of a local variable, once it gets popped off the stack your void* data will be pointing to garbage memory. — Marlon

Marlon Marlon · Accepted Answer · 2011-02-16T01:25:47

Is your code all in one function? If not, int int_data is getting popped off the stack (not your queue, the actual stack) which is probably why you are printing garbage; you are storing the address of a local variable.

I would suggest changing void* data to int data. (If you need to, you can store an address in an int and can cast it back to a pointer later.)

int int_data = 100;
push(q, int_data);

node* n = pop(q);
int num = n->data;

After reviewing your code again, you have the same problem when adding a new node. node new falls out of scope at the end of the function, so basically all of your nodes in your queue are pointing to invalid memory.

C -- (void*) to int

4 Answers