问Stack and push()函数

EN

struct upo_stack_node_s
{
    void *data;
    struct upo_stack_node_s *next; 
};
typedef struct upo_stack_node_s upo_stack_node_t;

struct upo_stack_s
{
    upo_stack_node_t *top;
    size_t size;
};

/*Function for create the stack*/
upo_stack_t upo_stack_create()
{
    upo_stack_t stack = malloc(sizeof(struct upo_stack_s));
    if (stack == NULL)
    {
        fprintf(stderr, "Unable to create a stack!\n");
        abort();
    }

    stack->top = NULL;
    stack->size = 0;

    return stack;
}

/*Push() function:*/
void upo_stack_push(upo_stack_t stack, void *data)
{
    /* TO STUDENTS:
    *  Remove the following two lines and put here your implementation
    */     
    upo_stack_node_t *node = malloc(sizeof(struct upo_stack_node_s));
    node->data = data; /*<-- Here's the problem */
    node->next = stack->top;
    stack->top = node;
    ++stack->size;
}

/*Top() function*/
void* upo_stack_top(const upo_stack_t stack)
{
    /* TO STUDENTS:
     *  Remove the following two lines and put here your implementation
     */
    return (void *)(stack->top); //<---
}

/*Function for testing (there are other functions in the code)*/
void test_top()
{
    int value1 = 1;
    int value2 = 2;
    upo_stack_t stack;

    stack = upo_stack_create();

    upo_stack_push(stack, &value1); //<----
    upo_stack_push(stack, &value2); //<----

    assert( upo_stack_top(stack) != NULL );
    assert( *((int*) upo_stack_top(stack)) == value2 ); <-- Here's the error

    upo_stack_pop(stack, 0);

    assert( upo_stack_top(stack) != NULL );
    assert( *((int*) upo_stack_top(stack)) == value1 );

    upo_stack_pop(stack, 0);

    assert( upo_stack_top(stack) == NULL );

    upo_stack_destroy(stack, 0);
}