Trouble accessing a struct member from a void pointer.

Question

I am not too experienced in C and I am having trouble with something that might be simple you for most of you. Basically, I have this structure that defines a 'generic' queue with a resizing array implementation:

typedef void (*free_fptr)(void *);

typedef struct {
    void **queue;       // pointer to generic type
    size_t first;       // head index in array
    size_t last;        // tail index in array
    size_t size;        // number of elements
    size_t capacity;    // capacity of array
    size_t elem_size;   // size in bytes of each element in queue
    free_fptr deleter;  // function used to free each element
} Queue;

Now, I have a data type that I want to put in the queue :

typedef struct {
    Process_state state;
    Queue time_queue;
    unsigned int start_time;
    unsigned int id;
} Process;

I also have a function 'Queue_destroy(Queue *q)' that I want to call when I need to free each element in the queue :

void
Queue_destroy(Queue *q)
{
    size_t i;

    for (i = 0; i < q->size; ++i) {
        q->deleter(q->queue[(q->first + i) % q->capacity]);
    }

    free(q->queue);

}

Now, my problem is that I don't know to access to the 'Process' queue inside the queue from a void pointer. For example :

void
Process_deleter(void *item)
{   
    // Here I want to access the queue inside (Process *)item
    free((Process *)item);
}

I tried many things without success such as :

    Queue_destroy((*(Process *)item).time_queue);
    Queue_destroy((Process *)item->time_queue);

It does not compile and I am clueless!

Any help would be greatly appreciated!

Answer 1

Your function:

Queue_destroy(Queue *q)

expects a Queue*

So, change:

Queue_destroy((*(Process *)item).time_queue); // Here you are passing the object 

to:

Queue_destroy(&(((Process *)item)->time_queue));

Assuming item is a pointer to your struct Process