C中的空指针的免费数组

Question

I'm implementing a min/max heap in C and am trying to do it in general, since I'll need it for a future project.

The idea is to use a 1D array of void* with a generic comparator, int (*cmp) (void*,void*) .

My structure looks like this:

typedef struct heap {
    size_t capacity;
    size_t size;
    int (*cmp)(void *, void *);
    void **data;
} Heap;

I malloc the space for void **data in a call to heapalloc:

Heap*
heapalloc(size_t capacity, int (*cmp)( void *,  void *))
{
    Heap* h = malloc(sizeof(Heap*));
    if (h == NULL) 
    {
        perror("heapalloc/h:");
        return h;
    }

h->cmp = cmp;
h->data = malloc(capacity * sizeof(void*) );

if (h->data == NULL)
{
    perror("heapalloc/h->data:");
    free(h);
    return NULL;
}

h->size = 0;
h->capacity = capacity;
return h;

}

Everything is running fine, until I have to free data. The following call gives a seg fault, and I don't understand why---I've only called malloc once, and data is the pointer returned from it!

void
heapfree(Heap *h)
{ 
    free(h->data);
    free(h);
}

Any help would be appreciated. I've seen a lot of posts on similar topics, but nothing I've found has actually worked so far. (Compiling with gcc; hence sizeof(void*))

Answer 1

Heap* h = malloc(sizeof(Heap*));

is wrong; you want

Heap *h = malloc(sizeof *h);