void * realloc(void * ptr, size_t size) {
    struct Malloc * head = malloc_begin;
    while (head) {
        if (head->a==ptr) {
            struct Malloc * newmem;
            if (head->size >= size) return head->a; /* no problem */
            /* otherwise allocate new and copy over */
            newmem = findFirstFree(size);
            if (!newmem) return NULL;
            memcpy(newmem->a,head->a,size);
            free(head->a);
            return newmem->a;
        }
        head=head->next;
    }
    fprintf(stderr,"attempt to realloc memory unknown to allocator\n"); return NULL;
}