size_t zmalloc_size(void *ptr) {
    void *realptr = (char*)ptr-PREFIX_SIZE;
    size_t size = *((size_t*)realptr);
    return size+PREFIX_SIZE;
}
size_t zmalloc_size(void *ptr) {
    void *realptr = (char*)ptr-PREFIX_SIZE;
    size_t size = *((size_t*)realptr);
    return size+PREFIX_SIZE;
}