c
struct pci_dn *PCI_DN(dn){
    if (dn == NULL) {
        // Handle NULL input error
        return NULL; // or propagate error
    }
    if (dn->data == NULL) {
        // Handle NULL data error
        return NULL; // or propagate error
    }
    // Optional: Validate that dn->data points to a struct pci_dn
    // This might involve checking the size or contents of dn->data
    return ((struct pci_dn *) (dn)->data);
}
