xenbus_write
if (xenbus_write(0, path_buffer, "error", printf_buffer) != 0) {
ret = xenbus_write(t, dir, node, printf_buffer);
extern int xenbus_write(xenbus_transaction_t t, const char *dir,
(void) xenbus_write(xbt, "control", "shutdown", "");
(void) xenbus_write(xbt, "control", "shutdown", "");