-size_t serial_read(int __fd, __ptr_t __buf, size_t __nbytes);
-size_t serial_write(int __fd, __const __ptr_t __buf, size_t __n);
+extern size_t serial_read(int __fd, __ptr_t __buf, size_t __nbytes);
+extern size_t serial_write(int __fd, __const __ptr_t __buf, size_t __n);