It is absolutely essential that a CDF that has been created or modified by an application be closed before the program exits. If the CDF is not closed it will in most cases be corrupted and unreadable. This is because the cache buffers maintained by the CDF library will not have been written to the CDF file(s). An existing CDF that has been opened and only read from should also be closed. In a C application CDFs are closed using either the CDFclose function (Standard Interface) or the <CLOSE_,CDF_> operation of the CDFlib function (Internal Interface). In a Fortran application CDFs are closed using either the CDF_close subroutine (Standard Interface) or the <CLOSE_,CDF_> operation of the CDF_lib subroutine (Internal Interface).