An application must open an existing CDF before access to that CDF is allowed by the CDF library. In a C application CDFs are opened using either the CDFopen function (Standard Interface) or the <OPEN_,CDF_> operation of the CDFlib function (Internal Interface). In a Fortran application CDFs are opened using either the CDF_open subroutine (Standard Interface) or the <OPEN_,CDF_> operation of the CDF_lib subroutine (Internal Interface).