Files are closed using the IDL_FileClose() function.
            IDL_FileClose()
            void IDL_FileClose(int argc, IDL_VPTR *argv, char *argk)
            argc
             The number of arguments in argv.
            argv
            The arguments to the close function. These should be scalar integer values giving the Logical Unit Numbers of the file units to close.
            argk
            Keywords. Set this argument to NULL.