Each subject routine called or otherwise referenced requires the “use” statement for an interface block designed for that subject routine. The contents of this interface block are the interfaces to the separate routines available for that subject. Packaged descriptive names for option numbers that modify documented optional data or internal parameters might also be provided in the interface block. Although this seems like an additional complication, many errors are avoided at an early stage in development through the use of these interface blocks. The “use” statement is required for each routine called in the user’s program. As illustrated in Examples 3 and 4 in routine lin_geig_gen, the “use” statement is required for defining the secondary option flags.

The function subprogram for s_NaN() or d_NaN() does not require an interface block because it has only a single “required” dummy argument. Also, if one is only using the Fortran 77 interfaces supplied for backwards compatibility then the “use” statements are not required.