To use any external storage format not supported by ESL in your ESL program, you must write your own external conversion routine. Your routine must reside in a DLL file, must be exported from the DLL, and must be defined as APIENTRY. See the documentation for the programming language you are using that describes how to build a DLL in that language.

An external conversion routine allows you to convert between ESL data formats, and storage formats used by other languages and other machines. There is only one absolute limit on what kind of storage format can be converted:  you are restricted to data stored in increments of eight-bit bytes.

The writer of the external conversion routine is responsible for ensuring that the routine can handle both conversions to the external storage format and conversions to any ESL type for which the conversion routine could be specified. If the conversion routine cannot convert to a given ESL type, it should write a message to the errorlog.