The external conversion routine should return a value of 0 only if the conversion is successful. If the conversion is unsuccessful, the conversion routine should return a non-zero value. In that case, ESL performs a default conversion, and an error message containing the value returned by the external conversion routine is printed in the errorlog. Note that if the conversion routine returns 0 for an unsuccessful conversion, it might cause unpredictable results. An external conversion routine can write to the standard error file handle to write its own messages to the ESL errorlog.

If the conversion routine is unsuccessful on a read reference to the field, ESL performs a default conversion, and supplies one of the following values for the field value:

If the conversion routine is unsuccessful on a write reference to the field, ESL performs no default conversion, and the external field is not changed. ESL does print an error message to the errorlog, containing the non-zero value returned by the conversion routine.