Synonym Fields
A synonym field provides an alternative reference for an existing field and, by doing so, logically reorders fields in a file. With synonym fields, you can access the same data under more than one field name and with more than one field sequence number.
For example, you can use a synonym to create a group field that includes a physically noncontiguous field. (APPX requires that the fields in a group are contiguous, based on sequence number.) You simply define a synonym for the noncontiguous field using a contiguous sequence number.
A synonym field occupies the same physical space as the field it relates to. For example, assume you wish to access three fields together as a group. One of these three fields contains a noncontiguous sequence number, a condition that is not permitted within a group field. To overcome the problem, you define a synonym of the noncontiguous field, and assign the synonym a contiguous sequence number. The synonym produces a logically contiguous series of bytes that, for purposes of group manipulation, APPX treats the same as a physically contiguous series. Because of the synonym definition, the physical spaces are not required to lie in the same order as the logical fields in the group field.
A synonym does not store a separate copy of data in a referenced field. Both a synonym and the field it references access the same physical data. As a result, a change to the data in either a synonym or the field it references affects both fields.
Most of the attributes for a synonym field are identical to the attributes of the physical field that a synonym references. Refer to the Special Uses for Synonym Fields section for additional information. |