The ILE RPG compiler allows you to define an area in storage and the layout of the fields, called subfields, within the area. This area in storage is called a data structure. You define a data structure by specifying DS in positions 24 through 25 on a definition specification.
You can use a data structure to:
In addition, there are four special data structures, each with a specific purpose:
Data structures can be either program-described or externally described, except for indicator data structures, which are program-described only. One data structure can be defined like another using the LIKEDS keyword.
A program-described data structure is identified by a blank in position 22 of the definition specification. The subfield definitions for a program-described data structure must immediately follow the data structure definition.
An externally described data structure, identified by an E in position 22 of the definition specification, has subfield descriptions contained in an externally described file. At compile time, the ILE RPG compiler uses the external name to locate and extract the external description of the data structure subfields. You specify the name of the external file either in positions 7 through 21, or as a parameter for the keyword EXTNAME .
An external subfield name can be renamed in the program using the keyword EXTFLD. The keyword PREFIX can be used to add a prefix to the external subfield names that have not been renamed with EXTFLD. Note that the data structure subfields are not affected by the PREFIX keyword specified on a file-description specification even if the file name is the same as the parameter specified in the EXTNAME keyword when defining the data structure using an external file name. Additional subfields can be added to an externally described data structure by specifying program-described subfields immediately after the list of external subfields.