The entry of ** is used for the lookahead function. This function lets you look at information in the next record in a file. You can look not only at the file currently selected for processing but also at other files present but not selected during this cycle.
Field description lines must contain From and To entries in the record, a field name, and decimal positions if the field is numeric. Note that a lookahead field may not be specified as a field name on input specifications or as a data structure name on definition specifications or as a Result Field on Calculation Specifications.
Positions 17 and 18 must contain an alphabetic entry. The lookahead fields are defined in positions 49 through 62 of the lines following the line containing ** in positions 21 and 22. Positions 63 through 80 must be blank.
Any or all of the fields in a record can be defined as lookahead fields. This definition applies to all records in the file, regardless of their type. If a field is used both as a lookahead field and as a normal input field, it must be defined twice with different names.
The lookahead function can be specified only for primary and secondary files and can be specified only once for a file. It cannot be used for full procedural files (identified by an F in position 18 of the file description specifications), or with AND or OR lines.
When a record is being processed from a combined file or an update file, the data in the lookahead field is the same as the data in the record being processed, not the data in the next record.
The lookahead function causes information in the file information data structure to be updated with data pertaining to the lookahead record, not to the current primary record.
If an array element is specified as a lookahead field, the entire array is classified as a lookahead field.
So that the end of the file can be recognized, lookahead fields are filled with a special value when all records in the file have been processed. For character fields, this value is all '9's; for all other data types, this value is the same as *HIVAL.