Records in database files can be described using either a field level description or record level description.
A field level description includes a description of all fields and their arrangement in this record. because the description of the fields and their arrangement is kept within a database file and not in your ILE C/C++ program, database files created with a field level description are referred to as externally described files. See Chapter 18, Using Externally Described Files in a Program.
A record level description describes only the length of the record, and not the contents of the record. Database files that are created with record level descriptions are referred to as program-described files. This means that your ILE C/C++ program must describe the fields in the record.
An ILE C/C++ program can use either externally described or program-described files. If it uses an externally described file, the ILE C/C++ compiler can extract information from the externally described file, and automatically include field information in your program. Your program does not need to define the field information. For further information see Chapter 18, Using Externally Described Files in a Program.
To define externally described database files, use one of the following:
A data description specification is a description of a database file that is entered into the system in a fixed form, and is used to create files. This description is composed of one or more record formats that define the fields that make up the record. It can also include access path information that determines the order in which records are retrieved from the file.
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