Specifying a File Type

Standard files

By far the most common type of file is the standard file, a structure comprised simply of a group of records beginning with record 0 and ending with record n - 1 (where n is the maximum specified in the filesize option). Examples of standard files are Editor files and program files. A standard file is the default file type created when you first open a file.

KSAM files

The keyed sequential access method (KSAM) is a method of organizing records in a file according to the content of key fields within each record. Every record in a KSAM file contains a primary key field whose contents determine the primary logical sequence of records in the file. Other key fields can also be defined so that the file can be sequenced in alternate orders. The order in that the records are physically written to the file, the chronological order, can be the same as the primary key sequence or it can be unrelated to any logical sequence.

KSAM files are not dealt with in this manual. Instead, the creation and application of KSAM files are discussed in great detail in the KSAM/3000 Reference Manual (30000-90079).

RIO files

RIO is a random access method that permits individual file records to be deactivated. These inactive records retain their relative position within the file. RIO files are intended for use primarily by COBOL programs; however, you can access these files by programs written in any language.

RIO files may be accessed in two ways: RIO access and non-RIO access. RIO access ignores the inactive records when the file is read sequentially using the FREAD intrinsic, and these records are transparent to you; however, they can be read by random access using FREADDIR. They may be overwritten both serially and randomly using FWRITE, FWRITEDIR, or FUPDATE. With RIO access, the internal structure of RIO blocks is transparent.

Circular files

Circular files are wrap-around structures that behave as standard sequential files until they are full. As records are written to a circular file, they are appended to the tail of the file; when the file is filled, the next record added causes the block at the head of the file to be deleted and all other blocks to be logically shifted toward the head of the file. Circular files may not be simultaneously accessed by both readers and writers. When the file has been closed by all writers, it may be read; a reader takes records from the circular file one at a time, starting at the head of the file.

Circular files are particularly useful as history files, when a user is interested in the information recently written to the file and is less concerned about earlier material that has been deleted. These history files are frequently used as debugging tools. Diagnostic information may be written to the file, and the most recent and relevant material can be saved and studied.

Creating a circular file is similar to creating a message file. When a user process opens a new file and indicates that it will be a circular file, the HPFOPEN/FOPEN intrinsic creates the new circular file. In order to create a circular file with the BUILD command, use the CIR keyword; for example, to build a circular file named CIRCLE, enter:

   BUILD CIRCLE;CIR

A new circular file may also be specified with a FILE command. Use the CIR keyword for a new file:

   FILE ROUND, NEW; CIR

A circular file named ROUND is indicated.

When you perform a LISTFILE,2 command, circular files are identified by an "O" in the TYP field; CIRCLE is identified here:

   FILENAME CODE -----------LOGICAL RECORD----------- ----SPACE----



                          SIZE TYP EOF LIMIT R/B      SECTORS #X MX



   CIRCLE                 256B FBO   0  1023   1          0    0  8

Message files

Message files are used by interprocess communication (IPC), a facility of the file system that permits multiple-user processes to communicate with one another in an easy and efficient manner. Message files act as first-in-first-out queues of records, with an entry made by FWRITE and a deletion made by FREAD; one process may submit records to the file with the FWRITE intrinsic while another process takes records from the file using the FREAD intrinsic.

Message files are not dealt with in this manual. Instead, the creation and application of message files are discussed in great detail in the Interprocess Communication Programmer's Guide (32650-90019).