Records and Files in the File System

The File System manages data being transferred or stored with peripheral devices. It handles I/O operations, such as passing information to and from user processes, compilers, and information management subsystems. Conceptually, information for data transfers is arranged as elements of data in a record. The record is input, processed, and output as a single unit. Logically related records are grouped into sets called files, which can be kept in any storage medium or sent to any I/O peripheral, as shown in Figure 6-2 “Records and Files Relationship”. In some cases, records may be physically grouped together in blocks when they are in a file residing on a non-disc device.

Figure 6-2 Records and Files Relationship

Since all I/O operations are done through the mechanism of files, you can access different devices in a standard, consistent way. The names assigned to a file when it is defined in a program do not restrict that file to residing on the same device every time the program is run; the program is device independent.

The File System recognizes two basic types of files, classified by the media on which they reside when processed: disc files and device files.

For detailed information on the File System interface, disc files, device files, and spooled device files, refer to Accessing Files Programmer's Guide (32650-90017).

Device Files

Device files are files currently being input to or output from any peripheral device, except a system or private domain disc volume. A foreign disc or serial disc file is considered to be a device file. A device file is nonsharable; it is accessed exclusively by the job or session that acquires it and is owned by that job or session until the job or session explicitly releases it or terminates.

A device file is a set of logically related records transferred to or from a non-random disc device such as a terminal, magnetic tape, or line printer. A device file is built differently from a disc file. You define the physical and operational characteristics each time you access the device. However, the File System is used to open, access, and close a device file, just as it does a disc file. When you have a program that is written to use a disc file and you want it to use a device file at run time, use the DEV= parameter of the :FILE command to specify a device class (such as TAPE) or a specific logical device number that corresponds to a device on your system. If you do not use the DEV= parameter, the :FILE command defaults to defining a disc file. Magnetic tape has no mandatory settings. You can use the REC= parameter of the :FILE command to specify physical characteristics.

The File System HPFOPEN and FOPEN intrinsics automatically set actual device characteristics that override any specifications passed from the program or the :FILE command. For example, the actual device characteristics of a line printer are always NEW, ASCII, UNDEFINED, and WRITE ONLY access.

If a device file characteristic is not specified through one of the following, then it has a default value:

Use of device characteristics.
:FILE command.
The way in which a program opens the file.

Figure 6-3 “Specifying Device File Characteristics” shows an example of specifying device file characteristics in a :FILE command. In this figure, the device specified by :DEV= for the file named PAYROLL is a line printer; the output priority of the spoolfile is 1; and the number of copies of output to print is 3. The ;FORMS= message is sent to the console when the file is ready to be output to a device.

Figure 6-3 Specifying Device File Characteristics

The most common medium for storage of a device file is magnetic tape. The File System has several capabilities in handling magnetic tape labels, and the Labeled Tape Facility provides for the handling of multireel tape sets. For detailed information on magnetic tape labels, refer to the MPE/iX Commands Reference Manual Volumes 1 and 2 (32650-90003 and 32650-90364) and Accessing Files Programmer's Guide (32650-90017).

Disc Files

A disc file is a set of logically related records that reside on disc. A disc file is immediately accessible by the system and is potentially sharable by several jobs or sessions at a time. The structure of a disc file is determined by the physical constraints of the disc, such as available space, and user-defined elements of the file. You use the File System to build, open, access, and close a disc file.

A disc file resides on one or more volumes. A file does not have to reside as a contiguous piece on one volume, but can be broken into many extents distributed on different volumes.

The File System must store data on disc in an efficient manner, since files are constantly manipulated: added, deleted, and updated. It uses demand paged virtual memory to manage virtual memory and the user interface to files. This occurs transparently from the user's perspective. Page-sized portions of files are swapped into memory only when needed.

The user-defined elements of a disc file are those within your control. Elements you can specify include:

Which disc (or discs) the file will reside on.
Record type, size, and access.

For detailed information on record types and elements you can specify, refer to the "Record Structure" subsection and Accessing Files Programmer's Guide (32650-90017).

Records and Files in the File System

Technical documentation

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Device Files

Disc Files