10.   HIGH-LEVEL INPUT-OUTPUT

10.1  INTRODUCTION

High-level I/O is necessary for two reasons: it provides long term information storage (as in a disk-based file system), and it provides access to physical devices (such as card readers, line printers, or interactive terminals). The high-level I/0 proposed for RED provides the necessary capabilities for both of these uses within a uniform framework.

Two major concepts in the RED I/0 design are the file variable, and the physical file (or device). A physical file is a repository for data. It consists of a linear sequence of data items, all of the same subtype. A physical file generally is a fairly static object and may exist for a long period of time. A file variable, however, is a connection to a particular physical file or device. Such a connection has both a position within the physical file and a directionality (input or output, or both). It is entirely possible for two different file variables to serve as input connections to the same physical file; however, depending on the implementation, two output or update connections to the same physical file might not work as expected.

10.2  FILE VARIABLES

A file variable has the following subtype:

      FILE [ cst ] (access, use)

where cst is the file's component subtype, access is the access mode (sequential or random), and use is the directionality of the connection (input, output, or update).

The type of the file's components is associated with the file type in order to permit type checking. Only data objects of type T can be read from or written to a file of type FILE[T]. Thus, unless a connection is established (via OPEN) to the wrong type of file, no type errors can occur.

The "access mode" and "use" properties have been associated with the file variable rather than with the physical file. This was done because they are logically part of the connection rather than of the data. Consider the following example:

This procedure advances a text file to the position following a given terminator character (or to the end of the file). Since the procedure only uses the EOF and READ file operations, it will work for sequential or random access files, and for any file use mode that allows reading.

Associating the access mode and use properties with a file variable documents the intended use of that variable. Making these properties distinguish subtype, rather than type, allows the scan procedure to be written as a simple non-generic procedure.

10.3  OPEN AND CLOSE

A file variable is connected to a particular physical file or device by invoking:

      OPEN (file_variable, file_name, status);

The file_name argument is a string that describes which physical file or device is desired. A status argument of 'OLD means that the physical file must currently exist. Typically, this is used with 'INPUT and 'UPDATE files, but it can also be used with 'OUTPUT files if the intention is to overwrite the existing information. A status argument of 'NEW means that the physical file need not currently exist; if it does not, it will be created. In either case, the physical file is initially empty. Typically, 'NEW is used for 'OUTPUT files, and occasionally for 'UPDATE files when it is desired to write and then reread what has been written.

Although 'OLD and 'NEW might not seem applicable to devices, RED requires that the status be specified. This policy leads to easy substitution of physical files for devices and vice versa.

A file variable is disconnected from a physical file or device by invoking:

      CLOSE (file_variable, disposition);

Closing a file variable is desirable for output files in order to indicate that the data are now logically complete. Closing a file is desirable from an efficiency standpoint, since any internal buffers can then be released for other uses.

The disposition argument to CLOSE comes from a recognition that some files are "permanent" and some are only temporary. The value 'SAVE means that the file is believed to have good data and should be preserved. The value 'DELETE means that the data are of no further use -- either because the file was only temporary or because the data are faulty. The 'DELETE specification causes the physical file to be deleted. Typically, 'SAVE is used for all permanent files and 'DELETE is used for temporaries. As with 'NEW and 'OLD, the 'SAVE and 'DELETE options are required for devices as well as physical files.

10.4  GENERALIZED FILE NAMES

The RED language does not stipulate a particular format for file names. It is felt that the file names used by RED programs should conform to whatever standards exist for the host system. However, programs should be able to name

1. particular physical files, e.g., " mail.txt" for the mail text file in Smith's directory,

2. particular physical devices, e.g., "TTY49:" for terminal number 49,

3. logical devices, such as "LPT:" for a temporary file to be queued for printing on some line printer, "TTY:" for the user's current terminal, and "SYS_lN:", "SYS_OUT:", and "SYS_ERR:" as the standard logical devices for input, output, and error reporting.

Since file names are strings, they can be obtained from a user's terminal or from a text file. They can also be computed. For example, the procedure

          gen_temp_file ("<Smith>", x);

might set x to be a guaranteed-unique name for a temporary file in Smith's directory.

The following program illustrates a simple file copying utility. The source and destination names are input from the user's terminal. Text can be transferred line-by-line between any two physical files or devices. For example, using " mail.txt" as the source and "TTY:" as the destination causes Smith's mail to be printed on the terminal.

10.5  READ AND WRITE

The operations for reading and writing data from files are procedures rather than statements. This means that they can be overloaded to transfer user-defined data values to and from text files. For example, a complex number could be printed as:

          3.07 + 99.2i

independently of how it was represented internally. As another example, consider the problem of generating an assembler listing. Within the assembler, it will be convenient to have a notion of a "generated machine word". This user-defined data abstraction would keep track of the value of a particular word, its address, and any relocation needed. The write operation for "generated machine word" would take care of the necessary formatting, and would encapsulate the knowledge that machine words representing instructions, pointers, character data, and floating point data all have different listing formats.

The use of READ and WRITE procedures provides a simple and easily extendable means of input and output. A complex "format description language" (as provided by FORTRAN) was not requested in Steelman, and has not been provided.

10.6  DEVICE DEPENDENT OPERATIONS

RED's high-level I/O design provides operations that will be useful in most environments, and for almost all file systems and devices. However, programs that make use of the unique properties of various devices will require other operations particularly suited for those devices. As an example, programs that use interactive terminals occasionally need to

1. test whether any unread input is available,

2. control whether input or output buffering occurs,

3. turn system-provided input editing features on or off, and

4. discover whether the terminal is a hard-copy printer or a sophisticated display terminal.

The RED input/output design assumes that any required device-specific operations will be available.

10.7  HIGH-LEVEL INPUT-OUTPUT IN THE TEST TRANSLATOR

The RED Test Translator will support a basic subset of the high-level I/O facilities. Text (ASCII) files may be read and written, either to disk or, interactively, on the user terminal.

The following will be available:

SYS_IN, SYS_OUT:	automatically declared files associated with the terminal
the FILE[ASCII] type, procedures OPEN and CLOSE:	for creating and manipulating other files
procedures READ, READLN, WRITE, WRITELN:	for transmitting information between the program and SYS_IN, SYS_DUT, or other specified files. Overload lines are available to allow READing and WRITEing of INT, BOOL, ENUM, and FLOAT data, as well as STRING[ASCII]
function EOF	.