Oberon-7 and the file system

By R. S. Doiel, 2020-05-09

This is the seventh post in the Mostly Oberon series. Mostly Oberon documents my exploration of the Oberon Language, Oberon System and the various rabbit holes I will inevitably fall into.

Working with files in Oberon-7

In a POSIX system we often talk of opening files, writing and reading files and close files. The Oberon language reflects a more Oberon System point of view.

The Oberon System generally avoids modality in favor of action. Modality is where a context must be set before a set of actions are possible. The vi text editor is a “modal” editor. You are in either edit (typing) mode or command mode. At the function level POSIX’s open(), is also modal. You can open a file for reading, open a file for writing, you can open a file for appending, etc. The Oberon language and base modules avoids modality.

The Oberon System is highly interactive but has a very different idea about code, data and computer resources. In POSIX the basic unit of code is a program and the basic unit of execution is a program. In Oberon the basic unit of code is the module and the basic unit of execution is the procedure. Modules are brought into memory and persist1. As a result it is common in the Oberon System to need to have file representations that can persist across procedure calls. It provides a set of abstractions that are a little bit like views and cursors found in database systems. In taking this approach Oberon language eschews modality at the procedure level.

Key Oberon Concepts

The following are exported in the Files module.

File
is a handle to the representation of a file, a File and Rider form a view into a file.
Rider
similar to a database cursor, it is the mechanism that lets you navigate in a file
New
Creates a new file (in memory but not on disc).
Registration
Associates a file handle created with New with the file system. A file must be registered to persist in the file system.
Old
Opens an existing file for use.
Set
Set the position of a rider in a file
Pos
Gets the position of a rider in a file
Close
Writes out unwritten buffers in file to disc, file handle is still value as is the rider.
Purge
Sets a file’s length to zero.
Delete
Unregister the filename with the file system.

In the Oberon Systems a file can be “opened” many times with only one copy maintained in memory. This allows efficient operations across a module’s procedures. Likewise a file can have one or more Riders associated with it. Each rider can move through the file independently operating on the common in memory file. If a file is created with New but not registered it can be treated like an in-memory temp file. Closing a file writes its buffers but the file remains accessible through it handle and riders. If a file is not modified it doesn’t need to be closed.

In POSIX we generally want to explicitly close the file when we’re through. In the Oberon language we only need to close a file if we’ve modified it.

The behavior of files and riders in Oberon creates interesting nuances around deleting files. The Delete operation can in principle happen multiple times before the file is deleted on disc. That is because the file handles and riders may still be operating on it. To know when a file is finally deleted when Delete procedure is called it includes a results parameter. When that value is set to zero by the Delete procedure you know your file has been deleted.

The Files module provides a number of methods to read and write basic Oberon types. These use the rider rather than the file handle. Calling them automatically updates the riders position. The procedures themselves map to what we’ve seen in the modules In and Out.
There are a few additional commands for file system house keeping such as Length, GetDate, Base. See the OBNC documentation for the Files module for details2

In the following examples we’ll be using the Files module to create, update and delete a file called HelloWorld.txt.

Creating a file

The recipe we want to follow for creating a file is New (creates an empty file in memory), Register (associations the filename with the file system), Set the rider position, with the rider write our content and with the file call close because we’ve have changed the file.

Like our origin SayingHi we’ll demonstrate this code in a new module called TypingHi.Mod. Below is a procedure called WriteHelloWorld. It shows how to create, write and close the new file called “HelloWorld.txt”.

  PROCEDURE WriteHelloWorld;
       VAR
         (* Define a file handle *)
         f : Files.File;
         (* Define a file rider *)
         r : Files.Rider;
     BEGIN
       (* Create our file, New returns a file handle *)
       f := Files.New("HelloWorld.txt");
       (* Register our file with the file system *)
       Files.Register(f);
       (* Set the position of the rider to the beginning *)
       Files.Set(r, f, 0);
       (* Use the rider to write out "Hello World!" *)
       Files.WriteString(r, "Hello World!");
       (* Write a end of line *)
       Files.WriteString(r, 0AX);
       (* Close our modified file *)
       Files.Close(f);
     END WriteHelloWorld;

Receipt in review

Working with an existing file

If we’re working with an existing file we swap New for a procedure named Old. We don’t need to register the file because it already exists. We still need to set our rider and we want to read back the string we previously wrote. We don’t need to close it because we haven’t modified it. To demonstrate a new procedure is added to our module called ReadHelloWorld.

  PROCEDURE ReadHelloWorld;
       VAR
         f : Files.File;
         r : Files.Rider;
         (* We need a string to store what we've read *)
         src : ARRAY 256 OF CHAR;
     BEGIN
       (* Create our file, New returns a file handle *)
       f := Files.Old("HelloWorld.txt");
       (* Set the position of the rider to the beginning *)
       Files.Set(r, f, 0);
       (* Use the rider to write out "Hello World!" *)
       Files.ReadString(r, src);
       (* Check the value we read, if it is not the name, 
          halt the program with an error *)
       ASSERT(src = "Hello World!");
     END ReadHelloWorld;

Receipt in review

Removing a file

Deleting the file only requires knowing the name of the file. Like in ReadHelloWorld we’ll use the built-in ASSERT procedure to check if the file was successfully removed.

  PROCEDURE DeleteHelloWorld;
       VAR
         result : INTEGER;
     BEGIN
       (* Delete our file *)
       Files.Delete("HelloWorld.txt", result);
       (* Check our result, if not zero then halt program with error *)
       ASSERT(result = 0);
     END DeleteHelloWorld;

Receipt in review

Putting it all together.

Here is the full listing of our module.

    MODULE TypingHi;
         IMPORT Files;
       
         PROCEDURE WriteHelloWorld;
           VAR
             (* Define a file handle *)
             f : Files.File;
             (* Define a file rider *)
             r : Files.Rider;
         BEGIN
           (* Create our file, New returns a file handle *)
           f := Files.New("HelloWorld.txt");
           (* Register our file with the file system *)
           Files.Register(f);
           (* Set the position of the rider to the beginning *)
           Files.Set(r, f, 0);
           (* Use the rider to write out "Hello World!" *)
           Files.WriteString(r, "Hello World!");
           (* Write a end of line *)
           Files.WriteString(r, 0AX);
           (* Close our modified file *)
           Files.Close(f);
         END WriteHelloWorld;
       
         PROCEDURE ReadHelloWorld;
           VAR
             f : Files.File;
             r : Files.Rider;
             (* We need a string to store what we've read *)
             src : ARRAY 256 OF CHAR;
         BEGIN
           (* Create our file, New returns a file handle *)
           f := Files.Old("HelloWorld.txt");
           (* Set the position of the rider to the beginning *)
           Files.Set(r, f, 0);
           (* Use the rider to write out "Hello World!" *)
           Files.ReadString(r, src);
           (* Check the value we read, if it is not the name, 
              halt the program with an error *)
           ASSERT(src = "Hello World!");
         END ReadHelloWorld;
       
         PROCEDURE DeleteHelloWorld;
           VAR
             result : INTEGER;
         BEGIN
           (* Delete our file *)
           Files.Delete("HelloWorld.txt", result);
           (* Check our result, if not zero then halt program with error *)
           ASSERT(result = 0);
         END DeleteHelloWorld;
       
       BEGIN
           WriteHelloWorld();
           ReadHelloWorld();
           DeleteHelloWorld();
       END TypingHi.

Next and Previous


  1. Modules can be explicitly unload otherwise they persist until the computer is turned off↩︎

  2. https://miasap.se/obnc/obncdoc/basic/Files.def.html↩︎