Computers: Universe: Directory Files

To create a directory file, the command is:

CREATE.FILE filename 1 or
CREATE.FILE filename 19

Directory files gain their name from the fact that they are implemented as directories under UNIX: each record in the directory file being a simple ASCII file under UNIX.

The different between type 1 and type 19 directory files is subtle. Type 1 files break up long record keys into sections not longer than 14 characters, and created a set of nested directories to store the entire name. This is a little difficult to explain except by example:

If you save a record called THISISAVERYLONGNAME in a type 1 file called TYPE1, the path to the UNIX file created will run .../TYPE1/THISISAVERYLON/GNAME. Thus the file will be called GNAME, and it will be the only file in a directory called THISISAVERYLON. This mechanism is a historical throwback to operating systems such as MSDOS which cannot handle long file and directory names.

In addition, keys to records in type 1 files are limited to 41 characters in length. A key of 123456789-123456789-123456789-123456789-12 would be automatically truncated to 123456789-123456789-123456789-123456789-1, and saved as ...TYPE1/123456789-1234/56789-12345678/9-123456789-1.

Unless, therefore, you have some reason to make your file structure, when viewed from UNIX, easily transportable to MSDOS, use type 19 files, which operate far more straightforwardly. The same record in a type 19 file is saved with a UNIX pathname of ...TYPE19/THISISAVERYLONGFILENAME.

Do not be confused by this into thinking that the key of a record in a type 1 file changes from the point of view in UniVerse: type 1 and type 19 files are indistinguishable from UniVerses point of view, and any UniVerse command or program would read the key THISISALONGFILENAME from either a type 1 or a type 19 file. It is only when you look at the storage under UNIX that the difference becomes apparent.

Directory files have a number of uses:

1. Only type 1 and type 19 files make their records and individually readable to UNIX. They are therefore sometimes used to communicate with UNIX programs.

2. The UniVerse Basic compiler will only compile records in a type 1 or type 19 file. All source code, therefore, has to be stored in such files.

3. There are a few other UniVerse commands which will work only with type 1 or type 19 files, but these are much less important than the basic compiler.

4. UniVerse Basic provides a group of statements for 'sequential file I/O', in which the fields of records are read or written in sequence. These only work on records in type 1 or type 19 files.

As can be seen, the uses of directory files are very specific, and unless you have a specific reason to use a directory file, do not create one. Above all, do not use them to contain randomly accessed application data (such as customer or invoice records), as they are extremely inefficient in this role.