Remark: In spring 2002, due to my attending a conference, showing all the parts of a disk and doing a problem involving I/O time are done here.

6.1.3: File types

Examples

1. (Regular) files.
   
   
2. Directories: studied below.
   
   
3. Special files (for devices). Uses the naming power of files to unify many actions.

       dir             # prints on screen
       dir > file      # result put in a file
       dir > /dev/tape # results written to tape
       

4. ``Symbolic'' Links (similar to ``shortcuts''): Also studied below.

``Magic number'': Identifies an executable file.

• There can be several different magic numbers for different types of executables.
• unix: #!/usr/bin/perl

Strongly typed files:

• The type of the file determines what you can do with the file.
• This make the easy and (hopefully) common case easier and, more importantly safer.
• It tends to make the unusual case harder. For example, you have a program that turns out data (.dat) files. But you want to use it to turn out a java file but the type of the output is data and cannot be easily converted to type java.

6.1.4: File access

There are basically two possibilities, sequential access and random access (a.k.a. direct access). Previously, files were declared to be sequential or random. Modern systems do not do this. Instead all files are random and optimizations are applied when the system dynamically determines that a file is (probably) being accessed sequentially.

1. With Sequential access the bytes (or records) are accessed in order (i.e., n-1, n, n+1, ...). Sequential access is the most common and gives the highest performance. For some devices (e.g. tapes) access ``must'' be sequential.
2. With random access, the bytes are accessed in any order. Thus each access must specify which bytes are desired.

6.1.5: File attributes

A laundry list of properties that can be specified for a file For example:

• hidden
• do not dump
• owner
• key length (for keyed files)