Review midterm answers

Hand out lab2

4.1.2: File structure

A file is a

1. Byte stream
   • Unix, dos, windows (I think)
   • Max flexibility
   • Min structure
   
   
2. (fixed size) Record stream: Out of date
   
   
3. Varied and complicated beast
   • Indexed sequential
   • B-trees
   • Supports rapidly finding a record with a specific key
   • Supports retrieving (varying size) records in key order.
   • Treated in depth in database courses

4.1.3: 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: Easy (hopefully normal) case easier (and safer) hard case harder.

4.1.4: File access

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.

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

4.1.5: File attributes

A laundry list of properties that can be specified for a file, e.g.

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

  4.1.6: File operations

  • Create: Essential if a system is to add files. Need not be a separate system call (can be merged with open).
  • Delete: Essential if a system is to delete files.
  • Open: Not essential. An optimization. Do the translation from file name to disk locations only once per file rather than once per access.
  • Close: Not essential. Free resources.
  • Read: Essential. Must specify filename, file location, number of bytes, where to put data read in. Several of these parameters can be set by other system calls and in many OS's they are.
  • Write: Essential if updates are to be supported. See read for parameters.
  • Seek: Not essential (could be in read/write). Specify the file offset of the next (read or write) access
  • Get attributes: Essential if attributes are to be used.
  • Set attributes: Essential if attributes are to be user settable.
  • Rename: Tanenbaum has strange words. Copy and delete is not acceptable for big files. Moreover copy-delete not atomic. Indeed link-delete is not atomic so even with link (discussed below) renaming a file adds functionality.

  Homework: 2, 3, 4.
  Read and understand ``copyfile'' on page 155.