The UNIX File System

Last time: Users and Files: who, cp

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Additional Credits

Announcements

• SQ6: Low 3, High 10, Mean 8.5
• Project #6 due today
• Homework #5 posted (due Friday)
• Project #7 should be posted tomorrow
• Today's Schedule
  • Efficiency of cp
  • Implementing tail
  • Implementing ls

Copy: does the size of the buffer matter?

• A buffer of size 1 works!
• Consider a filesize of 100,000 bytes
  • If buffer is 100 bytes => 1,000 read() and write() calls
  • If buffer is 10000 bytes => 10 read() and write() calls
  
  
  
• System calls are often resource intensive!
  • Requires a shift from user mode to kernel mode and back
  • Usually want to minimize system calls

Control flow in copying a file

I/O Efficiency

• Does this mean our who implementation is inefficient?
  • It might if we were accessing the file directly
  • Difficult to tell with the library
• In general, it is a good idea to read a bunch of records/bytes/etc at a time and then access them from the buffer

Can't the kernel do our buffering?

• Actually, it does (for disk access)
  • The kernel keeps copies of disk blocks in memory.
  • It writes those blocks to disk now and then.
  • The read() call actually copies data from kernel buffers, not from the disk.

Consequences of buffering

• Faster "disk" I/O
• Optimal disk reads and writes
• Need to sync disks before shutdown

Another example: tail

• What does tail do?
• How does tail work?

Reading arbitrary bytes from a file

• Q: How can a program jump to the end (or any arbitrary location) of a file?

Reading arbitrary bytes from a file

• Q: How can a program jump to the end (or any arbitrary location) of a file?
• A: lseek(fd, offset, base)

Directories, File Info, etc.

• We've now seen how to operate on the contents of a file
  • open, read, write, close, lseek
• But there is more to a file than just contents
  • Properties -- size, owner, permissions, type, ...
  • Location -- in a directory tree
• We will write a version of ls

Writing ls (Using same approach)

• What does ls do?

Writing ls (Using same approach)

• What does ls do?
  • Lists contents of directories and info about files
  • Has many options, such as -l -a -lu -s
  • Processes arguments of directories and filenames (or just current dir)
• Recall that the filesystem is organized like a tree

Writing ls (Using same approach)

• So writing ls should be easy...
  • Open directory
  • Repeatedly read directory entry and display entry
  • Close directory
• Much like the who command

Directories

• Q: What is a directory?
• A: A special kind of file that contains a list of files and/or other directories
• Note: Every directory contains special entries . and ..
• Q: Can we use file operators on directories? (e.g., open, read, etc.)
• A: Once, there was no other way

Reading Directories

• A2: Even if you could read all directories as files, you don't want to -- particularly since UNIX supports a variety of directory formats
  
• Q: So how do we read directories?

Reading Directories

• A2: Even if you could read all directories as files, you don't want to -- particularly since UNIX supports a variety of directory formats
  
• Q: So how do we read directories?

  man -k direct
  
  man -k direct | grep read
  

• A: opendir(), readdir(), closedir()

Using Directory Functions

#include <dirent.h>

opendir(name)
• Creates a connection, returns a DIR *
readdir(DIR *)
• Gets next record from directory, returns struct dirent *
closedir(DIR *)
• Closes the connection

So, this is enough to start implementing ls.

Writing first version of ls.c

main()
    opendir()
    while ( readdir() )
       print d_name

    closedir()
    return 0

• Let's flesh out, compile and run the code