Users and Files: cp, ls, and tail

Last time: Users and Files: copy

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Announcements

• Scripting project due Thursday
• Homework 5 online, due in one week
• Friday's quiz: unix programming
• Continue Understanding UNIX Programming
  • implement cp
  • tail
  • ls

Writing copy.c

• Want (logically):
  • open sourcefile
  • creat copyfile
  • read source to a buffer (until eof)
  • write buffer to the copy, repeat
  • close both files
• Let's do it... who is the first volunteer?

Control flow in copying a file

Does the size of the buffer matter?

• Could use a buffer of size 1!
• 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 resource intensive!
• Requires shift from user mode to kernel mode and back
• Usually want to minimize system calls

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
  • 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.

Control flow in copying a file

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?
• 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?
  • 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
  • Let's try cat /; more /tmp; od -c /etc

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 compile and run the code