Users and Files: who and cp

Last time: Users and Files: who

Prof. Brian D. Davison

Computer Science & Engineering, Lehigh University

Additional Credits

Announcements

• Project #6 is due next Monday.
• P4 returned on Wednesday: Low 30, high 100, mean 84
• Today's schedule
  • Short quiz #6
  • Continue Understanding UNIX Programming
    • Final version of who.c
    • Discuss copy (cp) implementation
    • implement cp

Final version of who.c

• We ended last time with this version.
• A cleaned up version looks like this.

Let's now consider: cp (read and write)

% cp sourcefile copyfile

• What does cp do?
  • Answer:
• How does it do so?
  • Answer:

cp (read and write)

% cp sourcefile copyfile

• What does cp do?
  • Answer: creates or truncates a file, then writes data into it from source file
• How does it do so?
  • Answer: let's find out by searching the manual pages

Creating/truncating a file

• Creating
  • fd = creat(name, 0644)
  • First creates or truncates a file
  • Then opens the file for writing
  • If create, set mode to 0644
  • returns -1 on error, otherwise a file descriptor
• Writing
  • n = write(fd, buffer, num)
  • sends num bytes to the file
  • returns actual number sent, or -1 on error

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?