Chapter 4: File I/O

• We will begin discussing File IO in the next few classes.
• Unix/Linux has what is called a Universal Model.
  • Access to all information sources is through the IO system.
• We will see that dealing with a file is the same as
  • Dealing with communications mechanisms.
  • Dealing with a terminal
  • This carries through with other systems as well.
• A file descriptor is a samll non-negative integer
  • It is most likely an index into a per-process file descriptor table.
  • It is created with a call to open
  • It is changed with read, write, seek, ....
  • It is closed with close.
• The shell normally operates with three open file descriptors
  • 0 is usually stdin, STDIN_FILENO
  • 1 is usually stdout, STDOUT_FILENO
  • 2 is usually stderr, STDERR_FILENO
  • These are normally the terminal.
  • But we will see how to change these to be other things.
• Be careful here, we are dropping to a much lower level of IO than cin and cout
  • streams in C++ can do these things, but it is not the default.
  • files in C can do these things, but it is not the default.
• The general form

  • int fd;
    
    fd = open(open, params, modes)
    
    // for output
    write(fd, values, size)
    
    // for input
    read(fd, values, size)
    
    close(fd)
    

  Look at simple.C.
  • Just exactaly what does this do?
• The system utility od or octal dump
  • This allows us to look at the contents of a file.
  • hex (od -x)
  • octal (od)
  • decimal (od -d/-i/-l)
  • float (od -f)
  • ascii (od -c)
  • or other formats
• Do an od -c, od -d, od -x ... on output1 from simple.C
• Look quickly at simple2.C
  • This is not good code, it ignores many problems.
  • But it does demonstrate what we need.
• Do an od -c, od -d, od -x ... on output2 from simple2.C
• What is the differenc?
  • What does cout << i; do?
  • What does sizeof do in c/c++
    • See the online reference.
  • What does write(fd, &i; sizeof(int)); do?