Chapter 4, File I/O

• This is at least part of chapter 4.
  • Give it a read as we work our way through it.
• I/O in general is far more complex than you think.
  • Many of the details of I/O are hidden in streams.
    • I believe that you can do most of this with streams.
    • But I don't know streams well enough to say for certain.
    • Streams are much more complex than what we present.
  • The title: The Universal I/O model
    • This is good for more than just "regular" files.
    • To accomplish this, the system has more depth than you might think.
  • It is going to take several chapters to understand the I/O system.
• First off, there are a number of different types of files
  • Regular files, this includes hard links.
  • Symbolic links.
  • Directories
  • FIFOs
  • Sockets
    • (see server.c and client.c from the man 7 unix pages.
    • (run the server, client 1 2 3 4, client down)
    • Block Special Devices
    • Character Special Devices
  • When I ask you for the different file types on the midterm I want the list above
    • They are not binary, text, ...
    • These are all examples of regular files.
  • Most IO is done through a basic set of commands
    • This changes slightly for different file types
    • But the basis is in chapter 4.
  • The basic unit for a file is a file descriptor
    • This is a small integer.
    • Actually it is an index into a per process table.
    • 
      • This is from chapter 5.
      • But it is ok to look at now.
    • The file descriptor maps to an open file in the kernel.
    • Which maps to the appropriate place in the file system.
    • A single process can have the same file open twice.
    • An implication of this is that two processes can have the same file open
  • By default three file descriptors are open for every process
    • 0 is mapped to standard in.
    • 1 is mapped to stdout
    • 2 is mapped to std err
  • The basic calls
    • open
      • takes a file name,
      • takes a set of arguments on how to open that file.
      • If successful returns a non-negative file descriptor.
      • On fail, returns a -1.
      • Open is the most complex.
    • write
      • Takes a fd
      • And a buffer
      • And a buffer size (in bytes)
      • And attempts to write from the buffer to the fd for up to the size.
      • It returns the number of bytes written.
    • read
      • Takes a fd
      • And a buffer
      • And a size
      • And attempts to read from that buffer for that size.
      • It returns the number of bytes read.
    • close
      • Takes a fd
      • Attempts to close the file
      • Returns a status.
  • Note that write and read read and write raw bits.
  • simple.cpp will demonstrate this.
  • Take a look at book.cpp
    • This is a modification of his code from the book.
    • He suggests
      • ./book ocap.txt new.txt
      • ./book ocap.txt /dev/tty
      • ./book ocap.txt /dev/pts/x - where x is another terminal.
      • ./book /dev/tty capture
      • Use a named pipe.
  • This is what they mean by a universal i/o system.