Objectives

1. Understand the basics of File I/O

Notes

• This is chapter 4, you should read it.
• This is the first of several chapters that cover I/O
• in the end, Figure 5-2 page 95 is important
  • It will take a while to understand
  • But once you do, it will be great
  • 
• Quick review of files in c++
• File Descriptors
  • A file descriptor is a non-negative integer.
  • As you can see above, it is just an index into a table associated with your process.
• The cool part of UNIX/Linux is that everything is a file
  • A file descriptor can point to
    • A regular file
      • Including hard links
      • And soft links
    • A pipe
    • A socket
    • Devices (/dev/pts/0) (block and character)
  • Missing from this list are directories
    • They have their own special methods.
• Most programs start with three open file descriptors
  • 0: standard input, (cin in c++) , STDIN_FILENO
  • 1: standard output, (cout in c++), STDOUT_FILENO
  • 2: standard error, (cerr in c++) , STDERR_FILENO
• The file names are from unistd.h
• These are inherited from the parent process.
• There are four main file manipulation routines
  • open takes a path and some flags and returns a file descriptor
  • close takes a file descriptor and returns the status
  • read takes a file descriptor, array and size and returns the number of bytes read.
  • write takes a file descriptor, array and size and returns the number of bytes written.
• There are some additional routines as ell
• creat a form of open for creating new files.
• lseek allows us to move around in a file.
• ioctl Additional file manipulation.
• Note, all of these operations are on the byte level, not character!
• I/O is refereed to as Universal as read/write/open/close work on most files.
• Take a look at demo1.cpp
  • od -x binaryFile
  • hexdump binaryFile