File I/O

• A file descriptor is an integer used to reference a file.
  • Actually it is a bit more complicated than that.
  • On the file system, the space is decomposed into a number of blocks.
    • Each of these blocks is of a fixed size and has an address.
    • Blocks are collected into files by inodes
    • For each file on the system, there is an inode.
    • inodes are identified by unique numbers.
    • Much more on this in chapter 14 if we get there.
    • The Inode contains information such as
      • File type : file, pipe, socket, ...
      • List of blocks in the file.
      • Access times and other meta information about the file
    • There is one inode per file.
    • But, strangely, multiple file names may point to the same inode.
      • This is called a hard link.
  • To keep track of open files, the system maintains a table of active files
    • This is a system wide table.
    • This table contains information about the files that are open
      • The inode number
      • The status of the file.
      • An offset into the file (currently position where the accessing processes is reading/writing)
    • There is at least one entry in this table per open file.
    • But there may be multiple entries per inode.
      • Program 1 and Program 2 are both accessing foo.txt
    • This table is probably a static array, built at kernel compile time
    • So there is a system wide limit on open files.
    • (Open can fail with ENFILE The system-wide limit on the total number of open files has been reached.)
  • Finally, there is a file descriptor table
    • This is a per process table.
    • It contains file flags
    • And a pointer into the system wide open file table.
    • It is most likely a static table as well.
    • So there is a limit on the number of open file descriptors a process may have
    • ( Open can fail with EMFILE The per-process limit on the number of open file descriptors has been reached )
    • A file descriptor points to an open file, which points to an inode.
    • Multiple file descriptors (in the same or different processes) can point to the same open file, which points to a single inode
  • 
• As indicated, file descriptors are small non-negative integers
  • They are indexes into the file descriptor table.
• File descriptors are created with a number of system commands
  • open
  • dup
• File descriptors are accessed with a number of system commands
  • read, write
  • lseek
• File descriptors are released with close
• Each process is created with three open file descriptors
  • standard input, fd = 0, STDIN_FILENO
  • standard output, fd = 1, STDOUT_FILENO
  • standard error, fd = 2, STDERR_FILENO
• Processes also inherits file open file descriptors from their parent.
  • discuss Code2HtML < a.C > a.html