File Descriptors, Files and Inodes

• 
• At the base of this system is the inode
  • This is a structure that holds information about a file.
  • Defined for various file systems in /usr/include/linux
  • ext2fs_fs.h is the clearest
    • Mode info
    • Owner
    • Access Info
    • Time info
    • number of links.
    • Permissions
    • Surprisingly, no name. (Symbolic,hard links)
    • A pointer to blocks on the disk.
      • This is a hierarchical system
  • There is a posix standard.
  • More on this later (possibly)
  • I have never really had to work directly with inodes.
  • Inodes are associated with the file system.
  • These are most likely stored in an in-memory table.
  • And on the file system
• Next is the open file table.
  • This is maintained by the kernel.
  • There is one entry in this table per call to open.
  • This will contain the flags set by open
  • A pointer to the inode table.
  • The file offset (lseek, ...)
  • Note: multiple processes can have the file open at any time
  • Or even one process twice.
• Finally the per-process file descriptor table.
• Some things to note on the picture:
  • fd2 of process A and B both point to the same file handle, ie the result of a fork.
  • fd1 and 20 of process A both refer to the same file handle, a dup
  • fd0 of process A and fd3 of process B both refer to the same file, Different opens but same file.