Chapter 5, Further File I/0 Details

• Look at the picture on page 95.
• The File Descriptor table contains
  • A few flags (Close On Exec)
  • A pointer into the Open File Table.
• The Open File Table contains
  • The current offset
    • Note, this is not per process
    • Or per file descriptor.
  • Most of the flags set by open
  • The file mode
  • A reference to the i-node table
• The i-node table connects to the real file
  • Includes the file type (regular, directory, fifo, ...)
  • Any "locks" on the file
  • Properties, including file size, time stamps, permissions, ...
• Truncate
  • man 2 truncate
  • Will set the length of a file to a given length.
  • Note two forms, truncate and ftruncate
• mkstemp
  • Because of race conditions, making temporary files is challenging
    • Search the directory for a unused temporary name.
    • Open the file.
  • mkstemp solves this problem
  • int mkstemp(char *template);
    • The last 6 characters of the template should be XXXXXX
    • These will be replaced with a unique random pattern.
  • The file is opened
    • O_EXCL, O_RDONLY,
    • READ/Write user, no other permissions
  • Typically the call is followed by unlink
  • When the file descriptor is closed, the file will go away.
  • Look at tmpMaker.C
• There are other topics in this chapter, large files, non-blocking i/o, gather/scatter but we will skip these.