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File/Resource Locking

  See also    perl/lock.hints   and   shell/file.hints

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File Locks

  NOTE: C and UNIX does not stop a program from accessing a file, all
  lock methods only work if all processes using the file use the same
  locking method. Usually this is by only one program but not always
  (EG: the passwd command with the /etc/passwd file).

   1/ flock() (BSD) is totally unreliable accross hosts.

   2/ lockf() (SysV) but appears to be NFS-specific it is actually
      and interface to the file locking functions fcntl().
      This may or may not be available on your host.

   3/ fcntl lock protocol is NFS complient but not always available

   4/ Create a lock file (open with create exclusive flags)
      Works from NFS version 3 will not work for root without exclusive.

   5/ rename() a file.  This should be atomic in all file systems
      Basically create a unique file and then try to rename it to
      the file lock.

      However internally it is really an atomic hard-link and a remove

   6/ Create a hard file link.  Essentually create a new directory entry
      which much be atomic (at some low level point) in all file systems.

   7/ Create a symbolic file link.
      Works for root, BSD, SysV, but unknown about NFS atomic

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