dotlock(8)                                             dotlock(8)



NAME
       dotlock - Apply dotlocking to a mailbox file

SYNOPSIS
       dotlock mailbox command [ args ...  ]

DESCRIPTION
       dotlock  locks  the file mailbox and runs command with the
       given arguments.  If command does  not  finish  within  55
       seconds,  it  is sent an ALRM signal.  It is the responsi-
       bility of command to take appropriate action  in  response
       to  this  signal; i.e., to restore mailbox to its original
       state.  When command exits, or in any case after  60  sec-
       onds, the lock is removed.


DOTLOCKING
       Dotlocking requires write access to the directory in which
       mailbox resides.  dotlock creates a temporary file with  a
       unique  name in that directory, and attempts to link it to
       mailbox.lock.   If  this  succeeds,  the  lock  has   been
       aquired.  A countdown timer is started, the temporary file
       is removed, and command is run.

       If locking failed due to the  existence  of  mailbox.lock,
       dotlock  will wait and retry.  However, if mailbox.lock is
       older than  60 seconds, it is consider fair game for being
       stolen,  in  which case dotlock will remove it and proceed
       as described before.

       The temporary file used by dotlock is given a  name  mail-
       box.lock  followed  by a string derived from dotlocks PID,
       the current time, and the name of the current host.


BUGS
       Dotlocking is subject to a serious race condition  in  the
       case  of  the  timer  expiring,  when  the lock is stolen.
       Therefore, dotlocking should  never  be  used  unless  the
       alternative  is worse.  Having to do file locking over NFS
       may be seen by some as worse, which is  why  this  program
       was  written  in  the first place.  Users with NFS mounted
       home directories are adviced to rely on  maildir  delivery
       whenever possible.


EXIT CODES
       Normally,  dotlock will exit using the same exit status as
       command.  Otherwise, the exit code is 100 for wrong usage,
       and 111 for other errors.


SEE ALSO
       dot-qmail(9), mboxappend(8)



                                                       dotlock(8)