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       lockmail - create mail lock files


       lockmail [-r] [-t timeout] {lockfile} {program} [argument...]


       lockmail is a helper utility for working with mailbox files. Mailbox
       files must be locked to prevent other applications from modifying the
       mailbox at the same time. Different system use different locking
       conventions.  lockmail uses two of the most common locking mechanisms
       in use, which should work reliably on most systems.

       lockfile is the pathname to an existing mailbox file. By default,
       lockmail tries to lock the mailbox every five seconds (if the mailbox
       is already locked), and will give up after three minutes. After the
       mailbox is succesfully locked, lockmail runs program as a child
       process, with any optional arguments. When program terminates, lockmail
       removes the mailbox lock, and terminates itself.


           If a regular lock fails, try a read-only lock. Use this option to
           lock mailbox files in a read-only directory.

       -t timeout
           If the lock attempt fails, try again for up to timeout seconds. The
           actual timeout is rounded up to the next five second interval (a
           lock attempt is tried every five seconds).


       This section briefly describes the locking mechanism used by lockmail.
       lockmail uses three different locking conventions in order to maximize
       compatibility with other mail software: C-Client folder locks,
       dot-locks, and file locks.

   C-Client folder locks
       Mail software based on the C-Client library creates lock files named
       /tmp/.dddddd.iiiiii. Here, dddddd and iiiiii are the device number and
       the inode number of the mailbox file (the st_dev and st_ino fields in
       the inode), in hexadecimal. If the process ID saved in the C-Client
       folder lock file is not valid, lockmail concludes that it´s a stale
       lock file, and will remove it.

           A race condition exists where a C-Client process is killed after it
           creates a lock file, but before saving its process ID in the lock
           file. The race window is very small, but it exists. The C-Client
           library does not appear to ever clear out the lock file.

           lockmail attempts to resolve this race condition by deleting
           zero-length lock files that are at least five minutes old.

       lockmail also creates, and honors dot-lock files. Dot-lock files are
       first created as temporary files, then linked to lockfile.lock. The
       link operation fails if the dot-lock file already exists.  lockmail
       uses an enhanced method of dot-locking, where its process ID, and the
       name of the server where lockmail is running is also saved in its
       dot-lock file. If the operation fails due to an existing dot-lock file
       that was created by another lockmail process on the same server, and
       the process ID no longer exists, this stale dot-lock file is removed
       immediately. In all other situations a dot-lock file older than five
       minutes is considered stale, and removed.

           A failure to create a dot-lock file is silently ignored if the
           reason for the failure is because lockmail does not have the write
           permission in the dot-lock file´s directory. The incoming mail
           spool directory (usually /var/mail) typically does not have global
           write permissions, so the attempt to create the dot-lock file in
           the spool directory will fail, and lockmail will be content with
           using file-locking only.

   File locks
       The final locking mechanism lockmail uses is the operating system´s
       file locking facility. If lockmail fails to obtain all three locks,
       lockmail will sleep for five seconds and try again. The only exception
       is a failure to create a dot-lock because of no write access to the
       dot-lock file´s directory, which is ignored. If lockmail still fails to
       obtain all required locks in the amount of time specified by the -t
       option (or its default value), lockmail will terminate with the
       EX_TEMPFAIL exit code.

       lockmail runs program after obtaining the last file lock, waits until
       program terminates, and releases all locks.  program must terminate
       before any of the locks obtained by lockmail expire, and are considered
       stale.  lockmail will then terminate with the same exit code as


       lockmail terminates with the same exit status as program lockmail
       terminates with the EX_TEMPFAIL exit status if it was unable to obtain
       a lock, or if program was killed by a signal.


       maildrop(1)[1], sendmail(8).


        1. maildrop(1)
           [set $man.base.url.for.relative.links]/maildrop.html