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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 successfully 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


       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

           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 program.


       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


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


       Sam Varshavchik


        1. maildrop(1)