1LOCKMAIL(1) Double Precision, Inc. LOCKMAIL(1)
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6 lockmail - create mail lock files
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9 lockmail [-r] [-t timeout] {lockfile} {program} [argument...]
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12 lockmail is a helper utility for working with mailbox files. Mailbox
13 files must be locked to prevent other applications from modifying the
14 mailbox at the same time. Different system use different locking
15 conventions. lockmail uses two of the most common locking mechanisms
16 in use, which should work reliably on most systems.
17 lockfile is the pathname to an existing mailbox file. By default,
19 lockmail tries to lock the mailbox every five seconds (if the mailbox
20 is already locked), and will give up after three minutes. After the
21 mailbox is successfully locked, lockmail runs program as a child
22 process, with any optional arguments. When program terminates, lockmail
23 removes the mailbox lock, and terminates itself.
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26 -r
27 If a regular lock fails, try a read-only lock. Use this option to
28 lock mailbox files in a read-only directory.
29 -t timeout
31 If the lock attempt fails, try again for up to timeout seconds. The
32 actual timeout is rounded up to the next five second interval (a
33 lock attempt is tried every five seconds).
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36 This section briefly describes the locking mechanism used by lockmail.
37 lockmail uses three different locking conventions in order to maximize
38 compatibility with other mail software: C-Client folder locks,
39 dot-locks, and file locks.
40 C-Client folder locks
42 Mail software based on the C-Client library creates lock files named
43 /tmp/.dddddd.iiiiii. Here, dddddd and iiiiii are the device number and
44 the inode number of the mailbox file (the st_dev and st_ino fields in
45 the inode), in hexadecimal. If the process ID saved in the C-Client
46 folder lock file is not valid, lockmail concludes that it's a stale
47 lock file, and will remove it.
48 Note
50 A race condition exists where a C-Client process is killed after it
51 creates a lock file, but before saving its process ID in the lock
52 file. The race window is very small, but it exists. The C-Client
53 library does not appear to ever clear out the lock file.
54 lockmail attempts to resolve this race condition by deleting
56 zero-length lock files that are at least five minutes old.
57 dot-locks
59 lockmail also creates, and honors dot-lock files. Dot-lock files are
60 first created as temporary files, then linked to lockfile.lock. The
61 link operation fails if the dot-lock file already exists. lockmail
62 uses an enhanced method of dot-locking, where its process ID, and the
63 name of the server where lockmail is running is also saved in its
64 dot-lock file. If the operation fails due to an existing dot-lock file
65 that was created by another lockmail process on the same server, and
66 the process ID no longer exists, this stale dot-lock file is removed
67 immediately. In all other situations a dot-lock file older than five
68 minutes is considered stale, and removed.
69 Note
71 A failure to create a dot-lock file is silently ignored if the
72 reason for the failure is because lockmail does not have the write
73 permission in the dot-lock file's directory. The incoming mail
74 spool directory (usually /var/mail) typically does not have global
75 write permissions, so the attempt to create the dot-lock file in
76 the spool directory will fail, and lockmail will be content with
77 using file-locking only.
78 File locks
80 The final locking mechanism lockmail uses is the operating system's
81 file locking facility. If lockmail fails to obtain all three locks,
82 lockmail will sleep for five seconds and try again. The only exception
83 is a failure to create a dot-lock because of no write access to the
84 dot-lock file's directory, which is ignored. If lockmail still fails to
85 obtain all required locks in the amount of time specified by the -t
86 option (or its default value), lockmail will terminate with the
87 EX_TEMPFAIL exit code.
88 lockmail runs program after obtaining the last file lock, waits until
90 program terminates, and releases all locks. program must terminate
91 before any of the locks obtained by lockmail expire, and are considered
92 stale. lockmail will then terminate with the same exit code as
93 program.
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96 lockmail terminates with the same exit status as program lockmail
97 terminates with the EX_TEMPFAIL exit status if it was unable to obtain
98 a lock, or if program was killed by a signal.
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101 maildrop(1)[1], sendmail(8).
102
104 Sam Varshavchik
105 Author
106
108 1. maildrop(1)
109 http://www.courier-mta.org/maildrop.html
110Courier Mail Server 11/04/2020 LOCKMAIL(1)