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32 .Nd execute a command while holding a file lock
43 utility acquires an exclusive lock on a
45 creating it if necessary,
47 and removing the file on exit unless explicitly told not to.
49 While holding the lock, it executes a
57 releases the lock, and removes the
63 locking is used, as described in
65 the mere existence of the
67 is not considered to constitute a lock.
71 utility is being used to facilitate concurrency between a number
72 of processes, it is recommended that the
75 This will guarantee lock ordering, as well as implement
76 a performance enhanced algorithm which minimizes CPU load associated
77 with concurrent unlink, drop and re-acquire activity.
81 option is not used, then no guarantees around lock ordering can be made.
83 The following options are supported:
84 .Bl -tag -width ".Fl t Ar seconds"
86 Causes the lock file to be kept (not removed) after the command
92 Failure to acquire the lock is indicated only in the exit status.
96 to fail if the specified lock
107 Specifies a timeout for waiting for the lock.
110 waits indefinitely to acquire the lock.
111 If a timeout is specified with this option,
113 will wait at most the given number of
116 A timeout of 0 may be given, in which case
118 will fail unless it can acquire the lock immediately.
119 When a lock times out,
129 for writing rather than reading.
130 This is necessary on filesystems (including NFSv4) where a file which
131 has been opened read-only cannot be exclusively locked.
136 break a lock that is held by another process.
140 successfully acquires the lock, it returns the exit status produced by
142 Otherwise, it returns one of the exit codes defined in
145 .Bl -tag -width ".Dv EX_CANTCREAT"
147 The specified lock file was already locked by another process.
152 was unable to create the lock file, e.g., because of insufficient access
154 .It Dv EX_UNAVAILABLE
157 option is specified and the specified lock file does not exist.
159 There was an error on the
169 did not exit normally,
170 but may have been signaled or stopped.
173 The first job takes a lock and sleeps for 5 seconds in the background.
174 The second job tries to get the lock and timeouts after 1 second (PID numbers
176 .Bd -literal -offset indent
177 $ lockf mylock sleep 5 & lockf -t 1 mylock echo "Success"
179 lockf: mylock: already locked
182 The first job takes a lock and sleeps for 1 second in the background.
183 The second job waits up to 5 seconds to take the lock and echoes the message on
184 success (PID numbers will differ):
185 .Bd -literal -offset indent
186 $ lockf mylock sleep 1 & lockf -t 5 mylock echo "Success"
189 [1]+ Done lockf mylock sleep 1
198 utility first appeared in
201 .An John Polstra Aq Mt jdp@polstra.com