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30 .Nd execute a command while holding a file lock
45 utility acquires an exclusive lock on a
47 creating it if necessary,
49 and removing the file on exit unless explicitly told not to.
51 While holding the lock, it executes a
59 releases the lock, and removes the
65 locking is used, as described in
67 the mere existence of the
69 is not considered to constitute a lock.
72 may also be used to operate on a file descriptor instead of a file.
77 must be a file descriptor.
80 may also be used with a file descriptor by supplying it as a path
82 where N is the desired file descriptor.
85 option is implied when a file descriptor is in use, and the
89 options are silently ignored.
90 This can be used to lock inside a shell script.
94 utility is being used to facilitate concurrency between a number
95 of processes, it is recommended that the
98 This will guarantee lock ordering, as well as implement
99 a performance enhanced algorithm which minimizes CPU load associated
100 with concurrent unlink, drop and re-acquire activity.
104 option is not used, then no guarantees around lock ordering can be made.
106 The following options are supported:
107 .Bl -tag -width ".Fl t Ar seconds"
109 Causes the lock file to be kept (not removed) after the command
115 Failure to acquire the lock is indicated only in the exit status.
119 to fail if the specified lock
130 Specifies a timeout for waiting for the lock.
133 waits indefinitely to acquire the lock.
134 If a timeout is specified with this option,
136 will wait at most the given number of
139 A timeout of 0 may be given, in which case
141 will fail unless it can acquire the lock immediately.
142 When a lock times out,
152 for writing rather than reading.
153 This is necessary on filesystems (including NFSv4) where a file which
154 has been opened read-only cannot be exclusively locked.
159 break a lock that is held by another process.
163 successfully acquires the lock, it returns the exit status produced by
165 Otherwise, it returns one of the exit codes defined in
168 .Bl -tag -width ".Dv EX_CANTCREAT"
170 The specified lock file was already locked by another process.
175 was unable to create the lock file, e.g., because of insufficient access
177 .It Dv EX_UNAVAILABLE
180 option is specified and the specified lock file does not exist.
182 There was an error on the
192 did not exit normally,
193 but may have been signaled or stopped.
196 The first job takes a lock and sleeps for 5 seconds in the background.
197 The second job tries to get the lock and timeouts after 1 second (PID numbers
199 .Bd -literal -offset indent
200 $ lockf mylock sleep 5 & lockf -t 1 mylock echo "Success"
202 lockf: mylock: already locked
205 The first job takes a lock and sleeps for 1 second in the background.
206 The second job waits up to 5 seconds to take the lock and echoes the message on
207 success (PID numbers will differ):
208 .Bd -literal -offset indent
209 $ lockf mylock sleep 1 & lockf -t 5 mylock echo "Success"
212 [1]+ Done lockf mylock sleep 1
214 Lock a file and run a script, return immediately if the lock is not
215 available. Do not delete the file afterward so lock order is
218 .Dl $ lockf -t 0 -k /tmp/my.lock myscript
220 Protect a section of a shell script with a lock, wait up to 5 seconds
221 for it to become available.
222 Note that the shell script has opened the lock file
226 is performing the lock call exclusively via the passed in file descriptor (9).
231 has no effect because the file has already been opened by the shell.
232 This example assumes that
234 is implemented in the shell by opening and truncating
236 rather than by replacing the lock file.
237 .Bd -literal -offset indent
240 if [ $? -ne 0 ]; then
241 echo "Failed to obtain lock"
257 utility first appeared in
260 .An John Polstra Aq Mt jdp@polstra.com