freebsd/usr.bin/lockf/lockf.1

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.Dd November 25, 2023
.Dt LOCKF 1
.Os
.Sh NAME
.Nm lockf
.Nd execute a command while holding a file lock
.Sh SYNOPSIS
.Nm
.Op Fl knsw
.Op Fl t Ar seconds
.Ar file
.Ar command
.Op Ar arguments
.Nm
.Op Fl s
.Op Fl t Ar seconds
.Ar fd
.Sh DESCRIPTION
The
.Nm
utility acquires an exclusive lock on a
.Ar file ,
creating it if necessary,
.Bf Em
and removing the file on exit unless explicitly told not to.
.Ef
While holding the lock, it executes a
.Ar command
with optional
.Ar arguments .
After the
.Ar command
completes,
.Nm
releases the lock, and removes the
.Ar file
unless the
.Fl k
option is specified.
.Bx Ns -style
locking is used, as described in
.Xr flock 2 ;
the mere existence of the
.Ar file
is not considered to constitute a lock.
.Pp
.Nm
may also be used to operate on a file descriptor instead of a file.
If no
.Ar command
is supplied, then
.Ar fd
must be a file descriptor.
The version with a
.Ar command
may also be used with a file descriptor by supplying it as a path
.Pa /dev/fd/N ,
where N is the desired file descriptor.
The
.Fl k
option is implied when a file descriptor is in use, and the
.Fl n
and
.Fl w
options are silently ignored.
This can be used to lock inside a shell script.
.Pp
If the
.Nm
utility is being used to facilitate concurrency between a number
of processes, it is recommended that the
.Fl k
option be used.
This will guarantee lock ordering, as well as implement
a performance enhanced algorithm which minimizes CPU load associated
with concurrent unlink, drop and re-acquire activity.
It should be noted
that if the
.Fl k
option is not used, then no guarantees around lock ordering can be made.
.Pp
The following options are supported:
.Bl -tag -width ".Fl t Ar seconds"
.It Fl k
Causes the lock file to be kept (not removed) after the command
completes.
.It Fl s
Causes
.Nm
to operate silently.
Failure to acquire the lock is indicated only in the exit status.
.It Fl n
Causes
.Nm
to fail if the specified lock
.Ar file
does not exist.
If
.Fl n
is not specified,
.Nm
will create
.Ar file
if necessary.
.It Fl t Ar seconds
Specifies a timeout for waiting for the lock.
By default,
.Nm
waits indefinitely to acquire the lock.
If a timeout is specified with this option,
.Nm
will wait at most the given number of
.Ar seconds
before giving up.
A timeout of 0 may be given, in which case
.Nm
will fail unless it can acquire the lock immediately.
When a lock times out,
.Ar command
is
.Em not
executed.
.It Fl w
Causes
.Nm
to open
.Ar file
for writing rather than reading.
This is necessary on filesystems (including NFSv4) where a file which
has been opened read-only cannot be exclusively locked.
.El
.Pp
In no event will
.Nm
break a lock that is held by another process.
.Sh EXIT STATUS
If
.Nm
successfully acquires the lock, it returns the exit status produced by
.Ar command .
Otherwise, it returns one of the exit codes defined in
.Xr sysexits 3 ,
as follows:
.Bl -tag -width ".Dv EX_CANTCREAT"
.It Dv EX_TEMPFAIL
The specified lock file was already locked by another process.
.It Dv EX_CANTCREAT
The
.Nm
utility
was unable to create the lock file, e.g., because of insufficient access
privileges.
.It Dv EX_UNAVAILABLE
The
.Fl n
option is specified and the specified lock file does not exist.
.It Dv EX_USAGE
There was an error on the
.Nm
command line.
.It Dv EX_OSERR
A system call (e.g.,
.Xr fork 2 )
failed unexpectedly.
.It Dv EX_SOFTWARE
The
.Ar command
did not exit normally,
but may have been signaled or stopped.
.El
.Sh EXAMPLES
The first job takes a lock and sleeps for 5 seconds in the background.
The second job tries to get the lock and timeouts after 1 second (PID numbers
will differ):
.Bd -literal -offset indent
$ lockf mylock sleep 5 & lockf -t 1 mylock echo "Success"
[1] 94410
lockf: mylock: already locked
.Ed
.Pp
The first job takes a lock and sleeps for 1 second in the background.
The second job waits up to 5 seconds to take the lock and echoes the message on
success (PID numbers will differ):
.Bd -literal -offset indent
$ lockf mylock sleep 1 & lockf -t 5 mylock echo "Success"
[1] 19995
Success
[1]+  Done                    lockf mylock sleep 1
.Ed
Lock a file and run a script, return immediately if the lock is not
available. Do not delete the file afterward so lock order is
guaranteed.
.Pp
.Dl $ lockf -t 0 -k /tmp/my.lock myscript
.Pp
Protect a section of a shell script with a lock, wait up to 5 seconds
for it to become available.
Note that the shell script has opened the lock file
.Fa /tmp/my.lock ,
and
.Nm
is performing the lock call exclusively via the passed in file descriptor (9).
In this case
.Fl k
is implied, and
.Fl w
has no effect because the file has already been opened by the shell.
This example assumes that
.Ql >
is implemented in the shell by opening and truncating
.Pa /tmp/my.lock ,
rather than by replacing the lock file.
.Bd -literal -offset indent
(
	lockf -s -t 5 9
	if [ $? -ne 0 ]; then
		echo "Failed to obtain lock"
		exit 1
	fi

	echo Start
	# Do some stuff
	echo End
) 9>/tmp/my.lock
.Ed
.Sh SEE ALSO
.Xr flock 2 ,
.Xr lockf 3 ,
.Xr sysexits 3
.Sh HISTORY
A
.Nm
utility first appeared in
.Fx 2.2 .
.Sh AUTHORS
.An John Polstra Aq Mt jdp@polstra.com