1.\" 2.\" Copyright (C) 1998 John D. Polstra. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED BY JOHN D. POLSTRA AND CONTRIBUTORS ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL JOHN D. POLSTRA OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD$ 26.\" 27.Dd August 26, 2020 28.Dt LOCKF 1 29.Os 30.Sh NAME 31.Nm lockf 32.Nd execute a command while holding a file lock 33.Sh SYNOPSIS 34.Nm 35.Op Fl knsw 36.Op Fl t Ar seconds 37.Ar file 38.Ar command 39.Op Ar arguments 40.Sh DESCRIPTION 41The 42.Nm 43utility acquires an exclusive lock on a 44.Ar file , 45creating it if necessary, 46.Bf Em 47and removing the file on exit unless explicitly told not to. 48.Ef 49While holding the lock, it executes a 50.Ar command 51with optional 52.Ar arguments . 53After the 54.Ar command 55completes, 56.Nm 57releases the lock, and removes the 58.Ar file 59unless the 60.Fl k 61option is specified. 62.Bx Ns -style 63locking is used, as described in 64.Xr flock 2 ; 65the mere existence of the 66.Ar file 67is not considered to constitute a lock. 68.Pp 69If the 70.Nm 71utility is being used to facilitate concurrency between a number 72of processes, it is recommended that the 73.Fl k 74option be used. 75This will guarantee lock ordering, as well as implement 76a performance enhanced algorithm which minimizes CPU load associated 77with concurrent unlink, drop and re-acquire activity. 78It should be noted 79that if the 80.Fl k 81option is not used, then no guarantees around lock ordering can be made. 82.Pp 83The following options are supported: 84.Bl -tag -width ".Fl t Ar seconds" 85.It Fl k 86Causes the lock file to be kept (not removed) after the command 87completes. 88.It Fl s 89Causes 90.Nm 91to operate silently. 92Failure to acquire the lock is indicated only in the exit status. 93.It Fl n 94Causes 95.Nm 96to fail if the specified lock 97.Ar file 98does not exist. 99If 100.Fl n 101is not specified, 102.Nm 103will create 104.Ar file 105if necessary. 106.It Fl t Ar seconds 107Specifies a timeout for waiting for the lock. 108By default, 109.Nm 110waits indefinitely to acquire the lock. 111If a timeout is specified with this option, 112.Nm 113will wait at most the given number of 114.Ar seconds 115before giving up. 116A timeout of 0 may be given, in which case 117.Nm 118will fail unless it can acquire the lock immediately. 119When a lock times out, 120.Ar command 121is 122.Em not 123executed. 124.It Fl w 125Causes 126.Nm 127to open 128.Ar file 129for writing rather than reading. 130This is necessary on filesystems (including NFSv4) where a file which 131has been opened read-only cannot be exclusively locked. 132.El 133.Pp 134In no event will 135.Nm 136break a lock that is held by another process. 137.Sh EXIT STATUS 138If 139.Nm 140successfully acquires the lock, it returns the exit status produced by 141.Ar command . 142Otherwise, it returns one of the exit codes defined in 143.Xr sysexits 3 , 144as follows: 145.Bl -tag -width ".Dv EX_CANTCREAT" 146.It Dv EX_TEMPFAIL 147The specified lock file was already locked by another process. 148.It Dv EX_CANTCREAT 149The 150.Nm 151utility 152was unable to create the lock file, e.g., because of insufficient access 153privileges. 154.It Dv EX_UNAVAILABLE 155The 156.Fl n 157option is specified and the specified lock file does not exist. 158.It Dv EX_USAGE 159There was an error on the 160.Nm 161command line. 162.It Dv EX_OSERR 163A system call (e.g., 164.Xr fork 2 ) 165failed unexpectedly. 166.It Dv EX_SOFTWARE 167The 168.Ar command 169did not exit normally, 170but may have been signaled or stopped. 171.El 172.Sh EXAMPLES 173The first job takes a lock and sleeps for 5 seconds in the background. 174The second job tries to get the lock and timeouts after 1 second (PID numbers 175will differ): 176.Bd -literal -offset indent 177$ lockf mylock sleep 5 & lockf -t 1 mylock echo "Success" 178[1] 94410 179lockf: mylock: already locked 180.Ed 181.Pp 182The first job takes a lock and sleeps for 1 second in the background. 183The second job waits up to 5 seconds to take the lock and echoes the message on 184success (PID numbers will differ): 185.Bd -literal -offset indent 186$ lockf mylock sleep 1 & lockf -t 5 mylock echo "Success" 187[1] 19995 188Success 189[1]+ Done lockf mylock sleep 1 190.Ed 191.Sh SEE ALSO 192.Xr flock 2 , 193.Xr lockf 3 , 194.Xr sysexits 3 195.Sh HISTORY 196A 197.Nm 198utility first appeared in 199.Fx 2.2 . 200.Sh AUTHORS 201.An John Polstra Aq Mt jdp@polstra.com 202