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.Dd August 26, 2020 26.Dt LOCKF 1 27.Os 28.Sh NAME 29.Nm lockf 30.Nd execute a command while holding a file lock 31.Sh SYNOPSIS 32.Nm 33.Op Fl knsw 34.Op Fl t Ar seconds 35.Ar file 36.Ar command 37.Op Ar arguments 38.Sh DESCRIPTION 39The 40.Nm 41utility acquires an exclusive lock on a 42.Ar file , 43creating it if necessary, 44.Bf Em 45and removing the file on exit unless explicitly told not to. 46.Ef 47While holding the lock, it executes a 48.Ar command 49with optional 50.Ar arguments . 51After the 52.Ar command 53completes, 54.Nm 55releases the lock, and removes the 56.Ar file 57unless the 58.Fl k 59option is specified. 60.Bx Ns -style 61locking is used, as described in 62.Xr flock 2 ; 63the mere existence of the 64.Ar file 65is not considered to constitute a lock. 66.Pp 67If the 68.Nm 69utility is being used to facilitate concurrency between a number 70of processes, it is recommended that the 71.Fl k 72option be used. 73This will guarantee lock ordering, as well as implement 74a performance enhanced algorithm which minimizes CPU load associated 75with concurrent unlink, drop and re-acquire activity. 76It should be noted 77that if the 78.Fl k 79option is not used, then no guarantees around lock ordering can be made. 80.Pp 81The following options are supported: 82.Bl -tag -width ".Fl t Ar seconds" 83.It Fl k 84Causes the lock file to be kept (not removed) after the command 85completes. 86.It Fl s 87Causes 88.Nm 89to operate silently. 90Failure to acquire the lock is indicated only in the exit status. 91.It Fl n 92Causes 93.Nm 94to fail if the specified lock 95.Ar file 96does not exist. 97If 98.Fl n 99is not specified, 100.Nm 101will create 102.Ar file 103if necessary. 104.It Fl t Ar seconds 105Specifies a timeout for waiting for the lock. 106By default, 107.Nm 108waits indefinitely to acquire the lock. 109If a timeout is specified with this option, 110.Nm 111will wait at most the given number of 112.Ar seconds 113before giving up. 114A timeout of 0 may be given, in which case 115.Nm 116will fail unless it can acquire the lock immediately. 117When a lock times out, 118.Ar command 119is 120.Em not 121executed. 122.It Fl w 123Causes 124.Nm 125to open 126.Ar file 127for writing rather than reading. 128This is necessary on filesystems (including NFSv4) where a file which 129has been opened read-only cannot be exclusively locked. 130.El 131.Pp 132In no event will 133.Nm 134break a lock that is held by another process. 135.Sh EXIT STATUS 136If 137.Nm 138successfully acquires the lock, it returns the exit status produced by 139.Ar command . 140Otherwise, it returns one of the exit codes defined in 141.Xr sysexits 3 , 142as follows: 143.Bl -tag -width ".Dv EX_CANTCREAT" 144.It Dv EX_TEMPFAIL 145The specified lock file was already locked by another process. 146.It Dv EX_CANTCREAT 147The 148.Nm 149utility 150was unable to create the lock file, e.g., because of insufficient access 151privileges. 152.It Dv EX_UNAVAILABLE 153The 154.Fl n 155option is specified and the specified lock file does not exist. 156.It Dv EX_USAGE 157There was an error on the 158.Nm 159command line. 160.It Dv EX_OSERR 161A system call (e.g., 162.Xr fork 2 ) 163failed unexpectedly. 164.It Dv EX_SOFTWARE 165The 166.Ar command 167did not exit normally, 168but may have been signaled or stopped. 169.El 170.Sh EXAMPLES 171The first job takes a lock and sleeps for 5 seconds in the background. 172The second job tries to get the lock and timeouts after 1 second (PID numbers 173will differ): 174.Bd -literal -offset indent 175$ lockf mylock sleep 5 & lockf -t 1 mylock echo "Success" 176[1] 94410 177lockf: mylock: already locked 178.Ed 179.Pp 180The first job takes a lock and sleeps for 1 second in the background. 181The second job waits up to 5 seconds to take the lock and echoes the message on 182success (PID numbers will differ): 183.Bd -literal -offset indent 184$ lockf mylock sleep 1 & lockf -t 5 mylock echo "Success" 185[1] 19995 186Success 187[1]+ Done lockf mylock sleep 1 188.Ed 189.Sh SEE ALSO 190.Xr flock 2 , 191.Xr lockf 3 , 192.Xr sysexits 3 193.Sh HISTORY 194A 195.Nm 196utility first appeared in 197.Fx 2.2 . 198.Sh AUTHORS 199.An John Polstra Aq Mt jdp@polstra.com 200