1.\" $NetBSD: lockf.3,v 1.10 2008/04/30 13:10:50 martin Exp $ 2.\" 3.\" Copyright (c) 1997 The NetBSD Foundation, Inc. 4.\" All rights reserved. 5.\" 6.\" This code is derived from software contributed to The NetBSD Foundation 7.\" by Klaus Klein and S.P. Zeidler. 8.\" 9.\" Redistribution and use in source and binary forms, with or without 10.\" modification, are permitted provided that the following conditions 11.\" are met: 12.\" 1. Redistributions of source code must retain the above copyright 13.\" notice, this list of conditions and the following disclaimer. 14.\" 2. Redistributions in binary form must reproduce the above copyright 15.\" notice, this list of conditions and the following disclaimer in the 16.\" documentation and/or other materials provided with the distribution. 17.\" 18.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 19.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 20.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 21.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 22.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 23.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 24.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 25.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 26.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 27.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 28.\" POSSIBILITY OF SUCH DAMAGE. 29.\" 30.Dd September 11, 2013 31.Dt LOCKF 3 32.Os 33.Sh NAME 34.Nm lockf 35.Nd record locking on files 36.Sh LIBRARY 37.Lb libc 38.Sh SYNOPSIS 39.In unistd.h 40.Ft int 41.Fn lockf "int fd" "int function" "off_t size" 42.Sh DESCRIPTION 43The 44.Fn lockf 45function allows sections of a file to be locked with advisory-mode locks. 46Calls to 47.Fn lockf 48from other processes which attempt to lock the locked file section will 49either return an error value or block until the section becomes unlocked. 50All the locks for a process are removed when the process terminates. 51.Pp 52The argument 53.Fa fd 54is an open file descriptor. 55The file descriptor must have been opened either for write-only 56.Dv ( O_WRONLY ) 57or read/write 58.Dv ( O_RDWR ) 59operation. 60.Pp 61The 62.Fa function 63argument is a control value which specifies the action to be taken. 64The permissible values for 65.Fa function 66are as follows: 67.Bl -tag -width F_ULOCKXX -compact -offset indent 68.It Sy Function 69.Sy Description 70.It Dv F_ULOCK 71unlock locked sections 72.It Dv F_LOCK 73lock a section for exclusive use 74.It Dv F_TLOCK 75test and lock a section for exclusive use 76.It Dv F_TEST 77test a section for locks by other processes 78.El 79.Pp 80.Dv F_ULOCK 81removes locks from a section of the file; 82.Dv F_LOCK 83and 84.Dv F_TLOCK 85both lock a section of a file if the section is available; 86.Dv F_TEST 87detects if a lock by another process is present on the specified section. 88.Pp 89The 90.Fa size 91argument is the number of contiguous bytes to be locked or 92unlocked. 93The section to be locked or unlocked starts at the current 94offset in the file and extends forward for a positive size or backward 95for a negative size (the preceding bytes up to but not including the 96current offset). 97However, it is not permitted to lock a section that 98starts or extends before the beginning of the file. 99If 100.Fa size 101is 0, the section from the current offset through the largest possible 102file offset is locked (that is, from the current offset through the 103present or any future end-of-file). 104.Pp 105The sections locked with 106.Dv F_LOCK 107or 108.Dv F_TLOCK 109may, in whole or in part, contain or be contained by a previously 110locked section for the same process. 111When this occurs, or if adjacent 112locked sections would occur, the sections are combined into a single 113locked section. 114If the request would cause the number of locks to 115exceed a system-imposed limit, the request will fail. 116.Pp 117.Dv F_LOCK 118and 119.Dv F_TLOCK 120requests differ only by the action taken if the section is not 121available. 122.Dv F_LOCK 123blocks the calling process until the section is available. 124.Dv F_TLOCK 125makes the function fail if the section is already locked by another 126process. 127.Pp 128File locks are released on first close by the locking process of any 129file descriptor for the file. 130.Pp 131.Dv F_ULOCK 132requests release (wholly or in part) one or more locked sections 133controlled by the process. 134Locked sections will be unlocked starting 135at the current file offset through 136.Fa size 137bytes or to the end of file if size is 0. 138When all of a locked section 139is not released (that is, when the beginning or end of the area to be 140unlocked falls within a locked section), the remaining portions of 141that section are still locked by the process. 142Releasing the center 143portion of a locked section will cause the remaining locked beginning 144and end portions to become two separate locked sections. 145If the 146request would cause the number of locks in the system to exceed a 147system-imposed limit, the request will fail. 148.Pp 149An 150.Dv F_ULOCK 151request in which size is non-zero and the offset of the last byte of 152the requested section is the maximum value for an object of type 153off_t, when the process has an existing lock in which size is 0 and 154which includes the last byte of the requested section, will be treated 155as a request to unlock from the start of the requested section with a 156size equal to 0. 157Otherwise an 158.Dv F_ULOCK 159request will attempt to unlock only the requested section. 160.Pp 161A potential for deadlock occurs if a process controlling a locked 162region is put to sleep by attempting to lock the locked region of 163another process. 164This implementation detects that sleeping until a 165locked region is unlocked would cause a deadlock and fails with an 166.Er EDEADLK 167error. 168.Pp 169The 170.Fn lockf , 171.Xr fcntl 2 , 172and 173.Xr flock 2 174locks are compatible. 175Processes using different locking interfaces can cooperate 176over the same file safely. 177However, only one of such interfaces should be used within 178the same process. 179If a file is locked by a process through 180.Xr flock 2 , 181any record within the file will be seen as locked 182from the viewpoint of another process using 183.Xr fcntl 2 184or 185.Fn lockf , 186and vice versa. 187.Pp 188Blocking on a section is interrupted by any signal. 189.Sh RETURN VALUES 190.Rv -std lockf 191In the case of a failure, existing locks are not changed. 192.Sh ERRORS 193The 194.Fn lockf 195function 196will fail if: 197.Bl -tag -width Er 198.It Bq Er EAGAIN 199The argument 200.Fa function 201is 202.Dv F_TLOCK 203or 204.Dv F_TEST 205and the section is already locked by another process. 206.It Bq Er EBADF 207The argument 208.Fa fd 209is not a valid open file descriptor. 210.Pp 211The argument 212.Fa function 213is 214.Dv F_LOCK 215or 216.Dv F_TLOCK , 217and 218.Fa fd 219is not a valid file descriptor open for writing. 220.It Bq Er EDEADLK 221The argument 222.Fa function 223is 224.Dv F_LOCK 225and a deadlock is detected. 226.It Bq Er EINTR 227The argument 228.Fa function 229is F_LOCK 230and 231.Fn lockf 232was interrupted by the delivery of a signal. 233.It Bq Er EINVAL 234The argument 235.Fa function 236is not one of 237.Dv F_ULOCK , 238.Dv F_LOCK , 239.Dv F_TLOCK 240or 241.Dv F_TEST . 242.Pp 243The argument 244.Fa fd 245refers to a file that does not support locking. 246.It Bq Er ENOLCK 247The argument 248.Fa function 249is 250.Dv F_ULOCK , 251.Dv F_LOCK 252or 253.Dv F_TLOCK , 254and satisfying the lock or unlock request would result in the number 255of locked regions in the system exceeding a system-imposed limit. 256.El 257.Sh SEE ALSO 258.Xr fcntl 2 , 259.Xr flock 2 260.Sh STANDARDS 261The 262.Fn lockf 263function conforms to 264.St -xpg4.2 . 265