1.\" $NetBSD: lockf.3,v 1.2 1998/02/05 18:47:28 perry 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.\" 3. All advertising materials mentioning features or use of this software 18.\" must display the following acknowledgement: 19.\" This product includes software developed by the NetBSD 20.\" Foundation, Inc. and its contributors. 21.\" 4. Neither the name of The NetBSD Foundation nor the names of its 22.\" contributors may be used to endorse or promote products derived 23.\" from this software without specific prior written permission. 24.\" 25.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 26.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 27.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 28.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 29.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 30.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 31.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 32.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 33.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 34.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 35.\" POSSIBILITY OF SUCH DAMAGE. 36.\" 37.\" $FreeBSD: src/lib/libc/gen/lockf.3,v 1.5.2.5 2003/03/13 18:05:37 trhodes Exp $ 38.\" 39.Dd December 19, 1997 40.Dt LOCKF 3 41.Os 42.Sh NAME 43.Nm lockf 44.Nd record locking on files 45.Sh LIBRARY 46.Lb libc 47.Sh SYNOPSIS 48.In unistd.h 49.Ft int 50.Fn lockf "int filedes" "int function" "off_t size" 51.Sh DESCRIPTION 52The 53.Fn lockf 54function allows sections of a file to be locked with advisory-mode locks. 55Calls to 56.Fn lockf 57from other processes which attempt to lock the locked file section will 58either return an error value or block until the section becomes unlocked. 59All the locks for a process are removed when the process terminates. 60.Pp 61The argument 62.Fa filedes 63is an open file descriptor. 64The file descriptor must have been opened either for write-only 65.Dv ( O_WRONLY ) 66or read/write 67.Dv ( O_RDWR ) 68operation. 69.Pp 70The 71.Fa function 72argument is a control value which specifies the action to be taken. 73The permissible values for 74.Fa function 75are as follows: 76.Bl -tag -width ".Dv F_ULOCK" -compact -offset indent 77.It Sy Function 78.Sy Description 79.It Dv F_ULOCK 80unlock locked sections 81.It Dv F_LOCK 82lock a section for exclusive use 83.It Dv F_TLOCK 84test and lock a section for exclusive use 85.It Dv F_TEST 86test a section for locks by other processes 87.El 88.Pp 89.Dv F_ULOCK 90removes locks from a section of the file; 91.Dv F_LOCK 92and 93.Dv F_TLOCK 94both lock a section of a file if the section is available; 95.Dv F_TEST 96detects if a lock by another process is present on the specified section. 97.Pp 98The 99.Fa size 100argument is the number of contiguous bytes to be locked or 101unlocked. 102The section to be locked or unlocked starts at the current 103offset in the file and extends forward for a positive size or backward 104for a negative size (the preceding bytes up to but not including the 105current offset). 106However, it is not permitted to lock a section that 107starts or extends before the beginning of the file. 108If 109.Fa size 110is 0, the section from the current offset through the largest possible 111file offset is locked (that is, from the current offset through the 112present or any future end-of-file). 113.Pp 114The sections locked with 115.Dv F_LOCK 116or 117.Dv F_TLOCK 118may, in whole or in part, contain or be contained by a previously 119locked section for the same process. 120When this occurs, or if adjacent 121locked sections would occur, the sections are combined into a single 122locked section. 123If the request would cause the number of locks to 124exceed a system-imposed limit, the request will fail. 125.Pp 126.Dv F_LOCK 127and 128.Dv F_TLOCK 129requests differ only by the action taken if the section is not 130available. 131.Dv F_LOCK 132blocks the calling process until the section is available. 133.Dv F_TLOCK 134makes the function fail if the section is already locked by another 135process. 136.Pp 137File locks are released on first close by the locking process of any 138file descriptor for the file. 139.Pp 140.Dv F_ULOCK 141requests release (wholly or in part) one or more locked sections 142controlled by the process. 143Locked sections will be unlocked starting 144at the current file offset through 145.Fa size 146bytes or to the end of file if size is 0. 147When all of a locked section 148is not released (that is, when the beginning or end of the area to be 149unlocked falls within a locked section), the remaining portions of 150that section are still locked by the process. 151Releasing the center 152portion of a locked section will cause the remaining locked beginning 153and end portions to become two separate locked sections. 154If the 155request would cause the number of locks in the system to exceed a 156system-imposed limit, the request will fail. 157.Pp 158An 159.Dv F_ULOCK 160request in which size is non-zero and the offset of the last byte of 161the requested section is the maximum value for an object of type 162off_t, when the process has an existing lock in which size is 0 and 163which includes the last byte of the requested section, will be treated 164as a request to unlock from the start of the requested section with a 165size equal to 0. 166Otherwise an 167.Dv F_ULOCK 168request will attempt to unlock only the requested section. 169.Pp 170A potential for deadlock occurs if a process controlling a locked 171region is put to sleep by attempting to lock the locked region of 172another process. 173This implementation detects that sleeping until a 174locked region is unlocked would cause a deadlock and fails with an 175.Er EDEADLK 176error. 177.Pp 178The 179.Fn lockf , 180.Xr fcntl 2 181and 182.Xr flock 2 183locks may be safely used concurrently. 184.Pp 185Blocking on a section is interrupted by any signal. 186.Sh RETURN VALUES 187.Rv -std lockf 188In the case of a failure, existing locks are not changed. 189.Sh ERRORS 190The 191.Fn lockf 192function 193will fail if: 194.Bl -tag -width Er 195.It Bq Er EAGAIN 196The argument 197.Fa function 198is 199.Dv F_TLOCK 200or 201.Dv F_TEST 202and the section is already locked by another process. 203.It Bq Er EBADF 204The argument 205.Fa filedes 206is not a valid open file descriptor. 207.Pp 208The argument 209.Fa function 210is 211.Dv F_LOCK 212or 213.Dv F_TLOCK , 214and 215.Fa filedes 216is not a valid file descriptor open for writing. 217.It Bq Er EDEADLK 218The argument 219.Fa function 220is 221.Dv F_LOCK 222and a deadlock is detected. 223.It Bq Er EINTR 224The argument 225.Fa function 226is 227.Dv F_LOCK 228and 229.Fn lockf 230was interrupted by the delivery of a signal. 231.It Bq Er EINVAL 232The argument 233.Fa function 234is not one of 235.Dv F_ULOCK , 236.Dv F_LOCK , 237.Dv F_TLOCK 238or 239.Dv F_TEST . 240.Pp 241The argument 242.Fa filedes 243refers to a file that does not support locking. 244.It Bq Er ENOLCK 245The argument 246.Fa function 247is 248.Dv F_ULOCK , 249.Dv F_LOCK 250or 251.Dv F_TLOCK , 252and satisfying the lock or unlock request would result in the number 253of locked regions in the system exceeding a system-imposed limit. 254.El 255.Sh SEE ALSO 256.Xr fcntl 2 , 257.Xr flock 2 258.Sh STANDARDS 259The 260.Fn lockf 261function conforms to 262.St -xpg4.2 . 263