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.\" $DragonFly: src/lib/libc/gen/lockf.3,v 1.2 2003/06/17 04:26:42 dillon Exp $ 39.\" 40.Dd December 19, 1997 41.Dt LOCKF 3 42.Os 43.Sh NAME 44.Nm lockf 45.Nd record locking on files 46.Sh LIBRARY 47.Lb libc 48.Sh SYNOPSIS 49.In unistd.h 50.Ft int 51.Fn lockf "int filedes" "int function" "off_t size" 52.Sh DESCRIPTION 53The 54.Fn lockf 55function allows sections of a file to be locked with advisory-mode locks. 56Calls to 57.Fn lockf 58from other processes which attempt to lock the locked file section will 59either return an error value or block until the section becomes unlocked. 60All the locks for a process are removed when the process terminates. 61.Pp 62The argument 63.Fa filedes 64is an open file descriptor. 65The file descriptor must have been opened either for write-only 66.Dv ( O_WRONLY ) 67or read/write 68.Dv ( O_RDWR ) 69operation. 70.Pp 71The 72.Fa function 73argument is a control value which specifies the action to be taken. 74The permissible values for 75.Fa function 76are as follows: 77.Bl -tag -width F_ULOCKXX -compact -offset indent 78.It Sy Function 79.Sy Description 80.It Dv F_ULOCK 81unlock locked sections 82.It Dv F_LOCK 83lock a section for exclusive use 84.It Dv F_TLOCK 85test and lock a section for exclusive use 86.It Dv F_TEST 87test a section for locks by other processes 88.El 89.Pp 90.Dv F_ULOCK 91removes locks from a section of the file; 92.Dv F_LOCK 93and 94.Dv F_TLOCK 95both lock a section of a file if the section is available; 96.Dv F_TEST 97detects if a lock by another process is present on the specified section. 98.Pp 99The 100.Fa size 101argument is the number of contiguous bytes to be locked or 102unlocked. 103The section to be locked or unlocked starts at the current 104offset in the file and extends forward for a positive size or backward 105for a negative size (the preceding bytes up to but not including the 106current offset). However, 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. When all of a locked section 147is not released (that is, when the beginning or end of the area to be 148unlocked falls within a locked section), the remaining portions of 149that section are still locked by the process. 150Releasing the center 151portion of a locked section will cause the remaining locked beginning 152and end portions to become two separate locked sections. 153If the 154request would cause the number of locks in the system to exceed a 155system-imposed limit, the request will fail. 156.Pp 157An 158.Dv F_ULOCK 159request in which size is non-zero and the offset of the last byte of 160the requested section is the maximum value for an object of type 161off_t, when the process has an existing lock in which size is 0 and 162which includes the last byte of the requested section, will be treated 163as a request to unlock from the start of the requested section with a 164size equal to 0. Otherwise an 165.Dv F_ULOCK 166request will attempt to unlock only the requested section. 167.Pp 168A potential for deadlock occurs if a process controlling a locked 169region is put to sleep by attempting to lock the locked region of 170another process. This implementation detects that sleeping until a 171locked region is unlocked would cause a deadlock and fails with an 172.Er EDEADLK 173error. 174.Pp 175The 176.Fn lockf , 177.Xr fcntl 2 178and 179.Xr flock 2 180locks may be safely used concurrently. 181.Pp 182Blocking on a section is interrupted by any signal. 183.Sh RETURN VALUES 184.Rv -std lockf 185In the case of a failure, existing locks are not changed. 186.Sh ERRORS 187The 188.Fn lockf 189function 190will fail if: 191.Bl -tag -width Er 192.It Bq Er EAGAIN 193The argument 194.Fa function 195is 196.Dv F_TLOCK 197or 198.Dv F_TEST 199and the section is already locked by another process. 200.It Bq Er EBADF 201The argument 202.Fa filedes 203is not a valid open file descriptor. 204.Pp 205The argument 206.Fa function 207is 208.Dv F_LOCK 209or 210.Dv F_TLOCK , 211and 212.Fa filedes 213is not a valid file descriptor open for writing. 214.It Bq Er EDEADLK 215The argument 216.Fa function 217is 218.Dv F_LOCK 219and a deadlock is detected. 220.It Bq Er EINTR 221The argument 222.Fa function 223is F_LOCK 224and 225.Fn lockf 226was interrupted by the delivery of a signal. 227.It Bq Er EINVAL 228The argument 229.Fa function 230is not one of 231.Dv F_ULOCK , 232.Dv F_LOCK , 233.Dv F_TLOCK 234or 235.Dv F_TEST . 236.Pp 237The argument 238.Fa filedes 239refers to a file that does not support locking. 240.It Bq Er ENOLCK 241The argument 242.Fa function 243is 244.Dv F_ULOCK , 245.Dv F_LOCK 246or 247.Dv F_TLOCK , 248and satisfying the lock or unlock request would result in the number 249of locked regions in the system exceeding a system-imposed limit. 250.El 251.Sh SEE ALSO 252.Xr fcntl 2 , 253.Xr flock 2 254.Sh STANDARDS 255The 256.Fn lockf 257function conforms to 258.St -xpg4.2 . 259