1.\" $NetBSD: lockf.3,v 1.6 2002/02/07 07:00:15 ross 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.Dd December 19, 1997 38.Dt LOCKF 3 39.Os 40.Sh NAME 41.Nm lockf 42.Nd record locking on files 43.Sh LIBRARY 44.Lb libc 45.Sh SYNOPSIS 46.Fd #include \*[Lt]unistd.h\*[Gt] 47.Ft int 48.Fn lockf "int filedes" "int function" "off_t size" 49.Sh DESCRIPTION 50The 51.Fn lockf 52function allows sections of a file to be locked with advisory-mode locks. 53Calls to 54.Fn lockf 55from other processes which attempt to lock the locked file section will 56either return an error value or block until the section becomes unlocked. 57All the locks for a process are removed when the process terminates. 58.Pp 59The argument 60.Fa filedes 61is an open file descriptor. 62The file descriptor must have been opened either for write-only 63.Dv ( O_WRONLY ) 64or read/write 65.Dv ( O_RDWR ) 66operation. 67.Pp 68The 69.Fa function 70argument is a control value which specifies the action to be taken. 71The permissible values for 72.Fa function 73are as follows: 74.Bl -tag -width F_ULOCKXX -compact -offset indent 75.It Sy Function 76.Sy Description 77.It Dv F_ULOCK 78unlock locked sections 79.It Dv F_LOCK 80lock a section for exclusive use 81.It Dv F_TLOCK 82test and lock a section for exclusive use 83.It Dv F_TEST 84test a section for locks by other processes 85.El 86.Pp 87.Dv F_ULOCK 88removes locks from a section of the file; 89.Dv F_LOCK 90and 91.Dv F_TLOCK 92both lock a section of a file if the section is available; 93.Dv F_TEST 94detects if a lock by another process is present on the specified section. 95.Pp 96The 97.Fa size 98argument is the number of contiguous bytes to be locked or 99unlocked. The section to be locked or unlocked starts at the current 100offset in the file and extends forward for a positive size or backward 101for a negative size (the preceding bytes up to but not including the 102current offset). However, it is not permitted to lock a section that 103starts or extends before the beginning of the file. 104If 105.Fa size 106is 0, the section from the current offset through the largest possible 107file offset is locked (that is, from the current offset through the 108present or any future end-of-file). 109.Pp 110The sections locked with 111.Dv F_LOCK 112or 113.Dv F_TLOCK 114may, in whole or in part, contain or be contained by a previously 115locked section for the same process. When this occurs, or if adjacent 116locked sections would occur, the sections are combined into a single 117locked section. If the request would cause the number of locks to 118exceed a system-imposed limit, the request will fail. 119.Pp 120.Dv F_LOCK 121and 122.Dv F_TLOCK 123requests differ only by the action taken if the section is not 124available. 125.Dv F_LOCK 126blocks the calling process until the section is available. 127.Dv F_TLOCK 128makes the function fail if the section is already locked by another 129process. 130.Pp 131File locks are released on first close by the locking process of any 132file descriptor for the file. 133.Pp 134.Dv F_ULOCK 135requests release (wholly or in part) one or more locked sections 136controlled by the process. Locked sections will be unlocked starting 137at the current file offset through 138.Fa size 139bytes or to the end of file if size is 0. When all of a locked section 140is not released (that is, when the beginning or end of the area to be 141unlocked falls within a locked section), the remaining portions of 142that section are still locked by the process. Releasing the center 143portion of a locked section will cause the remaining locked beginning 144and end portions to become two separate locked sections. If the 145request would cause the number of locks in the system to exceed a 146system-imposed limit, the request will fail. 147.Pp 148An 149.Dv F_ULOCK 150request in which size is non-zero and the offset of the last byte of 151the requested section is the maximum value for an object of type 152off_t, when the process has an existing lock in which size is 0 and 153which includes the last byte of the requested section, will be treated 154as a request to unlock from the start of the requested section with a 155size equal to 0. Otherwise an 156.Dv F_ULOCK 157request will attempt to unlock only the requested section. 158.Pp 159A potential for deadlock occurs if a process controlling a locked 160region is put to sleep by attempting to lock the locked region of 161another process. This implementation detects that sleeping until a 162locked region is unlocked would cause a deadlock and fails with an 163.Er EDEADLK 164error. 165.Pp 166.Fn lockf , 167.Xr fcntl 2 168and 169.Xr flock 2 170locks may be safely used concurrently. 171.Pp 172Blocking on a section is interrupted by any signal. 173.Sh RETURN VALUES 174If successful, the 175.Fn lockf 176function returns 0. 177Otherwise, it returns -1, sets 178.Va errno 179to indicate an error, and existing locks are not changed. 180.Sh ERRORS 181.Fn lockf 182will fail if: 183.Bl -tag -width Er 184.It Bq Er EAGAIN 185The argument 186.Fa function 187is 188.Dv F_TLOCK 189or 190.Dv F_TEST 191and the section is already locked by another process. 192.It Bq Er EBADF 193The argument 194.Fa filedes 195is not a valid open file descriptor. 196.Pp 197The argument 198.Fa function 199is 200.Dv F_LOCK 201or 202.Dv F_TLOCK , 203and 204.Fa filedes 205is not a valid file descriptor open for writing. 206.It Bq Er EDEADLK 207The argument 208.Fa function 209is 210.Dv F_LOCK 211and a deadlock is detected. 212.It Bq Er EINTR 213The argument 214.Fa function 215is F_LOCK 216and 217.Fn lockf 218was interrupted by the delivery of a signal. 219.It Bq Er EINVAL 220The argument 221.Fa function 222is not one of 223.Dv F_ULOCK , 224.Dv F_LOCK , 225.Dv F_TLOCK 226or 227.Dv F_TEST . 228.Pp 229The argument 230.Fa filedes 231refers to a file that does not support locking. 232.It Bq Er ENOLCK 233The argument 234.Fa function 235is 236.Dv F_ULOCK , 237.Dv F_LOCK 238or 239.Dv F_TLOCK , 240and satisfying the lock or unlock request would result in the number 241of locked regions in the system exceeding a system-imposed limit. 242.El 243.Sh SEE ALSO 244.Xr fcntl 2 , 245.Xr flock 2 246.Sh STANDARDS 247The 248.Fn lockf 249function conforms to 250.St -xpg4.2 . 251