.\" $NetBSD: lockf.3,v 1.10 2008/04/30 13:10:50 martin Exp $ .\" .\" Copyright (c) 1997 The NetBSD Foundation, Inc. .\" All rights reserved. .\" .\" This code is derived from software contributed to The NetBSD Foundation .\" by Klaus Klein and S.P. Zeidler. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS .\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED .\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR .\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS .\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR .\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF .\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS .\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN .\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE .\" POSSIBILITY OF SUCH DAMAGE. .\" .\" $FreeBSD: src/lib/libc/gen/lockf.3,v 1.14 2009/03/04 01:01:26 delphij Exp $ .\" .Dd December 19, 1997 .Dt LOCKF 3 .Os .Sh NAME .Nm lockf .Nd record locking on files .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In unistd.h .Ft int .Fn lockf "int fildes" "int function" "off_t size" .Sh DESCRIPTION The .Fn lockf function allows sections of a file to be locked with advisory-mode locks. Calls to .Fn lockf from other processes which attempt to lock the locked file section will either return an error value or block until the section becomes unlocked. All of the locks for a process are removed when the process terminates. .Pp The argument .Fa fildes is an open file descriptor. The file descriptor must have been opened either for write-only .Dv ( O_WRONLY ) or read/write .Dv ( O_RDWR ) operation. .Pp The .Fa function argument is a control value which specifies the action to be taken. The permissible values for .Fa function are as follows: .Bl -tag -width F_ULOCKXX -compact -offset indent .It Sy Function .Sy Description .It Dv F_ULOCK unlock locked sections .It Dv F_LOCK lock a section for exclusive use .It Dv F_TLOCK test and lock a section for exclusive use .It Dv F_TEST test a section for locks by other processes .El .Pp .Dv F_ULOCK removes locks from a section of the file; .Dv F_LOCK and .Dv F_TLOCK both lock a section of a file if the section is available; .Dv F_TEST detects if a lock by another process is present on the specified section. .Pp The .Fa size argument is the number of contiguous bytes to be locked or unlocked. The section to be locked or unlocked starts at the current offset in the file and extends forward for a positive size or backward for a negative size (the preceding bytes up to but not including the current offset). However, it is not permitted to lock a section that starts or extends before the beginning of the file. If .Fa size is 0, the section from the current offset through the largest possible file offset is locked (that is, from the current offset through the present or any future end-of-file). .Pp The sections locked with .Dv F_LOCK or .Dv F_TLOCK may, in whole or in part, contain or be contained by a previously locked section for the same process. When this occurs, or if adjacent locked sections would occur, the sections are combined into a single locked section. If the request would cause the number of locks to exceed a system-imposed limit, the request will fail. .Pp .Dv F_LOCK and .Dv F_TLOCK requests differ only by the action taken if the section is not available. .Dv F_LOCK blocks the calling process until the section is available. .Dv F_TLOCK makes the function fail if the section is already locked by another process. .Pp File locks are released on first close by the locking process of any file descriptor for the file. .Pp .Dv F_ULOCK requests release (wholly or in part) one or more locked sections controlled by the process. Locked sections will be unlocked starting at the current file offset through .Fa size bytes or to the end of file if size is 0. When all of a locked section is not released (that is, when the beginning or end of the area to be unlocked falls within a locked section), the remaining portions of that section are still locked by the process. Releasing the center portion of a locked section will cause the remaining locked beginning and end portions to become two separate locked sections. If the request would cause the number of locks in the system to exceed a system-imposed limit, the request will fail. .Pp An .Dv F_ULOCK request in which size is non-zero and the offset of the last byte of the requested section is the maximum value for an object of type off_t, when the process has an existing lock in which size is 0 and which includes the last byte of the requested section, will be treated as a request to unlock from the start of the requested section with a size equal to 0. Otherwise an .Dv F_ULOCK request will attempt to unlock only the requested section. .Pp A potential for deadlock occurs if a process controlling a locked region is put to sleep by attempting to lock the locked region of another process. This implementation detects that sleeping until a locked region is unlocked would cause a deadlock and fails with an .Er EDEADLK error. .Pp The .Fn lockf , .Xr fcntl 2 , and .Xr flock 2 locks are compatible. Processes using different locking interfaces can cooperate over the same file safely. However, only one of such interfaces should be used within the same process. If a file is locked by a process through .Xr flock 2 , any record within the file will be seen as locked from the viewpoint of another process using .Xr fcntl 2 or .Fn lockf , and vice versa. .Pp Blocking on a section is interrupted by any signal. .Sh RETURN VALUES .Rv -std lockf In the case of a failure, existing locks are not changed. .Sh ERRORS The .Fn lockf function will fail if: .Bl -tag -width Er .It Bq Er EAGAIN The argument .Fa function is .Dv F_TLOCK or .Dv F_TEST and the section is already locked by another process. .It Bq Er EBADF The argument .Fa fildes is not a valid open file descriptor. .Pp The argument .Fa function is .Dv F_LOCK or .Dv F_TLOCK , and .Fa fildes is not a valid file descriptor open for writing. .It Bq Er EDEADLK The argument .Fa function is .Dv F_LOCK and a deadlock is detected. .It Bq Er EINTR The argument .Fa function is F_LOCK and .Fn lockf was interrupted by the delivery of a signal. .It Bq Er EINVAL The argument .Fa function is not one of .Dv F_LOCK , .Dv F_TEST , .Dv F_TLOCK , or .Dv F_ULOCK . .Pp The argument .Fa fildes refers to a file that does not support advisory locking. .It Bq Er ENOLCK The argument .Fa function is .Dv F_LOCK , .Dv F_TLOCK , or .Dv F_ULOCK and satisfying the lock or unlock request would result in the number of locked regions in the system exceeding a system-imposed limit. .It Bq Er EOPNOTSUPP The argument .Fa fildes refers to a socket; these do not support advisory locking. .El .Sh SEE ALSO .Xr fcntl 2 , .Xr flock 2 .Sh STANDARDS The .Fn lockf function conforms to .St -xpg4.2 .