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.\" Copyright 1997 Nicolás Lichtmaier <nick@debian.org>
.\" Created Thu Aug 7 00:44:00 ART 1997
.\"
.\" This is free documentation; you can redistribute it and/or
.\" modify it under the terms of the GNU General Public License as
.\" published by the Free Software Foundation; either version 2 of
.\" the License, or (at your option) any later version.
.\"
.\" The GNU General Public License's references to "object code"
.\" and "executables" are to be interpreted as the output of any
.\" document formatting or typesetting system, including
.\" intermediate and printed output.
.\"
.\" This manual is distributed in the hope that it will be useful,
.\" but WITHOUT ANY WARRANTY; without even the implied warranty of
.\" MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
.\" GNU General Public License for more details.
.\"
.\" Added section stuff, aeb, 2002-04-22.
.\" Corrected include file, drepper, 2003-06-15.
.\"
.TH LOCKF 3 2002-04-22 "GNU" "Linux Programmer's Manual"
.SH NAME
lockf \- apply, test or remove a POSIX lock on an open file
.SH SYNOPSIS
.B #include <unistd.h>
.sp
.BI "int lockf(int " fd ", int " cmd ", off_t " len );
.SH DESCRIPTION
Apply, test or remove a POSIX lock on a section of an open file.
The file is specified by
.IR fd ,
a file descriptor open for writing, the action by
.IR cmd ,
and the section consists of byte positions
.IR pos .. pos + len \-1
if
.I len
is positive, and
.IR pos \- len .. pos \-1
if
.I len
is negative, where
.I pos
is the current file position, and if
.I len
is zero, the section extends from the current file position to
infinity, encompassing the present and future end-of-file positions.
In all cases, the section may extend past current end-of-file.
.LP
On Linux, this call is just an interface for
.BR fcntl (2).
(In general, the relation between
.BR lockf ()
and
.BR fcntl (2)
is unspecified.)
.LP
Valid operations are given below:
.TP
.B F_LOCK
Set an exclusive lock on the specified section of the file.
If (part of) this section is already locked, the call
blocks until the previous lock is released.
If this section overlaps an earlier locked section,
both are merged.
File locks are released as soon as the process holding the locks
closes some file descriptor for the file.
A child process does not inherit these locks.
.TP
.B F_TLOCK
Same as
.B F_LOCK
but the call never blocks and returns an error instead if the file is
already locked.
.TP
.B F_ULOCK
Unlock the indicated section of the file.
This may cause a locked section to be split into two locked sections.
.TP
.B F_TEST
Test the lock: return 0 if the specified section
is unlocked or locked by this process; return \-1, set
.I errno
to
.BR EAGAIN
.RB ( EACCES
on some other systems),
if another process holds a lock.
.SH "RETURN VALUE"
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
.TP
.BR EACCES " or " EAGAIN
The file is locked and
.B F_TLOCK
or
.B F_TEST
was specified, or the operation is prohibited because the file has
been memory-mapped by another process.
.TP
.B EBADF
.I fd
is not an open file descriptor.
.TP
.B EDEADLK
The command was
.B T_LOCK
and this lock operation would cause a deadlock.
.TP
.B EINVAL
An invalid operation was specified in
.IR fd .
.TP
.B ENOLCK
Too many segment locks open, lock table is full.
.SH "CONFORMING TO"
SVr4, POSIX.1-2001
.SH "SEE ALSO"
.BR fcntl (2),
.BR flock (2)
.br
There are also
.I locks.txt
and
.I mandatory.txt
in
.IR /usr/src/linux/Documentation .
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