1994-05-27 05:00:24 +00:00
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.\" Copyright (c) 1980, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" @(#)lseek.2 8.3 (Berkeley) 4/19/94
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1999-08-28 00:22:10 +00:00
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.\" $FreeBSD$
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1994-05-27 05:00:24 +00:00
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.\"
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2012-05-26 05:25:55 +00:00
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.Dd May 26, 2012
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1994-05-27 05:00:24 +00:00
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.Dt LSEEK 2
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2001-07-10 13:41:46 +00:00
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.Os
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1994-05-27 05:00:24 +00:00
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.Sh NAME
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.Nm lseek
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.Nd reposition read/write file offset
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2000-04-21 09:42:15 +00:00
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.Sh LIBRARY
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.Lb libc
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1994-05-27 05:00:24 +00:00
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.Sh SYNOPSIS
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2001-10-01 16:09:29 +00:00
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.In unistd.h
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1994-05-27 05:00:24 +00:00
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.Ft off_t
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.Fn lseek "int fildes" "off_t offset" "int whence"
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.Sh DESCRIPTION
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The
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.Fn lseek
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2002-12-18 09:22:32 +00:00
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system call repositions the offset of the file descriptor
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1994-05-27 05:00:24 +00:00
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.Fa fildes
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to the
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argument
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.Fa offset
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according to the directive
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2001-02-01 16:38:02 +00:00
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.Fa whence .
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1994-05-27 05:00:24 +00:00
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The argument
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.Fa fildes
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must be an open
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file descriptor.
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2002-12-18 09:22:32 +00:00
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The
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.Fn lseek
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system call
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1996-07-03 02:55:10 +00:00
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repositions the file position pointer associated with the file
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descriptor
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1994-05-27 05:00:24 +00:00
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.Fa fildes
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as follows:
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.Bl -item -offset indent
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.It
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If
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.Fa whence
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is
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.Dv SEEK_SET ,
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the offset is set to
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.Fa offset
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bytes.
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.It
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If
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.Fa whence
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is
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.Dv SEEK_CUR ,
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the offset is set to its current location plus
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.Fa offset
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bytes.
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.It
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If
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.Fa whence
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is
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.Dv SEEK_END ,
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the offset is set to the size of the
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file plus
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.Fa offset
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bytes.
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2007-04-05 21:10:53 +00:00
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.It
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If
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.Fa whence
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is
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.Dv SEEK_HOLE ,
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2012-05-26 05:25:55 +00:00
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the offset is set to the start of the next hole greater than or equal
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to the supplied
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.Fa offset .
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2007-04-05 21:10:53 +00:00
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The definition of a hole is provided below.
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.It
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If
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.Fa whence
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is
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.Dv SEEK_DATA ,
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the offset is set to the start of the next non-hole file region greater
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than or equal to the supplied
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.Fa offset .
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1994-05-27 05:00:24 +00:00
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.El
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.Pp
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The
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.Fn lseek
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2002-12-18 09:22:32 +00:00
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system call allows the file offset to be set beyond the end
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2000-03-02 09:14:21 +00:00
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of the existing end-of-file of the file.
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If data is later written
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1994-05-27 05:00:24 +00:00
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at this point, subsequent reads of the data in the gap return
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bytes of zeros (until data is actually written into the gap).
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.Pp
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2004-07-02 23:52:20 +00:00
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Some devices are incapable of seeking.
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The value of the pointer
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1994-05-27 05:00:24 +00:00
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associated with such a device is undefined.
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2007-04-05 21:10:53 +00:00
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.Pp
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A
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.Qq hole
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is defined as a contiguous range of bytes in a file, all having the value of
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zero, but not all zeros in a file are guaranteed to be represented as holes
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returned with
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.Dv SEEK_HOLE .
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File systems are allowed to expose ranges of zeros with
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.Dv SEEK_HOLE ,
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but not required to.
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Applications can use
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.Dv SEEK_HOLE
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to optimise their behavior for ranges of zeros, but must not depend on it to
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find all such ranges in a file.
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2016-02-18 18:41:40 +00:00
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Each file is presented as having a zero-size virtual hole at the very
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end of the file.
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2007-04-05 21:10:53 +00:00
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The existence of a hole at the end of every data region allows for easy
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2016-02-18 18:41:40 +00:00
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programming and also provides compatibility to the original imlementation
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in Solaris.
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It also causes the current file size (i.e. end-of-file offset) to be returned
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to indicate that there are no more holes past the supplied
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.Fa offset .
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2007-04-05 21:10:53 +00:00
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Applications should use
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.Fn fpathconf _PC_MIN_HOLE_SIZE
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or
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.Fn pathconf _PC_MIN_HOLE_SIZE
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to determine if a file system supports
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.Dv SEEK_HOLE .
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See
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.Xr pathconf 2 .
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.Pp
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For file systems that do not supply information about holes, the file will be
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represented as one entire data region.
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1994-05-27 05:00:24 +00:00
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.Sh RETURN VALUES
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Upon successful completion,
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.Fn lseek
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returns the resulting offset location as measured in bytes from the
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beginning of the file.
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Otherwise,
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a value of -1 is returned and
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.Va errno
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is set to indicate
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the error.
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.Sh ERRORS
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2002-12-18 09:22:32 +00:00
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The
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.Fn lseek
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system call
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1996-07-03 02:55:10 +00:00
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will fail and the file position pointer will remain unchanged if:
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2000-05-04 13:09:25 +00:00
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.Bl -tag -width Er
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1994-05-27 05:00:24 +00:00
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.It Bq Er EBADF
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2002-12-19 09:40:28 +00:00
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The
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2002-12-18 13:33:04 +00:00
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.Fa fildes
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2002-12-19 09:40:28 +00:00
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argument
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1994-05-27 05:00:24 +00:00
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is not an open file descriptor.
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2001-08-21 21:44:48 +00:00
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.It Bq Er EINVAL
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2002-12-19 09:40:28 +00:00
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The
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.Fa whence
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argument
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2001-08-21 21:44:48 +00:00
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is not a proper value
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or the resulting file offset would
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be negative for a non-character special file.
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2007-04-05 21:10:53 +00:00
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.It Bq Er ENXIO
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For
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.Dv SEEK_DATA ,
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there are no more data regions past the supplied offset.
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2016-02-18 18:41:40 +00:00
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Due to existence of the hole at the end of the file, for
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.Dv SEEK_HOLE
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this error is only returned when the
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.Fa offset
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already points to the end-of-file position.
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2001-08-21 21:44:48 +00:00
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.It Bq Er EOVERFLOW
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The resulting file offset would be a value which cannot be represented
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correctly in an object of type
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.Fa off_t .
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1994-05-27 05:00:24 +00:00
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.It Bq Er ESPIPE
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2002-12-19 09:40:28 +00:00
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The
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2002-12-18 13:33:04 +00:00
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.Fa fildes
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2002-12-19 09:40:28 +00:00
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argument
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1994-05-27 05:00:24 +00:00
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is associated with a pipe, socket, or FIFO.
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.El
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.Sh SEE ALSO
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.Xr dup 2 ,
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2007-04-05 21:10:53 +00:00
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.Xr open 2 ,
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.Xr pathconf 2
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1994-05-27 05:00:24 +00:00
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.Sh STANDARDS
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The
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.Fn lseek
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2002-12-18 09:22:32 +00:00
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system call is expected to conform to
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1996-12-02 20:03:58 +00:00
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.St -p1003.1-90 .
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1996-08-29 21:24:19 +00:00
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.Sh HISTORY
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2002-12-18 09:22:32 +00:00
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The
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1996-08-29 21:24:19 +00:00
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.Fn lseek
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2002-12-18 09:22:32 +00:00
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function appeared in
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1996-08-29 21:24:19 +00:00
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.At v7 .
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2005-01-20 09:17:07 +00:00
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.Sh BUGS
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This document's use of
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.Fa whence
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is incorrect English, but is maintained for historical reasons.
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