e7677232d6
In certain situations lseek(2) will return successful although if no seek was performed. This can happen when operating on devices that don't support seeking (older tape drives) or when operating on changeable media devices (such as DVD or Blu-ray devices) without a medium inserted. Document this within the man page and update the POSIX compliance while here. PR: 162765 Submitted by: arundel@ Reported by: arundel@ Reviewed by: bcr (mentor) Approved by: bcr (mentor) MFC after: 1 week Differential Revision: https://reviews.freebsd.org/D25646
247 lines
6.3 KiB
Groff
247 lines
6.3 KiB
Groff
.\" 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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.\" 3. 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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.\" $FreeBSD$
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.\"
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.Dd July 13, 2020
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.Dt LSEEK 2
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.Os
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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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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In unistd.h
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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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system call repositions the offset of the file descriptor
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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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.Fa whence .
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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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The
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.Fn lseek
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system call
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repositions the file position pointer associated with the file
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descriptor
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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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.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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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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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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.El
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.Pp
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The
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.Fn lseek
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system call allows the file offset to be set beyond the end
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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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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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However, the
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.Fn lseek
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system call does not, by itself, extend the size of a file.
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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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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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The existence of a hole at the end of every data region allows for easy
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programming and also provides compatibility to the original implementation
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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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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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.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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The
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.Fn lseek
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system call
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will fail and the file position pointer will remain unchanged if:
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.Bl -tag -width Er
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.It Bq Er EBADF
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The
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.Fa fildes
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argument
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is not an open file descriptor.
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.It Bq Er EINVAL
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The
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.Fa whence
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argument
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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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.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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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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.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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.It Bq Er ESPIPE
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The
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.Fa fildes
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argument
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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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.Xr open 2 ,
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.Xr pathconf 2
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.Sh STANDARDS
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The
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.Fn lseek
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system call is expected to conform to
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.St -p1003.1-2008 .
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.Pp
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The
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.Dv SEEK_HOLE
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and
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.Dv SEEK_DATA
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directives, along with the
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.Er ENXIO
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error, are extensions to that specification.
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.Sh HISTORY
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The
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.Fn lseek
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function appeared in
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.At v7 .
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.Sh BUGS
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If the
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.Fn lseek
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system call is operating on a device which is incapable of seeking,
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it will request the seek operation and return successfully,
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even though no seek was performed.
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Because the
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.Ar offset
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argument will be stored unconditionally in the file descriptor of that device,
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there is no way to confirm if the seek operation succeeded or not
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(e.g. using the
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.Fn ftell
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function).
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Device types which are known to be incapable of seeking include
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tape drives.
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.Pp
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The
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.Fn lseek
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system call will not detect whether media are present in changeable
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media devices such as DVD or Blu-ray devices.
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A requested seek operation will therefore return sucessfully when no
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medium is present.
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.Pp
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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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