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8269e7673c
Remove core system call implementations and documentation to lib/libsys and lib/libsys/<arch> from lib/libc/sys and lib/libc/<arch>/<sys>. Update paths to allow libc to find them in their new home. Reviewed by: kib, emaste, imp Pull Request: https://github.com/freebsd/freebsd-src/pull/908
244 lines
6.3 KiB
Groff
244 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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.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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