libc/sys/lseek.2

.\" Copyright (c) 1980, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" 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.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS 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 REGENTS 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.
.\"
.\"     @(#)lseek.2	8.3 (Berkeley) 4/19/94
.\" $FreeBSD: src/lib/libc/sys/lseek.2,v 1.10.2.6 2001/12/14 18:34:01 ru Exp $
.\" $DragonFly: src/lib/libc/sys/lseek.2,v 1.3 2006/02/17 19:35:06 swildner Exp $
.\"
.Dd April 19, 1994
.Dt LSEEK 2
.Os
.Sh NAME
.Nm lseek
.Nd reposition read/write file offset
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft off_t
.Fn lseek "int fildes" "off_t offset" "int whence"
.Sh DESCRIPTION
The
.Fn lseek
function repositions the offset of the file descriptor
.Fa fildes
to the
argument
.Fa offset
according to the directive
.Fa whence .
The argument
.Fa fildes
must be an open
file descriptor.
.Fn Lseek
repositions the file position pointer associated with the file
descriptor
.Fa fildes
as follows:
.Bl -item -offset indent
.It
If
.Fa whence
is
.Dv SEEK_SET ,
the offset is set to
.Fa offset
bytes.
.It
If
.Fa whence
is
.Dv SEEK_CUR ,
the offset is set to its current location plus
.Fa offset
bytes.
.It
If
.Fa whence
is
.Dv SEEK_END ,
the offset is set to the size of the
file plus
.Fa offset
bytes.
.El
.Pp
The
.Fn lseek
function allows the file offset to be set beyond the end
of the existing end-of-file of the file.
If data is later written
at this point, subsequent reads of the data in the gap return
bytes of zeros (until data is actually written into the gap).
.Pp
Some devices are incapable of seeking.  The value of the pointer
associated with such a device is undefined.
.Sh RETURN VALUES
Upon successful completion,
.Fn lseek
returns the resulting offset location as measured in bytes from the
beginning of the file.
Otherwise,
a value of -1 is returned and
.Va errno
is set to indicate
the error.
.Sh ERRORS
.Fn Lseek
will fail and the file position pointer will remain unchanged if:
.Bl -tag -width Er
.It Bq Er EBADF
.Em Fildes
is not an open file descriptor.
.It Bq Er ESPIPE
.Em Fildes
is associated with a pipe, socket, or FIFO.
.It Bq Er EINVAL
.Fa Whence
is not a proper value.
.El
.Sh SEE ALSO
.Xr dup 2 ,
.Xr open 2
.Sh STANDARDS
The
.Fn lseek
function call is expected to conform to
.St -p1003.1-90 .
.Sh HISTORY
A
.Fn lseek
function call appeared in
.At v7 .
.Sh BUGS
This document's use of
.Fa whence
is incorrect English, but is maintained for historical reasons.