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'\" t
.\" Copyright (c) 1980, 1991 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. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by the University of
.\" California, Berkeley and its contributors.
.\" 4. 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 6.5 (Berkeley) 3/10/91
.\"
.\" Modified Fri Jul 23 22:17:00 1993 by Rik Faith <faith@cs.unc.edu>
.\" Modified 10 June 1995 by Andries Brouwer <aeb@cwi.nl>
.\" Modified Thu Oct 31 15:18:33 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Sat Jan 17 13:00:32 MET 1998 by Michael Haardt <michael@cantor.informatik.rwth-aachen.de>
.\"
.TH LSEEK 2 "January 17, 1998" "Linux" "System calls"
.SH NAME
lseek \- reposition read/write file offset
.SH SYNOPSIS
.B #include <sys/types.h>
.br
.B #include <unistd.h>
.sp
.BI "off_t lseek(int " fildes ", off_t " offset ", int " whence );
.SH DESCRIPTION
The
.B lseek
function repositions the offset of the file descriptor
.I fildes
to the argument
.I offset
according to the directive
.I whence
as follows:
.IP \fBSEEK_SET\fP
The offset is set to
.I offset
bytes.
.IP \fBSEEK_CUR\fP
The offset is set to its current location plus
.I offset
bytes.
.IP \fBSEEK_END\fP
The offset is set to the size of the file plus
.I offset
bytes.
.PP
The
.B 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).
.SH "RETURN VALUES"
Upon successful completion,
.B lseek
returns the resulting offset location as measured in bytes from the
beginning of the file. Otherwise, a value of (off_t)\-1 is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I Fildes
is not an open file descriptor.
.TP
.B ESPIPE
.I Fildes
is associated with a pipe, socket, or FIFO.
.TP
.B EINVAL
.I Whence
is not a proper value.
.SH "CONFORMING TO"
SVr4, POSIX, BSD 4.3
.SH RESTRICTIONS
Some devices are incapable of seeking and POSIX does not specify which
devices must support it.
Linux specific restrictions: using \fBlseek\fP on a tty device returns
\fBESPIPE\fP. Other systems return the number of written characters,
using SEEK_SET to set the counter. Some devices, e.g. \fB/dev/null\fP
do not cause the error \fBESPIPE\fP, but return a pointer which value
is undefined.
.SH NOTES
This document's use of
.I whence
is incorrect English, but maintained for historical reasons.
When converting old code, substitute values for \fIwhence\fP with the
following macros:
.PP
.TS
c c
l l.
old new
0 SEEK_SET
1 SEEK_CUR
2 SEEK_END
L_SET SEEK_SET
L_INCR SEEK_CUR
L_XTND SEEK_END
.TE
.PP
SVR1-3 returns \fBlong\fP instead of \fBoff_t\fP, BSD returns \fBint\fP.
.SH "SEE ALSO"
.BR dup "(2), " open "(2), " fseek (3)
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