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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 1993-07-23 by Rik Faith <faith@cs.unc.edu>
.\" Modified 1995-06-10 by Andries Brouwer <aeb@cwi.nl>
.\" Modified 1996-10-31 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1998-01-17 by Michael Haardt
.\" <michael@cantor.informatik.rwth-aachen.de>
.\" Modified 2001-09-24 by Michael Haardt <michael@moria.de>
.\" Modified 2003-08-21 by Andries Brouwer <aeb@cwi.nl>
.\"
.TH LSEEK 2 2001-09-24 "Linux" "Linux Programmer's Manual"
.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
.BR lseek ()
function repositions the offset of the open file associated with the
file descriptor
.I fildes
to the argument
.I offset
according to the directive
.I whence
as follows:
.TP
.B SEEK_SET
The offset is set to
.I offset
bytes.
.TP
.B SEEK_CUR
The offset is set to its current location plus
.I offset
bytes.
.TP
.B SEEK_END
The offset is set to the size of the file plus
.I offset
bytes.
.PP
The
.BR lseek ()
function allows the file offset to be set beyond the end
of the file (but this does not change the size of the file).
If data is later written at this point, subsequent reads of the data
in the gap (a "hole") return null bytes ('\\0') until
data is actually written into the gap.
.SH "RETURN VALUE"
Upon successful completion,
.BR lseek ()
returns the resulting offset location as measured in bytes from the
beginning of the file.
Otherwise, a value of \fI(off_t)\-1\fP 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 EINVAL
.I whence
is not one of SEEK_SET, SEEK_CUR, SEEK_END;
or the resulting file offset would be negative,
or beyond the end of a seekable device.
.\" Some systems may allow negative offsets for character devices
.\" and/or for remote filesystems.
.TP
.B EOVERFLOW
.\" HP-UX 11 says EINVAL for this case (but POSIX.1 says EOVERFLOW)
The resulting file offset cannot be represented in an
.IR off_t .
.TP
.B ESPIPE
.I fildes
is associated with a pipe, socket, or FIFO.
.SH "CONFORMING TO"
SVr4, 4.3BSD, POSIX.1-2001.
.SH RESTRICTIONS
Some devices are incapable of seeking and POSIX does not specify which
devices must support
.BR lseek ().
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. (Of written characters.)
.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 \fIlong\fP instead of \fIoff_t\fP, BSD returns \fIint\fP.
.PP
Note that file descriptors created by
.BR dup (2)
or
.BR fork (2)
share the current file position pointer, so seeking on such files may be
subject to race conditions.
.SH "SEE ALSO"
.BR dup (2),
.BR fork (2),
.BR open (2),
.BR fseek (3),
.BR lseek64 (3),
.BR posix_fallocate (3)
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