path: root/man2/lseek.2

.\" Copyright (c) 1980, 1991 Regents of the University of California.
.\" and Copyright (c) 2011, Michael Kerrisk <mtk.manpages@gmail.com>
.\" All rights reserved.
.\"
.\" SPDX-License-Identifier: BSD-4-Clause-UC
.\"
.\"     @(#)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>
.\" 2011-09-18, mtk, Added SEEK_DATA + SEEK_HOLE
.\"
.TH lseek 2 2023-03-30 "Linux man-pages 6.05.01"
.SH NAME
lseek \- reposition read/write file offset
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.BI "off_t lseek(int " fd ", off_t " offset ", int " whence );
.fi
.SH DESCRIPTION
.BR lseek ()
repositions the file offset of the open file description
associated with the file descriptor
.I fd
to the argument
.I offset
according to the directive
.I whence
as follows:
.TP
.B SEEK_SET
The file offset is set to
.I offset
bytes.
.TP
.B SEEK_CUR
The file offset is set to its current location plus
.I offset
bytes.
.TP
.B SEEK_END
The file offset is set to the size of the file plus
.I offset
bytes.
.PP
.BR lseek ()
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 (\[aq]\e0\[aq]) until
data is actually written into the gap.
.SS Seeking file data and holes
Since Linux 3.1, Linux supports the following additional values for
.IR whence :
.TP
.B SEEK_DATA
Adjust the file offset to the next location
in the file greater than or equal to
.I offset
containing data.
If
.I offset
points to data,
then the file offset is set to
.IR offset .
.TP
.B SEEK_HOLE
Adjust the file offset to the next hole in the file
greater than or equal to
.IR offset .
If
.I offset
points into the middle of a hole,
then the file offset is set to
.IR offset .
If there is no hole past
.IR offset ,
then the file offset is adjusted to the end of the file
(i.e., there is an implicit hole at the end of any file).
.PP
In both of the above cases,
.BR lseek ()
fails if
.I offset
points past the end of the file.
.PP
These operations allow applications to map holes in a sparsely
allocated file.
This can be useful for applications such as file backup tools,
which can save space when creating backups and preserve holes,
if they have a mechanism for discovering holes.
.PP
For the purposes of these operations, a hole is a sequence of zeros that
(normally) has not been allocated in the underlying file storage.
However, a filesystem is not obliged to report holes,
so these operations are not a guaranteed mechanism for
mapping the storage space actually allocated to a file.
(Furthermore, a sequence of zeros that actually has been written
to the underlying storage may not be reported as a hole.)
In the simplest implementation,
a filesystem can support the operations by making
.B SEEK_HOLE
always return the offset of the end of the file,
and making
.B SEEK_DATA
always return
.I offset
(i.e., even if the location referred to by
.I offset
is a hole,
it can be considered to consist of data that is a sequence of zeros).
.\" https://lkml.org/lkml/2011/4/22/79
.\" http://lwn.net/Articles/440255/
.\" http://blogs.oracle.com/bonwick/entry/seek_hole_and_seek_data
.PP
The
.B _GNU_SOURCE
feature test macro must be defined in order to obtain the definitions of
.B SEEK_DATA
and
.B SEEK_HOLE
from
.IR <unistd.h> .
.PP
The
.B SEEK_HOLE
and
.B SEEK_DATA
operations are supported for the following filesystems:
.IP \[bu] 3
Btrfs (since Linux 3.1)
.IP \[bu]
OCFS (since Linux 3.2)
.\" commit 93862d5e1ab875664c6cc95254fc365028a48bb1
.IP \[bu]
XFS (since Linux 3.5)
.IP \[bu]
ext4 (since Linux 3.8)
.IP \[bu]
.BR tmpfs (5)
(since Linux 3.8)
.IP \[bu]
NFS (since Linux 3.18)
.\" commit 1c6dcbe5ceff81c2cf8d929646af675cd59fe7c0
.\" commit 24bab491220faa446d945624086d838af41d616c
.IP \[bu]
FUSE (since Linux 4.5)
.\" commit 0b5da8db145bfd44266ac964a2636a0cf8d7c286
.IP \[bu]
GFS2 (since Linux 4.15)
.\" commit 3a27411cb4bc3ce31db228e3569ad01b462a4310
.SH RETURN VALUE
Upon successful completion,
.BR lseek ()
returns the resulting offset location as measured in bytes from the
beginning of the file.
On error, the value \fI(off_t)\ \-1\fP is returned and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EBADF
.I fd
is not an open file descriptor.
.TP
.B EINVAL
.I whence
is not valid.
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 ENXIO
.I whence
is
.B SEEK_DATA
or
.BR SEEK_HOLE ,
and
.I offset
is beyond the end of the file, or
.I whence
is
.B SEEK_DATA
and
.I offset
is within a hole at the end of the file.
.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 fd
is associated with a pipe, socket, or FIFO.
.SH VERSIONS
On Linux, using
.BR lseek ()
on a terminal device fails with the error
.BR ESPIPE .
.\" Other systems return the number of written characters,
.\" using SEEK_SET to set the counter. (Of written characters.)
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, SVr4, 4.3BSD.
.PP
.B SEEK_DATA
and
.B SEEK_HOLE
are nonstandard extensions also present in Solaris,
FreeBSD, and DragonFly BSD;
they are proposed for inclusion in the next POSIX revision (Issue 8).
.\" FIXME . Review http://austingroupbugs.net/view.php?id=415 in the future
.SH NOTES
See
.BR open (2)
for a discussion of the relationship between file descriptors,
open file descriptions, and files.
.PP
If the
.B O_APPEND
file status flag is set on the open file description,
then a
.BR write (2)
.I always
moves the file offset to the end of the file, regardless of the use of
.BR lseek ().
.PP
Some devices are incapable of seeking and POSIX does not specify which
devices must support
.BR lseek ().
.SH SEE ALSO
.BR dup (2),
.BR fallocate (2),
.BR fork (2),
.BR open (2),
.BR fseek (3),
.BR lseek64 (3),
.BR posix_fallocate (3)