'\" t

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.\"

.\"     @(#)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 2017-09-15 "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>

.PP

.BI "off_t lseek(int " fd ", off_t " offset ", int " whence );

.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\\0\(aq) until

data is actually written into the gap.

.SS Seeking file data and holes

Since version 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

.BR SEEK_HOLE

always return the offset of the end of the file,

and making

.BR SEEK_DATA

always return

.IR 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

.BR _GNU_SOURCE

feature test macro must be defined in order to obtain the definitions of

.BR SEEK_DATA

and

.BR SEEK_HOLE

from

.IR <unistd.h> .

.PP

The

.BR SEEK_HOLE

and

.BR SEEK_DATA

operations are supported for the following filesystems:

.IP * 3

Btrfs (since Linux 3.1)

.IP * 3

OCFS (since Linux 3.2)

.\" commit 93862d5e1ab875664c6cc95254fc365028a48bb1

.IP *

XFS (since Linux 3.5)

.IP *

ext4 (since Linux 3.8)

.IP *

.BR tmpfs "(5) (since Linux 3.8)"

.IP *

NFS (since Linux 3.18)

.\" commit 1c6dcbe5ceff81c2cf8d929646af675cd59fe7c0

.\" commit 24bab491220faa446d945624086d838af41d616c

.IP *

FUSE (since Linux 4.5)

.\" commit 0b5da8db145bfd44266ac964a2636a0cf8d7c286

.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 the file offset is beyond 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 CONFORMING TO

POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.

.PP

.BR SEEK_DATA

and

.BR 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

The

.I off_t

data type is a signed integer data type specified by POSIX.1.

.PP

Some devices are incapable of seeking and POSIX does not specify which

devices must support

.BR lseek ().

.PP

On Linux, using

.BR lseek ()

on a terminal device fails with the error

\fBESPIPE\fP.

.\" Other systems return the number of written characters,

.\" using SEEK_SET to set the counter. (Of written characters.)

.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)

.SH COLOPHON

This page is part of release 4.15 of the Linux

.I man-pages

project.

A description of the project,

information about reporting bugs,

and the latest version of this page,

can be found at

\%https://www.kernel.org/doc/man\-pages/.