LSEEK
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME
lseek - reposition read/write file offset
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);
DESCRIPTION
The
lseek()
function repositions the file offset of the open file description
associated with the file descriptor
fd
to the argument
offset
according to the directive
whence
as follows:
- SEEK_SET
-
The file offset is set to
offset
bytes.
- SEEK_CUR
-
The file offset is set to its current location plus
offset
bytes.
- SEEK_END
-
The file offset is set to the size of the file plus
offset
bytes.
The
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 (aq\0aq) until
data is actually written into the gap.
Seeking file data and holes
Since version 3.1, Linux supports the following additional values for
whence:
- SEEK_DATA
-
Adjust the file offset to the next location
in the file greater than or equal to
offset
containing data.
If
offset
points to data,
then the file offset is set to
offset.
- SEEK_HOLE
-
Adjust the file offset to the next hole in the file
greater than or equal to
offset.
If
offset
points into the middle of a hole,
then the file offset is set to
offset.
If there is no hole past
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).
In both of the above cases,
lseek()
fails if
offset
points past the end of the file.
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.
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
SEEK_HOLE
always return the offset of the end of the file,
and making
SEEK_DATA
always return
offset
(i.e., even if the location referred to by
offset
is a hole,
it can be considered to consist of data that is a sequence of zeros).
The
_GNU_SOURCE
feature test macro must be defined in order to obtain the definitions of
SEEK_DATA
and
SEEK_HOLE
from
<unistd.h>.
The
SEEK_HOLE
and
SEEK_DATA
operations are supported for the following filesystems:
- *
-
Btrfs (since Linux 3.1)
- *
-
OCFS (since Linux 3.2)
- *
-
XFS (since Linux 3.5)
- *
-
ext4 (since Linux 3.8)
- *
-
tmpfs(5) (since Linux 3.8)
- *
-
NFS (since Linux 3.18)
- *
-
FUSE (since Linux 4.5)
RETURN VALUE
Upon successful completion,
lseek()
returns the resulting offset location as measured in bytes from the
beginning of the file.
On error, the value
(off_t) -1 is returned and
errno
is set to indicate the error.
ERRORS
- EBADF
-
fd
is not an open file descriptor.
- EINVAL
-
whence
is not valid.
Or: the resulting file offset would be negative,
or beyond the end of a seekable device.
- ENXIO
-
whence
is
SEEK_DATA
or
SEEK_HOLE,
and the file offset is beyond the end of the file.
- EOVERFLOW
-
The resulting file offset cannot be represented in an
off_t.
- ESPIPE
-
fd
is associated with a pipe, socket, or FIFO.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
SEEK_DATA
and
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).
NOTES
See
open(2)
for a discussion of the relationship between file descriptors,
open file descriptions, and files.
If the
O_APPEND
file status flag is set on the open file description,
then a
write(2)
always
moves the file offset to the end of the file, regardless of the use of
lseek().
The
off_t
data type is a signed integer data type specified by POSIX.1.
Some devices are incapable of seeking and POSIX does not specify which
devices must support
lseek().
On Linux, using
lseek()
on a terminal device fails with the error
ESPIPE.
SEE ALSO
dup(2),
fallocate(2),
fork(2),
open(2),
fseek(3),
lseek64(3),
posix_fallocate(3)
COLOPHON
This page is part of release 4.13 of the Linux
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/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- Seeking file data and holes
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- SEE ALSO
-
- COLOPHON
-
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