Section: Linux Programmer's Manual (2)
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lseek - reposition read/write file offset
off_t lseek(int fd, off_t offset, int whence);
function repositions the offset of the open file associated with the
to the argument
according to the directive
The offset is set to
The offset is set to its current location plus
The offset is set to the size of the file plus
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.
Seeking file data and holes
Since version 3.1, Linux supports the following additional values for
Adjust the file offset to the next location
in the file greater than or equal to
points to data,
then the file offset is set to
Adjust the file offset to the next hole in the file
greater than or equal to
points into the middle of a hole,
then the file offset is set to
If there is no hole past
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,
points past the end of the file.
These operations allow applications to map holes in a sparsely
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
always return the offset of the end of the file,
(i.e., even if the location referred to by
is a hole,
it can be considered to consist of data that is a sequence of zeros).
feature test macro must be defined in order to obtain the definitions of
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 (since Linux 3.8)
Upon successful completion,
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
is set to indicate the error.
is not an open file descriptor.
is not valid.
Or: the resulting file offset would be negative,
or beyond the end of a seekable device.
The resulting file offset cannot be represented in an
is associated with a pipe, socket, or FIFO.
and the current file offset is beyond the end of the file.
SVr4, 4.3BSD, POSIX.1-2001.
are nonstandard extensions also present in Solaris,
FreeBSD, and DragonFly BSD;
they are proposed for inclusion in the next POSIX revision (Issue 8).
for a discussion of the relationship between file descriptors,
open file descriptions, and files.
Some devices are incapable of seeking and POSIX does not specify which
devices must support
On Linux, using
on a terminal device returns
When converting old code, substitute values for whence with the
Note that file descriptors created by
share the current file position pointer, so seeking on such files may be
subject to race conditions.
This page is part of release 3.74 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
- Seeking file data and holes
- RETURN VALUE
- CONFORMING TO
- SEE ALSO