lseek - reposition read/write file offset
off_t lseek(int fd, off_t offset, int
() repositions the file offset of the open file description
associated with the file descriptor fd
to the argument offset
according to the directive whence
- The file offset is set to offset bytes.
- The file offset is set to its current location plus offset
- The file offset is set to the size of the file plus offset
() 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.
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 offset containing data. If offset points to data,
then the file offset is set to offset.
- 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
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
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, and making
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).
feature test macro must be defined in order to obtain the
definitions of SEEK_DATA
operations are supported for the
- 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)
Upon successful completion, lseek
() returns the resulting offset location
as measured in bytes from the beginning of the file. On error, the value
is returned and errno
is set to indicate the
- fd is not an open file descriptor.
- whence is not valid. Or: the resulting file offset would be
negative, or beyond the end of a seekable device.
- whence is SEEK_DATA or SEEK_HOLE, and the file offset
is beyond the end of the file.
- The resulting file offset cannot be represented in an off_t.
- fd is associated with a pipe, socket, or FIFO.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
are nonstandard extensions also present in
Solaris, FreeBSD, and DragonFly BSD; they are proposed for inclusion in the
next POSIX revision (Issue 8).
(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
moves the file offset to the end of the
file, regardless of the use of lseek
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