Section (2) lseek
lseek — reposition read/write file offset
#include <sys/types.h> #include <unistd.h>
lseek() repositions the file
offset of the open file description associated with the file
fd to the
according to the directive
whence as follows:
The file offset is set to
The file offset is set to its current location plus
The file offset is set to the size of the file plus
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 (_zsingle_quotesz_ _zsingle_quotesz_) 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
offsetcontaining data. If
offsetpoints 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
offsetpoints 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
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
test macro must be defined in order to obtain the
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)
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.
fdis not an open file descriptor.
whenceis not valid. Or: the resulting file offset would be negative, or beyond the end of a seekable device.
SEEK_HOLE, and the file offset is beyond the end of the file.
The resulting file offset cannot be represented in an off_t.
fdis associated with a pipe, socket, or FIFO.
POSIX.1-2001, POSIX.1-2008, SVr4, 4.3BSD.
SEEK_HOLE are nonstandard extensions also
present in Solaris, FreeBSD, and DragonFly BSD; they are
proposed for inclusion in the next POSIX revision (Issue
See open(2) for a discussion of the relationship between file descriptors, open file descriptions, and files.
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
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
On Linux, using
lseek() on a
terminal device fails with the error ESPIPE.
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(#)lseek.2 6.5 (Berkeley) 3/10/91
Modified 1993-07-23 by Rik Faith <faithcs.unc.edu>
Modified 1995-06-10 by Andries Brouwer <aebcwi.nl>
Modified 1996-10-31 by Eric S. Raymond <esrthyrsus.com>
Modified 1998-01-17 by Michael Haardt
Modified 2001-09-24 by Michael Haardt <michaelmoria.de>
Modified 2003-08-21 by Andries Brouwer <aebcwi.nl>
2011-09-18, mtk, Added SEEK_DATA + SEEK_HOLE