lseek - reposition read/write file offset
Standard C library (libc, -lc)
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);lseek() 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:
The file offset is set to offset bytes.
The file offset is set to its current location plus offset
bytes.
The file offset is set to the size of the file plus offset
bytes.
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 ('\0') until data is actually written into the gap.
Since Linux 3.1, Linux supports the following additional values for
whence:
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 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)
GFS2 (since Linux 4.15)
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.
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 offset is beyond the end of the
file, or whence is SEEK_DATA and
offset is within a hole at 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.
On Linux, using lseek() on a terminal device fails with the error ESPIPE.
POSIX.1-2008.
POSIX.1-2001, 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).
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().
Some devices are incapable of seeking and POSIX does not specify which devices must support lseek().