lseek(2) System Calls Manual lseek(2)
NAME
lseek - reposition read/write file offset
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);
DESCRIPTION
lseek() repositions the file offset of the open file description associ-
ated 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.
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.
Seeking file data and holes
Since Linux 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. How-
ever, 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 op-
erations 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 loca-
tion 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)
RETURN VALUE
Upon successful completion, lseek() returns the resulting offset loca-
tion 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 neg-
ative, or beyond the end of a seekable device.
ENXIO 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.
EOVERFLOW
The resulting file offset cannot be represented in an off_t.
ESPIPE fd is associated with a pipe, socket, or FIFO.
VERSIONS
On Linux, using lseek() on a terminal device fails with the error ES-
PIPE.
STANDARDS
POSIX.1-2008.
HISTORY
POSIX.1-2001, SVr4, 4.3BSD.
SEEK_DATA and SEEK_HOLE are nonstandard extensions also present in So-
laris, 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 descrip-
tors, 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, re-
gardless of the use of lseek().
Some devices are incapable of seeking and POSIX does not specify which
devices must support lseek().
SEE ALSO
dup(2), fallocate(2), fork(2), open(2), fseek(3), lseek64(3), posix_fal-
locate(3)
Linux man-pages 6.9.1 2024-06-15 lseek(2)
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