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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON |
LSEEK(2) Linux Programmer's Manual LSEEK(2)
lseek - reposition read/write file offset
#include <sys/types.h>
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);
The lseek() function 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:
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.
The lseek() 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
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. 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)
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.
EBADF fd is not an open file descriptor.
EINVAL whence is not valid. Or: the resulting file offset would be
negative, or beyond the end of a seekable device.
ENXIO whence is SEEK_DATA or SEEK_HOLE, and the file offset is
beyond 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.
POSIX.1-2001, POSIX.1-2008, 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().
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 lseek().
On Linux, using lseek() on a terminal device fails with the error
ESPIPE.
dup(2), fallocate(2), fork(2), open(2), fseek(3), lseek64(3),
posix_fallocate(3)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-05-03 LSEEK(2)
Pages that refer to this page: copy_file_range(2), dup(2), llseek(2), open(2), pread(2), read(2), readahead(2), readv(2), syscalls(2), write(2), fseek(3), getdirentries(3), lseek64(3), posix_fallocate(3), seekdir(3), stdin(3), cpuid(4), proc(5), pipe(7), signal-safety(7), spufs(7), user_namespaces(7), xfs_io(8)