переместить смещение файла чтения / записи (reposition read/write file offset)
Имя (Name)
lseek - reposition read/write file offset
Синопсис (Synopsis)
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);
Описание (Description)
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:
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 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)
* GFS2 (since Linux 4.15)
Возвращаемое значение (Return value)
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.
Ошибки (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 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.
Стандарты (Conforming to)
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).
Примечание (Note)
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.
Смотри также (See also)
dup(2), fallocate(2), fork(2), open(2), fseek(3), lseek64(3),
posix_fallocate(3)
