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Name

Synopsis

off_t lseek(int fd, off_t offset, int whence);

Description

SEEK_SET

The offset is set to offset bytes.

SEEK_CUR

The offset is set to its current location plus offset bytes.

SEEK_END

The 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 (aq\0aq) until data is actually written into the gap.

Seeking file data and holes

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 (since Linux 3.8)

Return Value

Errors

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.

EOVERFLOW

The resulting file offset cannot be represented in an off_t.

ESPIPE

fd is associated with a pipe, socket, or FIFO.

ENXIO

whence is SEEK_DATA or SEEK_HOLE, and the current file offset is beyond the end of the file.

Conforming to

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).

Notes

Some devices are incapable of seeking and POSIX does not specify which devices must support lseek().

On Linux, using lseek() on a terminal device returns ESPIPE.

When converting old code, substitute values for whence with the following macros:

old	new
0	SEEK_SET
1	SEEK_CUR
2	SEEK_END
L_SET	SEEK_SET
L_INCR	SEEK_CUR
L_XTND	SEEK_END

Note that file descriptors created by dup(2) or fork(2) share the current file position pointer, so seeking on such files may be subject to race conditions.

See Also

Colophon

Table of Contents

• Name
• Synopsis
• Description
• Seeking file data and holes
• Return Value
• Errors
• Conforming to
• Notes
• See Also
• Colophon