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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 offset of the open file associated
with the file descriptor fd to the argument offset according to the
directive whence as follows:
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 ('\0') until data is actually written into the gap.
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 file system 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 file system 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).
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.
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.
SVr4, 4.3BSD, POSIX.1-2001.
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).
Some devices are incapable of seeking and POSIX does not specify which
devices must support lseek().
On Linux, using lseek() on a tty 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.
dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)
This page is part of release 3.41 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2011-09-25 LSEEK(2)
HTML rendering created 2012-05-11 by Michael Kerrisk,
author of
The Linux Programming Interface,
maintainer of the
Linux man-pages project