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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_SETThe offset is set to offset bytes.
SEEK_CURThe offset is set to its current location plus
offset
bytes.
SEEK_ENDThe 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_DATAAdjust 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_HOLEAdjust 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>
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.
fd is not an
open file descriptor.
whence is
not valid. Or: the resulting file offset would be
negative, or beyond the end of a seekable device.
The resulting file offset cannot be represented in an off_t.
fd is
associated with a pipe, socket, or FIFO.
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
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.
This page is part of release 3.54 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/.
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t Copyright (c) 1980, 1991 Regents of the University of California. and Copyright (c) 2011, Michael Kerrisk <mtk.manpagesgmail.com> All rights reserved. %%%LICENSE_START(BSD_4_CLAUSE_UCB) Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met: 1. Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer. 2. Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution. 3. All advertising materials mentioning features or use of this software must display the following acknowledgement: This product includes software developed by the University of California, Berkeley and its contributors. 4. Neither the name of the University nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission. THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. %%%LICENSE_END (#)lseek.2 6.5 (Berkeley) 3/10/91 Modified 1993-07-23 by Rik Faith <faithcs.unc.edu> Modified 1995-06-10 by Andries Brouwer <aebcwi.nl> Modified 1996-10-31 by Eric S. Raymond <esrthyrsus.com> Modified 1998-01-17 by Michael Haardt <michaelcantor.informatik.rwth-aachen.de> Modified 2001-09-24 by Michael Haardt <michaelmoria.de> Modified 2003-08-21 by Andries Brouwer <aebcwi.nl> 2011-09-18, mtk, Added SEEK_DATA + SEEK_HOLE |