LSEEK
Section: Linux Programmer's Manual (2)
Updated: 2001-09-24
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NAME
lseek - reposition read/write file offset
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
off_t lseek(int fd, off_t offset, int whence);
DESCRIPTION
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 (aq\0aq) until
data is actually written into the gap.
RETURN VALUE
Upon successful completion,
lseek()
returns the resulting offset location as measured in bytes from the
beginning of the file.
Otherwise, a value of (off_t) -1 is returned and
errno
is set to indicate the error.
ERRORS
- EBADF
-
fd
is not an open file descriptor.
- EINVAL
-
whence
is not one of
SEEK_SET,
SEEK_CUR,
SEEK_END;
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.
CONFORMING TO
SVr4, 4.3BSD, POSIX.1-2001.
NOTES
This document's use of
whence
is incorrect English, but maintained for historical reasons.
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
|
SVr1-3 returns long instead of off_t, BSD returns int.
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
dup(2),
fork(2),
open(2),
fseek(3),
lseek64(3),
posix_fallocate(3)
COLOPHON
This page is part of release 3.22 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/.