lseek
NAME
lseek - reposition read/write file offset
SYNOPSIS
#include <sys/types.h>
#include <unistd.h> I off_t lseek(int fildes , off_t offset , int whence );
#include <unistd.h> I off_t lseek(int fildes , off_t offset , int whence );
DESCRIPTION
The
R lseek ()
function repositions the offset of the open file associated with the
file descriptor
fildes
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
R 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.
RETURN VALUE
Upon successful completion,
R 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
fildes
is not an open file descriptor.
EINVAL
whence
is not one of
R SEEK_SET ,
R SEEK_CUR ,
R 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
R off_t .
ESPIPE
fildes
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
R lseek ().
On Linux, using
R lseek ()
on a tty device returns
ESPIPE.
When converting old code, substitute values for whence with the
following macros:
c c
l l.
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.
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