LSEEK (2)
reposition read/write file offset
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
off_t lseek(int fildes , off_t offset , int whence );
DESCRIPTION
The
lseek
function repositions the offset of 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
lseek
function allows the file offset to be set beyond the end of the existing
end-of-file of the file. If data is later written at this point, subsequent
reads of the data in the gap return bytes of zeros (until data is actually
written into the gap).
RETURN VALUES
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
Fildes
is not an open file descriptor.
ESPIPE
Fildes
is associated with a pipe, socket, or FIFO.
EINVAL
Whence
is not a proper value.
CONFORMING TO
RESTRICTIONS
Some devices are incapable of seeking and POSIX does not specify which
devices must support it.
Linux specific restrictions: using lseek on a tty device returns
ESPIPE. Other systems return the number of written characters,
using SEEK_SET to set the counter. Some devices, e.g. /dev/null
do not cause the error ESPIPE, but return a pointer which value
is undefined.
NOTES
This document's use of
whence
is incorrect English, but maintained for historical reasons.
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.
SEE ALSO
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