NAME
lseek,
seek —
reposition read/write file offset
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#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. The argument
fildes must
be an open file descriptor.
lseek() repositions the file
pointer
fildes as follows:
- If whence is
SEEK_SET
, the offset is set to
offset bytes.
- If whence is
SEEK_CUR
, the offset is set to its current
location plus offset bytes.
- If whence is
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).
Some devices are incapable of seeking. The value of the pointer associated with
such a device is undefined.
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 -1 is returned and
errno is set to
indicate the error.
ERRORS
lseek() will fail and the file pointer will remain unchanged
if:
-
-
- [
EBADF
]
- fildes is not an open file
descriptor.
-
-
- [
EINVAL
]
- whence is not a proper value, or the
resulting file offset would be invalid.
-
-
- [
ESPIPE
]
- fildes is associated with a pipe,
socket, or FIFO.
SEE ALSO
dup(2),
open(2)
STANDARDS
The
lseek() function conforms to
IEEE Std
1003.1-1990 (“POSIX.1”).
HISTORY
A
seek() function appeared in
Version 2
AT&T UNIX, later renamed into
lseek() for
“long seek” due to a larger
offset argument
type.
BUGS
This document's use of
whence is incorrect English, but is
maintained for historical reasons.