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 (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 return bytes of zeros (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
fildes is not an open file descriptor.
ESPIPE
fildes is associated with a pipe, socket, or FIFO.
EINVAL
whence is not one of SEEK_SET, SEEK_CUR, SEEK_END,
or the resulting file offset would be negative.
EOVERFLOW
The resulting file offset cannot be represented in an off_t.
CONFORMING TO
SVr4, POSIX, BSD 4.3
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.
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:
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.
