NAME
lseek —
reposition read/write file
offset
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.
- [
ESPIPE] - fildes is associated with a pipe, socket, or FIFO.
- [
EINVAL] - whence is not a proper value or the resulting offset would be negative on a file system or special device that does not allow negative offsets to be used.
SEE ALSO
STANDARDS
The lseek() function conforms to
IEEE Std 1003.1-2008 (“POSIX.1”).
HISTORY
A seek() system call first appeared in
Version 1 AT&T UNIX. In
Version 7 AT&T UNIX it was renamed to
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.