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.\"     @(#)lseek.2	8.3 (Berkeley) 4/19/94
.\" $FreeBSD$
.\"
.Dd April 19, 1994
.Dt LSEEK 2
.Os
.Sh NAME
.Nm lseek
.Nd reposition read/write file offset
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In unistd.h
.Ft off_t
.Fn lseek "int fildes" "off_t offset" "int whence"
.Sh DESCRIPTION
The
.Fn lseek
system call repositions the offset of the file descriptor
.Fa fildes
to the
argument
.Fa offset
according to the directive
.Fa whence .
The argument
.Fa fildes
must be an open
file descriptor.
The
.Fn lseek
system call
repositions the file position pointer associated with the file
descriptor
.Fa fildes
as follows:
.Bl -item -offset indent
.It
If
.Fa whence
is
.Dv SEEK_SET ,
the offset is set to
.Fa offset
bytes.
.It
If
.Fa whence
is
.Dv SEEK_CUR ,
the offset is set to its current location plus
.Fa offset
bytes.
.It
If
.Fa whence
is
.Dv SEEK_END ,
the offset is set to the size of the
file plus
.Fa offset
bytes.
.El
.Pp
The
.Fn lseek
system call 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).
.Pp
Some devices are incapable of seeking.
The value of the pointer
associated with such a device is undefined.
.Sh RETURN VALUES
Upon successful completion,
.Fn lseek
returns the resulting offset location as measured in bytes from the
beginning of the file.
Otherwise,
a value of -1 is returned and
.Va errno
is set to indicate
the error.
.Sh ERRORS
The
.Fn lseek
system call
will fail and the file position pointer will remain unchanged if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa fildes
argument
is not an open file descriptor.
.It Bq Er EINVAL
The
.Fa whence
argument
is not a proper value
or the resulting file offset would
be negative for a non-character special file.
.It Bq Er EOVERFLOW
The resulting file offset would be a value which cannot be represented
correctly in an object of type
.Fa off_t .
.It Bq Er ESPIPE
The
.Fa fildes
argument
is associated with a pipe, socket, or FIFO.
.El
.Sh SEE ALSO
.Xr dup 2 ,
.Xr open 2
.Sh STANDARDS
The
.Fn lseek
system call is expected to conform to
.St -p1003.1-90 .
.Sh HISTORY
The
.Fn lseek
function appeared in
.At v7 .
.Sh BUGS
This document's use of
.Fa whence
is incorrect English, but is maintained for historical reasons.