.\"	$OpenBSD: disklabel.5,v 1.15 2003/06/02 20:06:14 millert Exp $
.\"	$NetBSD: disklabel.5,v 1.3 1995/03/18 14:54:36 cgd Exp $
.\"
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.\"     @(#)disklabel.5.5	8.1 (Berkeley) 6/5/93
.\"
.Dd August 6, 2001
.Dt DISKLABEL 5
.Os
.Sh NAME
.Nm disklabel
.Nd disk pack label
.Sh SYNOPSIS
.Fd #include <sys/disklabel.h>
.Sh DESCRIPTION
Each disk or disk pack on a system may contain a disk label
which provides detailed information
about the geometry of the disk and the partitions into which the disk
is divided.
It should be initialized when the disk is formatted,
and may be changed later with the
.Xr disklabel 8
program.
This information is used by the system disk driver and by the bootstrap
program to determine how to program the drive
and where to find the filesystems on the disk partitions.
Additional information is used by the filesystem in order
to use the disk most efficiently and to locate important filesystem information.
The description of each partition contains an identifier for the partition
type (standard filesystem, swap area, etc.).
The filesystem updates the in-core copy of the label if it contains
incomplete information about the filesystem.
.Pp
The label is located in sector number
.Dv LABELSECTOR
of the drive, usually sector 0 where it may be found
without any information about the disk geometry.
It is at an offset
.Dv LABELOFFSET
from the beginning of the sector, to allow room for the initial bootstrap.
The disk sector containing the label is normally made read-only
so that it is not accidentally overwritten by pack-to-pack copies
or swap operations;
the
.Dv DIOCWLABEL
.Xr ioctl 2 ,
which is done as needed by the
.Em disklabel
program, allows modification of the label sector.
.Pp
A copy of the in-core label for a disk can be obtained with the
.Dv DIOCGDINFO
.Em ioctl ;
this works with a file descriptor for a block or character
.Pq Dq raw
device for any partition of the disk.
The in-core copy of the label is set by the
.Dv DIOCSDINFO
.Em ioctl .
The offset of a partition cannot generally be changed while it is open,
nor can it be made smaller while it is open.
One exception is that any change is allowed if no label was found
on the disk, and the driver was able to construct only a skeletal label
without partition information.
The
.Dv DIOCWDINFO
.Em ioctl
operation sets the in-core label and then updates the on-disk label;
there must be an existing label on the disk for this operation to succeed.
Thus, the initial label for a disk or disk pack must be installed
by writing to the raw disk.
The
.Dv DIOCGPDINFO
.Em ioctl
operation gets the default label for a disk.
This simulates the case
where there is no physical label on the disk itself and can be used to
see the label the kernel would construct in that case.
The
.Dv DIOCRLDINFO
.Em ioctl
operation causes the kernel to update its copy of label based on the
physical label on the disk.
It can be used when the on-disk version
of the label was changed directly or, if there is no physical label,
to update the kernel's skeletal label if some variable affecting label
generation has changed (e.g. the fdisk partition table).
All of these operations are normally done using
.Em disklabel .
.Sh SEE ALSO
.Xr disktab 5 ,
.Xr disklabel 8