The label is located in sector number LABELSECTOR of the drive, usually sector 0 (zero) where it may be found without any information about the disk geometry. It is at an offset 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 DIOCWLABEL ioctl, which is done as needed by the disklabel program, allows modification of the label sector.
A copy of the in-core label for a disk can be obtained with the DIOCGDINFO ioctl; this works with a file descriptor for a block or character (raw) device for any partition of the disk. The in-core copy of the label is set by the DIOCSDINFO ioctl. The offset of a partition cannot generally be changed, nor 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. Finally, the DIOCWDINFO 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. All of these operations are normally done using the disklabel program.
Commands: disklabel(8) delim off