from Symmetric Computer Systems.
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@(#)disklabel.5.5 6.2 (Berkeley) 06/13/88

C 7 disklabel - disk pack label #include <sys/disklabel.h> 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 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.

The label is located in sector number LABELSECTOR of the drive, usually sector 0 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 (8) program.

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 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. 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 disklabel (8).

The format of the disk label, as specified in < sys/disklabel.h >, is

.so  /usr/include/sys/disklabel.h
 "SEE ALSO"
disktab(5), disklabel(8)