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CD(4) Device Drivers Manual CD(4)

cdATAPI and SCSI CD-ROM driver

cd* at scsibus?
#cd0 at scsibus0 target 6 lun 0 (fixed-configuration example)

The cd driver provides support for ATAPI and SCSI CD-ROM (Compact Disc Read-Only Memory) drives, via scsibus(4). In an attempt to look like a regular disk, the cd driver synthesizes a partition table, with one partition covering the entire CD-ROM. It is possible to modify this partition table using disklabel(8), but it will only last until the CD-ROM is unmounted. In general the interfaces are similar to those described by wd(4) and sd(4).

As the SCSI adapter is probed during boot, the SCSI bus is scanned for devices. Any devices found which answer as “Read-only” and “removable” CD-ROM or WORM devices will be “attached” to the cd driver.

The system utility disklabel(8) may be used to read the synthesized disk label structure, which will contain correct figures for the size of the CD-ROM should that information be required.

The following ioctl(2) calls which apply to CD-ROM drives are defined in the header files <sys/cdio.h> and <sys/dkio.h>.

, DIOCSDINFO struct disklabel *
Read or write the in-core copy of the disklabel for the drive. The disklabel is initialized with information read from the SCSI inquiry commands, and should be the same as the information printed at boot. This structure is defined in disklabel(5).
struct ioc_play_track *
Start audio playback given a track address and length. The structure is defined as follows:
struct ioc_play_track {
	u_char	start_track;
	u_char	start_index;
	u_char	end_track;
	u_char	end_index;
struct ioc_play_blocks *
Start audio playback given a block address and length. The structure is defined as follows:
struct ioc_play_blocks {
	int	blk;
	int	len;
struct ioc_play_msf *
Start audio playback given a “minutes-seconds-frames” address and length. The structure is defined as follows:
struct ioc_play_msf {
	u_char	start_m;
	u_char	start_s;
	u_char	start_f;
	u_char	end_m;
	u_char	end_s;
	u_char	end_f;
struct ioc_read_subchannel *
Read information from the subchannel at the location specified by this structure:
struct ioc_read_subchannel {
	u_char address_format;
#define CD_LBA_FORMAT	1
#define CD_MSF_FORMAT	2
	u_char data_format;
#define CD_SUBQ_DATA		0
#define CD_TRACK_INFO		3
	u_char track;
	int	data_len;
	struct  cd_sub_channel_info *data;
struct ioc_toc_header *
Return summary information about the table of contents for the mounted CD-ROM. The information is returned into the following structure:
struct ioc_toc_header {
	u_short len;
	u_char  starting_track;
	u_char  ending_track;
struct ioc_read_toc_entry *
Return information from the table of contents entries mentioned. The argument structure is defined as follows:
struct ioc_read_toc_entry {
	u_char	address_format;
	u_char	starting_track;
	u_short	data_len;
	struct  cd_toc_entry *data;

The requested data is written into an area of size data_len and pointed to by data.

struct ioc_patch *
Attach various audio channels to various output channels. The argument structure is defined thusly:
struct ioc_patch {
	u_char	patch[4];
	/* one for each channel */
, CDIOCSETVOL struct ioc_vol *
Get (set) information about the volume settings of the output channels. The argument structure is as follows:
struct	ioc_vol {
	u_char	vol[4];
	/* one for each channel */
Patch all output channels to all source channels.
Patch left source channel to the left output channel and the right source channel to the right output channel.
Mute output without changing the volume settings.
Attach both output channels to the left (right) source channel.
Turn on (off) debugging for the appropriate device.
Pause (resume) audio play, without resetting the location of the read-head.
Reset the drive.
Tell the drive to spin-up (-down) the CD-ROM.
Tell the drive to allow (prevent) manual ejection of the CD-ROM disc. Not all drives support this feature.
Eject the CD-ROM.

In addition the general scsi(4) ioctls may be used with the cd driver, if used against the `whole disk' partition (i.e., /dev/rcd0c).

When a CD-ROM is changed in a drive controlled by the cd driver, then the act of changing the media will invalidate the disklabel and information held within the kernel. To stop corruption, all accesses to the device will be discarded until there are no more open file descriptors referencing the device. During this period, all new open attempts will be rejected. When no more open file descriptors reference the device, the first next open will load a new set of parameters (including disklabel) for the drive.

The audio code in the cd driver only supports SCSI-2 standard audio commands. Because many CD-ROM manufacturers have not followed the standard, there are many CD-ROM drives for which audio will not work. Some work is planned to support some of the more common “broken” CD-ROM drives; however, this is not yet under way.

block mode CD-ROM devices
raw mode CD-ROM devices


cdio(1), eject(1), ioctl(2), intro(4), scsi(4), scsibus(4), sd(4), wd(4), disklabel(5), disklabel(8)

The cd driver appeared in 386BSD 0.1.

The names of the structures used for the third argument to ioctl() were poorly chosen, and a number of spelling errors have survived in the names of the ioctl() commands.

August 27, 2015 OpenBSD-6.4