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CD (4) | Special files and drivers | Unix Manual Pages | :man


cd - SCSI CD-ROM driver


Kernel Configuration
Changer Operation
See Also


.Cd device cd
.Cd "options " "CHANGER_MIN_BUSY_SECONDS=3" ""
.Cd "options " "CHANGER_MAX_BUSY_SECONDS=11" "


The cd driver provides support for a SCSI CD-ROM (Compact Disc-Read Only Memory) drive. 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 ad(4) and da(4).

As the SCSI adapter is probed during boot, the SCSI bus is scanned for devices. Any devices found which answer as CDROM (type 5) or WORM (type 4) type devices will be ‘attached’ to the cd driver. Prior to
.Fx 2.1 , the first device found will be attached as cd0 the next, cd1, etc. Beginning in
.Fx 2.1 it is possible to specify what cd unit a device should come on line as; refer to scsi(4) for details on kernel configuration.

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.


Any number of CD-ROM devices may be attached to the system regardless of system configuration as all resources are dynamically allocated.


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

("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).
CDIOCCAPABILITY ("struct ioc_capability") Retrieve information from the drive on what features it supports. The information is returned in the following structure:
struct ioc_capability {
u_long play_function;
#define CDDOPLAYTRK0x00000001
/* Can play tracks/index */
#define CDDOPLAYMSF0x00000002
/* Can play msf to msf */
#define CDDOPLAYBLOCKS 0x00000004
/* Can play range of blocks */
#define CDDOPAUSE 0x00000100
/* Output can be paused */
#define CDDORESUME 0x00000200
/* Output can be resumed */
#define CDDORESET 0x00000400
/* Drive can be completely reset */
#define CDDOSTART 0x00000800
/* Audio can be started */
#define CDDOSTOP 0x00001000
/* Audio can be stopped */
#define CDDOPITCH 0x00002000
/* Audio pitch can be changed */

u_long routing_function;
#define CDREADVOLUME 0x00000001
/* Volume settings can be read */
#define CDSETVOLUME0x00000002
/* Volume settings can be set */
#define CDSETMONO 0x00000100
/* Output can be set to mono */
#define CDSETSTEREO0x00000200
/* Output can be set to stereo (def) */
#define CDSETLEFT 0x00000400
/* Output can be set to left only */
#define CDSETRIGHT 0x00000800
/* Output can be set to right only */
#define CDSETMUTE 0x00001000
/* Output can be muted */
#define CDSETPATCH 0x00008000
/* Direct routing control allowed */

u_long special_function;
#define CDDOEJECT 0x00000001
/* The tray can be opened */
#define CDDOCLOSE 0x00000002
/* The tray can be closed */
#define CDDOLOCK 0x00000004
/* The tray can be locked */
#define CDREADHEADER 0x00000100
/* Can read Table of Contents */
#define CDREADENTRIES 0x00000200
/* Can read TOC Entries */
#define CDREADSUBQ 0x00000200
/* Can read Subchannel info */
#define CDREADRW 0x00000400
/* Can read subcodes R-W */
#define CDHASDEBUG 0x00004000
/* The tray has dynamic debugging */

CDIOCPLAYTRACKS ("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;

CDIOCPLAYBLOCKS ("struct ioc_play_blocks") Start audio playback given a block address and length. The structure is defined as follows:
struct ioc_play_blocks

CDIOCPLAYMSF ("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;
struct cd_sub_channel_info *data;

CDIOREADTOCHEADER ("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;

CDIOREADTOCENTRYS ("struct ioc_read_toc_entry") Return information from the table of contents entries mentioned. (Yes, this command name is misspelled.) 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.
CDIOCSETPATCH ("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 */

("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 */

CDIOCSETMONO Patch all output channels to all source channels.
CDIOCSETSTEREO Patch left source channel to the left output channel and the right source channel to the right output channel.
CDIOCSETMUTE 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.
CDIOCRESET 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.
CDIOCCLOSE Tell the drive to close its door and load the media. Not all drives support this feature.
CDIOCPITCH ("struct ioc_pitch") For drives that support it, this command instructs the drive to play the audio at a faster or slower rate than normal. Values of speed between -32767 and -1 result in slower playback; a zero value indicates normal speed; and values from 1 to 32767 give faster playback. Drives with less than 16 bits of resolution will silently ignore less-significant bits. The structure is defined thusly:
struct ioc_pitch
short speed;


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 support SCSI-2 standard audio commands. As 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.

The cd driver attempts to automatically determine whether the drive it is talking to supports 6 byte or 10 byte MODE SENSE/MODE SELECT operations. Many SCSI drives only support 6 byte commands, and ATAPI drives only support 10 byte commands. The cd driver first attempts to determine whether the protocol in use typically supports 6 byte commands by issuing a CAM Path Inquiry CCB. It will then default to 6 byte or 10 byte commands as appropriate. After that, the cd driver defaults to using 6 byte commands (assuming the protocol the drive speaks claims to support 6 byte commands), until one fails with a SCSI ILLEGAL REQUEST error. Then it tries the 10 byte version of the command to see if that works instead. Users can change the default via per-drive sysctl variables and loader tunables. The variable names are the same in both instances:


Where "%d" is the unit number of the drive in question. Valid minimum command sizes are 6 and 10. Any value above 6 will be rounded to 10, and any value below 6 will be rounded to 6.


This driver has built-in support for LUN-based CD changers. A LUN-based CD changer is a drive that can hold two or more CDs, but only has one CD player mechanism. Each CD in the drive shows up as a separate logical unit on the SCSI bus. The cd driver automatically recognizes LUN-based changers, and routes commands for changers through an internal scheduler. The scheduler prevents changer "thrashing", which is caused by sending commands to different LUNs in the changer at the same time.

The scheduler honors minimum and maximum time quanta that the driver will spend on a particular LUN. The minimum time is the guaranteed minimum amount of time that the driver will spend on a given LUN, even if there is no outstanding I/O for that LUN. The maximum time is the maximum amount of time the changer will spend on a LUN if there is outstanding I/O for another LUN. If there is no outstanding I/O for another LUN, the driver will allow indefinite access to a given LUN.

The minimum and maximum time quanta are configurable via kernel options and also via sysctl and kernel tunable variables. The kernel options are:

.Cd "options " "CHANGER_MIN_BUSY_SECONDS=3" ""

.Cd "options " "CHANGER_MAX_BUSY_SECONDS=11" ""

The sysctl/kernel tunable variables are:


It is suggested that the user try experimenting with the minimum and maximum timeouts via the sysctl variables to arrive at the proper values for your changer. Once you have settled on the proper timeouts for your changer, you can then put them in your kernel config file.

If your system does have a LUN-based changer, you may notice that the probe messages for the various LUNs of the changer will continue to appear while the boot process is going on. This is normal, and is caused by the changer scheduling code.


raw mode CD-ROM devices




da(4), scsi(4), disklabel(5), disklabel(8), cd(9)




Created by Blin Media, 2008-2013