READCD(1) Schily´s USER COMMANDS READCD(1)NAMEreadcd - read or write data Compact Discs
SYNOPSISreadcd dev=device [ options ]
Readcd is used to read or write Compact Discs.
The device refers to scsibus/target/lun of the drive. Communication on
SunOS is done with the SCSI general driver scg. Other operating sys‐
tems are using a library simulation of this driver. Possible syntax
is: dev= scsibus,target,lun or dev= target,lun. In the latter case,
the drive has to be connected to the default SCSI bus of the machine.
Scsibus, target and lun are integer numbers. Some operating systems or
SCSI transport implementations may require to specify a filename in
addition. In this case the correct syntax for the device is: dev=
devicename:scsibus,target,lun or dev= devicename:target,lun. If the
name of the device node that has been specified on such a system refers
to exactly one SCSI device, a shorthand in the form dev= devicename:@
or dev= devicename:@,lun may be used instead of dev= devicename:scsi‐
To access remote SCSI devices, you need to prepend the SCSI device name
by a remote device indicator. The remote device indicator is either
REMOTE:user@host: or REMOTE:host:
A valid remote SCSI device name may be: REMOTE:user@host: to allow
remote SCSI bus scanning or REMOTE:user@host:1,0,0 to access the SCSI
device at host connected to SCSI bus # 1,target 0 lun 0.
To access SCSI devices via alternate transport layers, you need to
prepend the SCSI device name by a transport layer indicator. The
transport layer indicator may be something like USCSI: or ATAPI:. To
get a list of supported transport layers for your platform, use dev=
To make readcd portable to all UNIX platforms, the syntax dev= device‐
name:scsibus,target,lun is preferred as is hides OS specific knowledge
about device names from the user. A specific OS must not necessarily
support a way to specify a real device file name nor a way to specify
Scsibus 0 is the default SCSI bus on the machine. Watch the boot mes‐
sages for more information or look into /var/adm/messages for more
information about the SCSI configuration of your machine. If you have
problems to figure out what values for scsibus,target,lun should be
used, try the -scanbus option of cdrecord.
If no options except the dev= option have been specified, readcd goes
into interactive mode. Select a primary function and then follow the
Print version information and exit.
Sets the SCSI target for the drive, see notes above. A typical
device specification is dev=6,0 . If a filename must be pro‐
vided together with the numerical target specification, the
filename is implementation specific. The correct filename in
this case can be found in the system specific manuals of the
target operating system. On a FreeBSD system without CAM sup‐
port, you need to use the control device (e.g. /dev/rcd0.ctl).
A correct device specification in this case may be
On Linux, drives connected to a parallel port adapter are mapped
to a virtual SCSI bus. Different adapters are mapped to differ‐
ent targets on this virtual SCSI bus.
If no dev option is present, cdrecord will try to get the device
from the CDR_DEVICE environment.
If the argument to the dev= option does not contain the charac‐
ters ',', '/', '@' or ':', it is interpreted as an label name
that may be found in the file /etc/default/cdrecord (see FILES
Set the default SCSI command timeout value to # seconds. The
default SCSI command timeout is the minimum timeout used for
sending SCSI commands. If a SCSI command fails due to a time‐
out, you may try to raise the default SCSI command timeout above
the timeout value of the failed command. If the command runs
correctly with a raised command timeout, please report the bet‐
ter timeout value and the corresponding command to the author of
the program. If no timeout option is present, a default timeout
of 40 seconds is used.
Set the misc debug value to # (with debug=#) or increment the
misc debug level by one (with -d). If you specify -dd, this
equals to debug=2. This may help to find problems while opening
a driver for libscg. as well as with sector sizes and sector
types. Using -debug slows down the process and may be the rea‐
son for a buffer underrun.
Tell the scg-driver to modify the kernel debug value while SCSI
commands are running.
Do not print out a status report for failed SCSI commands.
-v Increment the level of general verbosity by one. This is used
e.g. to display the progress of the process.
-V Increment the verbose level with respect of SCSI command trans‐
port by one. This helps to debug problems during the process,
that occur in the CD-Recorder. If you get incomprehensible
error messages you should use this flag to get more detailed
output. -VV will show data buffer content in addition. Using
-V or -VV slows down the process.
f=file Specify the filename where the output should be written or the
inout should be taken from. Using '-' as filename will cause
readcd to use stdout resp. stdin.
-w Switch to write mode. If this option is not present, readcd
reads from the specified device.
Scans the whole CD or the range specified by the sectors=range
for C2 errors. C2 errors are errors that are uncorrectable after
the second stage of the 24/28 + 28/32 Reed Solomon correction
system at audio level (2352 bytes sector size). If an audio CD
has C2 errors, interpolation is needed to hide the errors. If a
data CD has C2 errors, these errors are in most cases corrected
by the ECC/EDC code that makes 2352 bytes out of 2048 data
bytes. The ECC/EDC code should be able to correct about 100 C2
error bytes per sector.
If you find C2 errors you may want to reduce the speed using the
speed= option as C2 errors may be a result of dynamic unbalance
on the medium.
Scan all SCSI devices on all SCSI busses and print the inquiry
strings. This option may be used to find SCSI address of the
devices on a system. The numbers printed out as labels are com‐
puted by: bus * 100 + target
Specify a sector range that should be read. The range is speci‐
fied by the starting sector number, a minus sign and the ending
sector number. The end sector is not included in the list, so
sectors=0-0 will not read anything and may be used to check for
a CD in the drive.
Set the speed factor of the read or write process to #. # is an
integer, representing a multiple of the audio speed. This is
about 150 KB/s for CD-ROM and about 172 KB/s for CD-Audio. If
no speed option is present, readcd will use maximum speed. Only
MMC compliant drives will benefit from this option. The speed
of non MMC drives is not changed.
Using a lower speed may increase the readability of a CD or DVD.
ts=# Set the maximum transfer size for a single SCSI command to #.
The syntax for the ts= option is the same as for cdrecord fs=#
or sdd bs=#.
If no ts= option has been specified, readcd defaults to a trans‐
fer size of 256 kB. If libscg gets lower values from the operat‐
ing system, the value is reduced to the maimum value that is
possible with the current operating system. Sometimes, it may
help to further reduce the transfer size or to enhance it, but
note that it may take a long time to find a better value by
experimenting with the ts= option.
Do not truncate the output file when opening it.
Retrieve a full TOC from the current disk and print it in hex.
-clone Do a clone read. Read the CD with all sub-channel data and a
full TOC. The full TOC data will be put into a file with simi‐
lar name as with the f= option but the suffix .toc added.
Do not abort if the high level error checking in readcd found an
uncorrectable error in the data stream.
Switch the drive into a mode where it ignores read errors in
data sectors that are a result of uncorrectable ECC/EDC errors
before reading. If readcd completes, the error recovery mode of
the drive is switched back to the remembered old mode.
Set the retry count for high level retries in readcd to #. The
default is to do 128 retries which may be too much if you like
to read a CD with many unreadable sectors.
Meter the SCSI command overhead time. This is done by executing
several commands 1000 times and printing the total time used. If
you divide the displayed times by 1000, you get the average
overhead time for a single command.
Print read-speed at # locations. The purpose of this option is
to create a list of read speed values suitable for e.g. gnu‐
plot. The speed values are calculated assuming that 1000 bytes
are one kilobyte as documented in the SCSI standard. The ouput
data created for this purpose is written to stdout.
Output the speed values for meshpoints=# as factor based on sin‐
gle speed of the current medium. This only works if readcd is
able to determine the current medium type.
For all examples below, it will be assumed that the drive is connected
to the primary SCSI bus of the machine. The SCSI target id is set to 2.
To read the complete media from a CD-ROM writing the data to the file
readcd dev=2,0 f=cdimage.raw
To read sectors from range 150 ... 10000 from a CD-ROM writing the data
to the file cdimage.raw:
readcd dev=2,0 sectors=150-10000 f=cdimage.raw
To write the data from the file cdimage.raw (e.g. a filesystem image
from mkisofs) to a DVD-RAM, call:
readcd dev=2,0 -w f=cdimage.raw
RSH If the RSH environment is present, the remote connection will
not be created via rcmd(3) but by calling the program pointed to
by RSH. Use e.g. RSH=/usr/bin/ssh to create a secure shell
Note that this forces cdrecord to create a pipe to the rsh(1)
program and disallows cdrecord to directly access the network
socket to the remote server. This makes it impossible to set up
performance parameters and slows down the connection compared to
a root initiated rcmd(3) connection.
RSCSI If the RSCSI environment is present, the remote SCSI server will
not be the program /opt/schily/sbin/rscsi but the program
pointed to by RSCSI. Note that the remote SCSI server program
name will be ignored if you log in using an account that has
been created with a remote SCSI server program as login shell.
FILESSEE ALSOcdrecord(1), mkisofs(1), scg(7), fbk(7), rcmd(3), ssh(1).
If you don't want to allow users to become root on your system, readcd
may safely be installed suid root. This allows all users or a group of
users with no root privileges to use readcd. Readcd in this case will
only allow access to CD-ROM type drives- To give all user access to use
chown root /usr/local/bin/readcd
chmod 4711 /usr/local/bin/readcd
To give a restricted group of users access to readcd enter:
chown root /usr/local/bin/readcd
chgrp cdburners /usr/local/bin/readcd
chmod 4710 /usr/local/bin/readcd
and add a group cdburners on your system.
Never give write permissions for non root users to the /dev/scg?
devices unless you would allow anybody to read/write/format all your
You should not connect old drives that do not support disconnect/recon‐
nect to either the SCSI bus that is connected to the CD-Recorder or the
When using readcd with the broken Linux SCSI generic driver. You
should note that readcd uses a hack, that tries to emulate the func‐
tionality of the scg driver. Unfortunately, the sg driver on Linux has
several severe bugs:
· It cannot see if a SCSI command could not be sent at all.
· It cannot get the SCSI status byte. Readcd for that reason can‐
not report failing SCSI commands in some situations.
· It cannot get real DMA count of transfer. Readcd cannot tell
you if there is an DMA residual count.
· It cannot get number of bytes valid in auto sense data. Readcd
cannot tell you if device transfers no sense data at all.
· It fetches to few data in auto request sense (CCS/SCSI-2/SCSI-3
needs >= 18).
A typical error message for a SCSI command looks like:
readcd: I/O error. test unit ready: scsi sendcmd: no error
CDB: 00 20 00 00 00 00
status: 0x2 (CHECK CONDITION)
Sense Bytes: 70 00 05 00 00 00 00 0A 00 00 00 00 25 00 00 00 00 00
Sense Key: 0x5 Illegal Request, Segment 0
Sense Code: 0x25 Qual 0x00 (logical unit not supported) Fru 0x0
Sense flags: Blk 0 (not valid)
cmd finished after 0.002s timeout 40s
The first line gives information about the transport of the command.
The text after the first colon gives the error text for the system call
from the view of the kernel. It usually is: I/O error unless other
problems happen. The next words contain a short description for the
SCSI command that fails. The rest of the line tells you if there were
any problems for the transport of the command over the SCSI bus. fatal
error means that it was not possible to transport the command (i.e. no
device present at the requested SCSI address).
The second line prints the SCSI command descriptor block for the failed
The third line gives information on the SCSI status code returned by
the command, if the transport of the command succeeds. This is error
information from the SCSI device.
The fourth line is a hex dump of the auto request sense information for
The fifth line is the error text for the sense key if available, fol‐
lowed by the segment number that is only valid if the command was a
copy command. If the error message is not directly related to the cur‐
rent command, the text deferred error is appended.
The sixth line is the error text for the sense code and the sense qual‐
ifier if available. If the type of the device is known, the sense data
is decoded from tables in scsierrs.c . The text is followed by the
error value for a field replaceable unit.
The seventh line prints the block number that is related to the failed
command and text for several error flags. The block number may not be
The eight line reports the timeout set up for this command and the time
that the command really needed to complete.
If you want to actively take part on the development of cdrecord, you
may join the cdwriting mailing list by sending mail to:
and include the word subscribe in the body. The mail address of the
Additional information can be found on:
If you have support questions, send them to:
If you have definitely found a bug, send a mail to:
To subscribe, use:
Joerg Schilling Version 2.0 READCD(1)