Can not send SCSI command to the cdrom device.

Dear all:

I have a CDROM device, and after I plug it into the Linux system, the Linux system will generate three device files in the directory of /dev/, such as:
scd1: block special (11/0)
sr1: link to scd1
sg1: character special(21/1)

And I write a c program to send the SCSI command of SPACE to my CDROM device. Such as :

   fd = open("/dev/sg1", 0_RDWR);
   ioctl(fd, SCSI_IOCTL_SEND_COMMAND, cmdbuff);

And I found that I can send the SPACE command to my CDROM device, if I open the file of /dev/sg1. But if I open the file of /dev/scd1, then I can not send the SPACE command to my CDROM device.

Why I can not send SCSI command through /dev/scd1 ?

What difference between /dev/sg1 and /dev/scd1 ?

And how can I send send the command through /dev/scd1?

Wait for the help, thanks.

Best Regards,

Franko Fang
2009-1-17

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Probably because you don't have media in the drive in which case open("/dev/scd1", ...) will block. You might try or-ing in O_NONBLOCK as in open("/dev/scd1", 0_RDWR | O_NONBLOCK).

If you are using any Linux system made in the last 5 years then you should _not_ be using the SCSI_IOCTL_SEND_COMMAND ioctl. Try the SG_IO ioctl instead.

The sg driver makes a lot less assumptions about the SCSI device and is designed for lower-level, pass-through access at the SCSI command level. The bsg driver in recent kernels plays a similar role to the older sg driver which dates from 1992.

Answered above. BTW In the lk 2.6 series the SG_IO ioctl will work on both "sg" and "scd/sr" drivers.

For examples on the use of SG_IO have a look at my sg3_utils package, specifically the examples directory. You may find the sg_get_config utility interesting to use on a CD/DVD/BD drive.

Doug Gilbert