Here is my setup:

root@ubuntu:~# cdrskin drive_scsi_dev_family=sg --devices
cdrskin 1.3.8 : limited cdrecord compatibility wrapper for libburn
cdrskin: scanning for devices ...
cdrskin: ... scanning for devices done
cdrskin: Overview of accessible drives (1 found) :
-----------------------------------------------------------------------------
0  dev='/dev/sg1'  rwrw-- :  'HL-DT-ST'  'DVDRAM GH24LS50'
-----------------------------------------------------------------------------
root@ubuntu:~# cdrskin drive_scsi_dev_family=sr --devices
cdrskin 1.3.8 : limited cdrecord compatibility wrapper for libburn
cdrskin: scanning for devices ...
cdrskin: ... scanning for devices done
cdrskin: Overview of accessible drives (1 found) :
-----------------------------------------------------------------------------
0  dev='/dev/sr0'  rwrw-- :  'HL-DT-ST'  'DVDRAM GH24LS50'
-----------------------------------------------------------------------------


The optical drive does contain a media with data on it but it is not mounted.


Here is the testcode (compiled with "gcc -Wall -o test -pedantic test.c"):

#include <fcntl.h>
#include <stdio.h>
#include <unistd.h>

int main(int argc, char **argv)
{
	int file;

	if(argc < 2)
	{
		printf("Missing argument \"device\"\n");
		return 1;
	}
	if((file = open(argv[1], O_RDWR | O_EXCL)) < 0)
	{
		printf("Error\n");
		return 1;
	}
	printf("Device locked\n");
	sleep(15);
	close(file);
	return 0;
}


Test on /dev/sg*:

On executing "./test /dev/sg1" on the first terminal I'm seeing "Device locked" and on executing the same command on the second terminal it blocks (this is what I would expect).


Test on /dev/sr*:

On executing "./test /dev/sr0" on the first terminal I'm seeing "Device locked" and on executing the same command on the second terminal I'm seeing "Error" (but I would expect a blocking behavior).