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11. Example Using Sense Buffer

Here we will use the TEST UNIT READY command to check whether media is loaded into our device. The header declarations and function handle_SCSI_cmd from the inquiry example will be needed as well.

                        Table 73: TEST UNIT READY Command
+=====-========-========-========-========-========-========-========-========+
|  Bit|   7    |   6    |   5    |   4    |   3    |   2    |   1    |   0    |
|Byte |        |        |        |        |        |        |        |        |
|=====+=======================================================================|
| 0   |                           Operation Code (00h)                        |
|-----+-----------------------------------------------------------------------|
| 1   | Logical Unit Number      |                  Reserved                  |
|-----+-----------------------------------------------------------------------|
| 2   |                           Reserved                                    |
|-----+-----------------------------------------------------------------------|
| 3   |                           Reserved                                    |
|-----+-----------------------------------------------------------------------|
| 4   |                           Reserved                                    |
|-----+-----------------------------------------------------------------------|
| 5   |                           Control                                     |
+=============================================================================+

Here is the function which implements it:

#define TESTUNITREADY_CMD 0
#define TESTUNITREADY_CMDLEN 6

#define ADD_SENSECODE 12
#define ADD_SC_QUALIFIER 13
#define NO_MEDIA_SC 0x3a
#define NO_MEDIA_SCQ 0x00

int TestForMedium ( void )
{
  /* request READY status */
  static unsigned char cmdblk [TESTUNITREADY_CMDLEN] = {
      TESTUNITREADY_CMD, /* command */
                      0, /* lun/reserved */
                      0, /* reserved */
                      0, /* reserved */
                      0, /* reserved */
                      0};/* control */

  memcpy( cmd + SCSI_OFF, cmdblk, sizeof(cmdblk) );

  /*
   * +------------------+
   * | struct sg_header | <- cmd
   * +------------------+
   * | copy of cmdblk   | <- cmd + SCSI_OFF
   * +------------------+
   */

  if (handle_SCSI_cmd(sizeof(cmdblk), 0, cmd, 
                            0, NULL)) {
      fprintf (stderr, "Test unit ready failed\n");
      exit(2);
  }

  return
   *(((struct sg_header*)cmd)->sense_buffer +ADD_SENSECODE) != 
                                                        NO_MEDIA_SC ||
   *(((struct sg_header*)cmd)->sense_buffer +ADD_SC_QUALIFIER) != 
                                                        NO_MEDIA_SCQ;
}

Combined with this main function we can do the check.

void main( void )
{
  fd = open(DEVICE, O_RDWR);
  if (fd < 0) {
    fprintf( stderr, "Need read/write permissions for "DEVICE".\n" );
    exit(1);
  }

  /* look if medium is loaded */

  if (!TestForMedium()) {
    printf("device is unloaded\n");
  } else {
    printf("device is loaded\n");
  }
}

The file generic_demo.c from the appendix contains both examples.


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