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<TITLE>The Linux SCSI programming HOWTO: Example Using Sense Buffer</TITLE>
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<H2><A NAME="s11">11. Example Using Sense Buffer</A></H2>
<P>Here we will use the TEST UNIT READY command to check whether media is
loaded into our device. The header declarations and function
<CODE>handle_SCSI_cmd</CODE> from the inquiry example will be needed as well.
<P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
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 |
+=============================================================================+
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>
<P>Here is the function which implements it:
<P>
<BLOCKQUOTE><CODE>
<PRE>
#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;
}
</PRE>
</CODE></BLOCKQUOTE>
<P>Combined with this <CODE>main</CODE> function we can do the check.
<P>
<BLOCKQUOTE><CODE>
<PRE>
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");
}
}
</PRE>
</CODE></BLOCKQUOTE>
<P>
<P>The file <CODE>generic_demo.c</CODE> from the appendix contains both examples.
<P>
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