Re: [Mtools] Reading label from a large capacity memory card

[Alain Knaff]

Simon Large wrote:
> Hi David,
>
> Thanks for your reply and sorry for my lack of response. Stupid Exchange
> Server throws all Bcc mail in the bin, so your mail never reached me.

So, why again do you use Exchange?... :-)

[...]
> > Hi folks,
> >
> > I am try to read the volume label from a memory card. 2 questions:
> >
> > 1. Using mlabel -s a: with a 4GB card I get the message:
> > "Big disks not supported on this architecture."

The reason for this is that the compiler that you are using on that 
architecture (ARM?) does not have a data type (such as long long) big 
enough to support offsets found on "big" disks (larger than 2G).

The following data types are tried:
        off_t
        loff_t
        offset_t
        long long

If none of these are 64 bit capable, then the maximal allowable size is 2G 
(the biggest amount representable in a signed 32 bit integer).

If there is another data type capable of holding more than 32 bits, you may 
add it to llong.h by replacing long long in lines 35 and 36:

#  define MT_OFF_T long long
#  define MT_SIZE_T unsigned long long

In order to test out this hypothesis, try using mlabel on the same card on 
a PC with an Intel processor.

Theoretically, I could "hand-design" such a type using a struct, but then 
the problem would still exist of passing such offsets to the kernel, 
because its lseek would not support these size either.

If all you need is mlabel, you might get by by commenting out the check in
init.c:
if (tot_sectors >= (maxSize >> This->sectorShift)) {
        fprintf(stderr, "Big disks not supported on this architecture\n");
        exit(1);
}

becomes:

if (tot_sectors >= (maxSize >> This->sectorShift)) {
         tot_sectors = (maxSize >> This->sectorShift);
}

Indeed, for mlabel this would probably be good enough, as it merely needs 
to read the root directory, which would usually be at the beginning of the 
partition. Theoretically a FAT32 could have it anywhere, but usually, 
that's rather unlikely.

[...]
> > 2. I only ever want to read the volume label. I don't need to write
> > anything to the card, or use any other mtools great features (sorry,
> > nothing against mtools). Is there an easy way to make a cut down app
> > which just does this? I am cross compiling for ARM and it has to go
> onto

The (short) label is stored in the boot sector starting at byte position 43.

Try using the following command to read the (short) volume label.

dd bs=1 skip=43 count=11 if=/dev/mmvblk0p1 2>/dev/null

where img.bin is the image whose label you want to read.

A small equivalent C program could achieve the same.

For long labels (more than 11 characters), it gets a little bit more 
complicated, because there it is stored in the root directory.

Regards,

Alain