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SubjectGetting the number of 512-byte sectors?
Andries, LKML,

Referring to an old email I was just rereading:

At 01:29 02/10/2000, Andries Brouwer wrote:
On Mon, Oct 02, 2000 at 02:33:20AM +0200, Daniel Phillips wrote:
> >unsigned long maxsector = (blk_size[major][MINOR(bh->b_rdev)] << 1) + 1;
>blk_size[][] gives a block count
>blk_size[][]<<1 gives a sector count
>but if the device had an odd number of sectors, the sector count
>is one larger than twice the block count.
>(thus, this is not the precisely correct test; the knowledge about
>the number of sectors lives in the dev->sizes array; here we only
>have a rough check)

But according to drivers/block/blkpg.c, (direct quote): "hd_struct has the
number of 512-byte sectors, g->sizes[] and blk_size[][] have the number of
1024-byte blocks". Assuming the "dev" you refer to == the "g" blkpg.c
refers to, the dev->sizes array is also not what is needed for the correct
test? Looking at blkpg.c, g->sizes is initialized to plength >>
(BLOCK_SIZE_BITS - 9) so it is 1024 byte blocks, indeed.

My question is how to get the _real_ number of sectors of a partition from
within a file system. I.e. we are starting only with the knowledge of the
struct super_block for the partition.

Also, I need to know the starting offset of the data inside the partition.
- I am probably explaining this wrong but what I mean is that I want the
values "First Sector", "Last Sector" and "Offset" as shown by running
cfdisk /dev/hda
and selecting "Print", "Sectors".

Alternatively, it would suffice to obtain the subtracted values, ie. the
data starting sector = "First Sector + Offset" and the number of sectors =
"Last Sector" - ("First Sector" + "Offset"). I.e. what is shown by:
fdisk /dev/hda
and selecting: u (to change to units = sectors), followed by p (to display
the partition table) and then the values of interest are "Start" and "End",
from which the number of sectors can be calculated from.

Are these "subtracted" values what is present in the "struct hd_struct
*part" that can be found inside "struct gendisk"?

Assuming that hd_struct does contain the subtracted values as described
above, is the following procedure A) correct? and B) the best way to obtain

Locate the correct gendisk structure by performing a walk of gendisk_head
as done in drivers/block/blkpg.c::get_gendisk(kdev_t), (Could we make this
function to not be private to blkpg.c?), then get the hd_struct for the
partition and obtain the values from that. In code:

struct gendisk *g = get_gendisk(my_super_block->s_dev)
struct hd_struct *hd = &g->part[MINOR(my_super_block->s_dev)];

wanted "subtracted" values:

Am I missing something? Is the meaning of the values in hd_struct different
from what I think it is?

Note: I know I can normally get the number of sectors from the boot sector
of the partition but in this case please assume that there is no boot
sector present or that it is corrupt or that the values in it cannot be
relied on due to weird things done by the 'other OS'.

Thanks a lot for your help in advance.

Best regards,


"Programmers never die. They do a GOSUB without RETURN." - Unknown source
Anton Altaparmakov <aia21 at> (replace at with @)
Linux NTFS Maintainer
ICQ: 8561279 / WWW:

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