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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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