The disk volume header... again

[Shoshana Abrass]

  Last week I posted a question and answer about how to read the volume
  header into a c program. My code looks like this (extras and error  
  checking removed for brevity):

    struct volume_header  *vh;

    fd = open("/dev/rvh", O_RDONLY);
    ioctl(fd, DIOCGETVH, vh);

  The ioctl call fills "vh" with useful information, and I can then read
  the volume directory and determine the length and logical-block-number
  of the file I want to read.  But... what now?  I'd like to read this 
  file in off the disk.  I've looked at the man pages for vh, dksc,
  and the include files dvh.h and dkio.h, and nothing jumps out at me.

  I've tried doing an lseek to the logical block number, but I get either 
  zeros or garbage. I know zilch about how devices work at this level, so 
  I tried the following lseeks on both /dev/vh and /dev/rvh:

    lbn
    sizeof(struct volume_header) + lbn

  I then do a read of length-of-file, and I get 512 bytes back - of nothing.

  Perhaps there's an ioctl command for this that I haven't found yet, or I'm
  not doing the lseeks-reads correctly. 

  By the way, if I copy the file from the volume directory onto disk using
  dvhtool, I get an ascii file containing what I expect. The length and lbn
  in my program also match the values returned by dvhtool:

        File name        Length     Block #
        sgilabel            280           1
        diskno              512         414     <== the file I want
        ide              500224         415
        sash             200704        1392

  -shoshana
  shoshana at pdi.com