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    SubjectRe: procfs feature or bug?
    At 3:53 PM +0530 2001-09-19, Sandip Bhattacharya wrote:
    >I was trying out the multipage procfs entries, when i found out that i
    >was having a problem.
    >I am trying to use the "hack" by Paul Russell to allow mangling of
    >filepos using ``*start'' entries as my personal page offset. Now, for
    >multipage entries, whatever i set as *start should be coming back to
    >me as offset when the next page is called.
    >But in fs/proc/generic.c in proc_file_read() at the end we have (
    >after the first page has been "copied_to_user")
    > *ppos += start < page ? (long) start : n;
    >But this _adds_ the contents of start to the offset, in the case where
    >I am supplying the offset in ``start''. Shouldn't this just be
    >_replacing_ ? 'Cause in this case the offsets get cumulatively added,
    >causing an oops at the end for me :(
    >Or am I missing something big???

    Per the comment:

    > /* This is a hack to allow mangling of file pos independent
    > * of actual bytes read. Simply place the data at page,
    > * return the bytes, and set `start' to the desired offset
    > * as an unsigned int. -
    > */

    *ppos represents the offset in the *file*, not the buffer, so
    cumulative is correct. So when you do your read, you need to
    interpret it thus. I've been using it in one case as a record number,
    where the records are variable length. I set *start to 1, and treat
    is as an index into a (virtual) array of records.
    /Jonathan Lundell.
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